Evolución y Clima

7/23/2019 Evolucin y Clima

1/44

Environmental Hypotheses of Hominin EvolutionRICHARD POTTSHum an Or ig ins Pr ogr am , Nat i onal M useum of Natur al H istor y,Smi thsonian In stituti on, Washington, DC 20560-0112

K E Y W O R D S c limat e; habita t ; ada ptat ion; varia bility select ion;fossil huma ns; st one tools

A B S T R A C T The study of human evolut ion ha s long sought to explainm a j or a d a p t a t i on s a n d t r en d s t h a t l ed t o t h e or i gi n o f H omo sapiens.Environmental scenarios have played a pivotal role in this endeavor. Theyrepresent statements or, more commonly, assumptions concerning the adap-t ive context in which key hominin tra its emerged. In m any cases, however,these scenarios are based on very li t t le if any da ta about the past set t ings inwhich early hominins lived. Several environmental hypotheses of humanevolution are presented in this paper. Explicit test expectations are laid out,and a preliminary assessment of the hypotheses is made by examining theenvironment a l records of Olduvai, Turka na , Olorgesa ilie, Zhoukoudia n, CombeG renal, a nd other hominin localit ies. H abita t-specifi c hypotheses have pre-vailed in almost all previous accounts of human adaptive history. The rise ofAfrican dry savanna is often cited as the crit ical event behind the develop-ment of terrestrial bipedality , stone toolmaking, and encephalized brains,am ong other t rait s . This sa vanna hypothesis has been countered recently bythe woodland/forest hypothesis, w hich claims tha t P liocene hominins hade volve d in an d we re p r ima ri ly a t t ra ct e d t o c los ed h abi t a t s . Th e ide as t h a th u ma n e v o lu t io n wa s fo s t e re d by c o ld h abi t a t s in h ig h e r la t i t u de s o r byseasonal variat ions in tropical and temperate zones also have their propo-nents. An alternat ive view, the variability select ion hypothesis, states that

large disparities in environmental conditions were responsible for importantepisodes of a daptive evolut ion. The result ing a dapta t ions enha nced behav-ioral versat il i ty and ult imately ecological diversity in the human lineage.Global environmental records for the late Cenozoic and specific records athominin sites show the following: 1) early human habitats were subject tolar ge-scale remodeling over t ime; 2) the evidence for environment a l insta bil-ity does not support habitat-specific explanat ions of key adaptive changes;3) the ra nge of environmenta l change over t ime wa s more extensive and t hetempo far more prolonged than allowed by the seasonality hypothesis; and4)t he var iability selection hypothesis is strongly support ed by the persistenceof hominins through long sequences of environmental remodeling and theorigin of important adaptat ions in periods of wide habitat diversity . Earlybipedality, stone tra nsport , diversificat ion of ar t ifact contexts, encephaliza-t ion, and enhanced cognit ive and social funct ioning all may reflect adapta-

t ions to environmenta l novelty an d highly va rying select ive contexts. YrbkP hy s Anth ropol 41:93136, 1998. 1998 Wiley-Liss, I nc.

TABLE OF CONTENTSA P a leoenvironmenta l P rimer ........ . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. 95

Ma rine oxygen isotope a na lysis ....... . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . 96Ocea n dust records ........ .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . 98Ice cores .............................................................................................................................. 99

Y E AR BOOK OF PH Y SI CA L AN TH ROPOL OGY 41:93136 (1998)

1998 WIL EY-L I SS, INC.


2/44

Sediment a ry environment ........ .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . 99La ke sediments ........ . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . 99Loess sequences ................................................................................................................101Fossil pollen sequences .....................................................................................................101

St able isotope a na lysis of soils ....... . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . .101Fa una l a na lysis ....... . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .104Sum ma ry ........ . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .104

A Fra mework for Ana lyzing Ada pta tion ........ .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . .104En vironmenta l H ypotheses of Hominin Evolution ........ . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . .106

A concise hist ory .............. .............. .............. ............... .............. .............. ............... ...........106Cha llenges of t he sa van na ........ . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .106Int rinsic a nd extrinsic a pproa ches ........ . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . .106Turn over hypoth eses .............. .............. .............. ............... .............. .............. .............. ..108

Adaptive evolution in hominins .......................................................................................109Ha bita t-specifi c hypotheses ........ .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . .109Sh ort-term var iability hypothesis ........ . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . .111Long-term (variability selection) hypothesis ................................................................111

Test implica tions .............. .............. ............... .............. .............. .............. ............... ...........112Ha bita t-specifi c hypotheses ........ . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . .112Sh ort-term var iat ion (seasona lity) hypothesis ........ . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . .113Variability selection hypothesis ....................................................................................113

En vironmenta l Records at Ea rly H uma n Localities ........ . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . .1146.0 to 2.5 million years ago ...............................................................................................114

Ea stern a nd southern Africa ........ . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . .114Discussion ......................................................................................................................116

2.3 to 1.7 million years ago ...............................................................................................118Turka na a nd Olduvai ba sins ........ . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . .118Discussion ......................................................................................................................119

1 million to 250,000 years ago ..........................................................................................122Olorgesailie basin ..........................................................................................................122Zhoukoudia n, the Loess Pla tea u, an d B ose bas in, China ........ .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . .122Dis cussion ............... .............. .............. ............... .............. .............. ............... .............. ..125

140,000 to 50,000 years ago ..............................................................................................125Combe G rena l, Fra nce ........ . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . .125Dis cussion ............... .............. .............. ............... .............. .............. ............... .............. ..127

En vironmenta l P erspectives on Adaptive C ha nge ........ . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .128Conclusions ...........................................................................................................................130Acknowledgments .................................................................................................................131Litera tur e Cit ed ........ . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . ..132

Regarding t he study of human evolut ion,i t m a y b e r e m a r k e d t h a t e v e r y b o d y t a l k sabo ut t h e e n viron me n t , bu t n obody doe smuch a bout i t . The environment of human

evolution has been a source of speculations in ce D arw in . An a ly s e s o f a n ima l bon e s,pollen, and sediments have rendered a gen-e ra l p ort ra i t of P l ioce n e a n d P leis t oce n eh abi t a t s . Ye t fi e ld p roje ct s are ra re ly de -signed specifically to recover high-resolutiondat a on the ecological set t ings inha bited bye ar ly h u man s . T h u s , t h e re is a p a u c i t y o ffi n e -s ca l e s p a t i a l a n d t e mp or a l d a t a t h a t

can be linked to in situ evidence of homi-nins.1 U s u a l ly, fi n d i n g f os si ls a n d d a t i n gthem ha ve oversha dowed an y effort to deter-

1Although it will take getting used to, the termh o m i n i n

is nowpreferred in place of hominid to indicate th e group of bipeda l apesto which huma ns belong (genera Homo, Australophitecus, Paran-thropus, a nd A r d i p i t h i c u s ) (Begun, 1992; Andrews, 1992; An-drews et al. , 1996, Delson a nd Tat tersa ll, 1997). The systema ticsof apes, including huma ns, now recognizes the growing body ofevidence that hu mans a nd chimpanzees are s is ter taxa . Withinthe superfamily Hominoidea (apes) is the family of great apes(Hominidae). The hominid fa mily includes the subfamily ofAfrican apes (Homininae), or hominines. The t r ibe ra nk ofHominini, then, is reserved for humans and related bipeds sincethe spl i t from the chimpanzee l ineage. Because early Pl iocenebipeds no longer comprise a val id subfam ily, I use the informaltermaustralopi thsto refer to them.

94 Y E A R B O OK O F P H Y S I C A L A N T H R O PO L O G Y [Vol. 41, 1998


3/44

mine ha bitat contexts. A related problem ist h a t mos t p roje ct s s t i ll focu s on s u rfac ecollect ing, with unavoidably poorer tapho-nomic and contextual control than obtain-

able by dig g in g . I t i s , h o we v e r , o n ly wi t hexact stra t igraphic relat ing of environmen-t a l da t a , fo ss i ls , a n d a r t i fact s t h a t a n s we rsm a y e me rg e a b o ut t h e r a n g e of h a b it a t sas sociat ed with par ticular species, wh en keyexpa nsions of habita t use occurred, or others u bs t an t iv e q u e st ion s re la t e d t o h omin inada ptive change.

As fossil discoveries have sparked moretha n a century of ideas on the phylogenetichistory of hominins, so research on the con-t e x t s o f fo s s i ls an d ar t i fac t s ma y c o me t ostimula te a long phase of careful work on theecological history of hominins. The immedi-at e goal is to ma ke this endeavor as syst em-at ic, empirically ba sed, and compara t ive aspossible. Clear definition of hypotheses andtheir test implicat ions can help serve thisgoal.

The fi rst pur pose of this pa per is to defi nepivota l hypotheses about t he context of ear lyh u man e v o lu t io n a n d t o la y o u t t h e ir t e s timplications. Second, I will present some ofthe best-known environmental sequencesfrom hominin sites as a preliminar y series oftests. En vironmenta l hypotheses can be di-vided into tw o types: those concerning a da p-

tat ion and others concerning species turn-over. Although the lat ter will be discussedbriefl y, our focus is on environm enta l hypoth-e s e s as t h e y re la t e t o a dap t iv e e v o lu t io n .Th is t o pic is of mark e d in t ere s t becau s eadaptive explanat ions relate direct ly to theprocesses of natural selection and thereforehelp to discern how important evolutionarytransformations took place.

Historically, the search for a convincingn arra t iv e o f h u ma n e v o lu t io n h a s s t imu -l a t e d s t r o n g i n t e r e s t a n d g u e ss w o r k a bout the ada ptive history of hominins. Thus,essentially every account of hominin evolu-

t ion describes or a ssumes something a boutthe milieu of human ancestors and the envi-ro n me n t a l p ro ble ms t h at u n iq u e ly h u manfeatures supposedly solved or overcame. Ourfocus here on environment a l hypotheses withan a daptive fla vor requires a framew ork forana lyzing adapta t ion. So a third goal of thispaper is to outline such a framework, whichwill show how paleoenvironmental studies

ma y contribute decisive dat a to the study ofhominin evolution.

A fi na l aim is to offer a br ief primer to helpant hropologists comprehend the barra ge ofpaleoenvironmental methods, data, and in-t e rpre t a t ion s p u bl is h ed a lmo st we e kly injo u rn als s u c h as Nature, Science, Quater-nary Research, a nd other scientific periodi-ca ls . A bas ic famil iar i t y wi t h is ot o pe da t a ,deep-sea cores, pollen analysis, loess climatec u rv e s , o rbi t a l p arame t e rs , an d t h e ma insources of paleoenvironment a l evidence a ndhabitat change is essential for understand-ing th e contexts in wh ich hominins evolved.To the extent t ha t context is importa nta ndD arw in ian an d la t e r e volu t ion ary biolog yestablished its fundamenta l role in organis-

mal evolutionthe field of paleoanthropol-ogy is , in this view, on the brink of novelideas and da ta sets concerning the h ow a n dw h y of hominin evolution. Tremendous a d-v an c e s in t h e e n v iro n me n t a l s c ie n c e s a reforcing th ese developments t o occur.

A PALEOENVIRONMENTAL PRIMER

An intricate system of heat and moisturecirculat ion determines Earths climate andhas strong influence over the distribution ofe n viron me n t a l zon e s an d g radien t s . Th ise nor m ou s h ea t i n g a n d p lu m bi n g s y st e m

links the atmosphere, oceans, and land. It isaffected by orbital cycles that determine thep lan e t s v ar iable do s e o f s olar radia t io n .Volcanic activity and uplift of the crust alsoimpact i t . The basic processes and factorstha t therefore contribute to t his circulat ionsystem are

t h e a m ou n t a n d y ea r l y p a t t e r n o f s ol a rra diat ion, determined largely by the plan-et s dista nce, orbital sha pe, a nd a xial t i l trelat ive to the sun;

t h e amo u n t o f s o lar radia t io n abs o rbe dan d re fle c t e d by t h e a t mo s p h e re an d by

the ground surface; atmospheric processes, including evapora-tion, cloud formation, and rainfall;

ocean a nd wind currents, two relat ed pat -terns of circulat ion tha t r edistribute heatand moisture in a biased manner aroundthe globe;

the size a nd distribution of ice caps, landma sses, and ma jor bodies of w at er, a ll of

95E N V I R O N M E N T A L H Y PO T H E S E S OF H O M I N I N E V O L U T I O N P ot t s]


4/44

wh ich crea te complex feedback with a tmo-spheric processes;

lan d uplift , which may interr upt or deflectocean and at mospheric circulat ion cur-

r en t s a n d m a y cr ea t e r a i n s h a d ow s t h a tbias th e distribution of rainfall near high-lan ds ; a n d

v o lc a n is m, wh ic h a l t e rs c l imat e by t h ee ru pt ion of a irborn e p ar t icles , an d re -g ion a l lan ds cap e s by t h e de pos i t ion oflava or tephra (e.g., ash, pum ice).

A series of methods ha s been developed tomeas ure the intera ctions a nd effects of theseprocesses on Ea rths environments. All ofthese meth ods ar e applicable to the period ofhuma n evolution; indeed, some of their mostfruit ful applicat ions have been in the con-

text of paleoant hropologica l resear ch.Marine oxygen isotope analysis

Interact ion between the ocean and atmo-s p h e re l ie s a t t h e h e a r t o f o x y g e n s t ableisotope a na lysis, a technique tha t ha s revolu-tionized pa leoclimat e stud ies since the 1950s.Dur ing evaporat ion, the sta ble isotope 18 O i senriched in ocean water as the lighter 16O ispreferentia lly relea sed into the at mosphere.2

In building their carbonate skeletons, cer-tain marine microorganisms, such as fora-minifera, absorb oxygen isotopes in propor-t io n t o t h e s u rro u n din g s e awa t e r . An 18O

-enriched ocean will thus result in a higherrat io of 18O t o 16O in the microscopic shells.This t ra nsfer from ocean to foraminifera isalso affected, though, by temperature. Fore v e ry 1 C dro p in wat e r t e mp e ra t u re , t h eproportion of 18O t o 16O w i t h in t h e f or a mshells is increased by about 0.22 parts permil (). Thus the oxygen isotope ra tio18O (delta 18-O)in foraminifera is a mea-s u re, or p rox y, of bot h t e mpe ra t u re an devaporation. 3

During glacial periods, oxygen (biased to-w a r d 16O) tha t ha s evaporat ed from the seabecomes locked up in ice sheets a s precipita -

t ion fa l ls o n la n d. Th u s , fo ram 18O fu r-

nishes a record of gla cial ice volume (whichcontains the sequestered 16O) an d tempera-ture.4 P la nkt onic species of foram s give infor-ma tion a bout sea-surface condit ions, whilebenthic forams provide data about the deep-ocean environment , w hich is considered toreflect a wider record of ice (evaporat ion)an d t e mpe rat u re h is t ory. Th is h is t ory isreconstructed by measuring 18O in fo ra mskeletons found in cores dr illed from thedeep sea . 18O curves are often thought toprovide a mea sure of global clima te, but t hisidea only refers to ocean temperature andice volume, a history t ha t ma y be decoupledfrom environments on lan d (e.g., deMenoca let al., 1993).

F ig u re 1 de p ic t s t h e av e rag e c u rv e fo r

benthic forams from 70 Ma to the present .According to this curve, substant ial globalcool in g of oce an s t o ok p lac e du rin g t h isperiod, with especially marked ice-sheet for-ma tion over the pa st 3 myr or so. By lookingat specifi c ocean cores, however, one seest h at t h is av e rag e cool in g /dry in g t re n d isactually composed of many oscillations, theam plitude of which ha s risen over the past 6myr (Fig. 2A). Sets of oscillations occurredwit hin wa rmer a nd colder periods, forming as e q u en ce o f ox y g en is ot o pe s t ag e s (odd-n u m be re d s t a g es r ep re se nt w a r m t i m es ;even refer to cold). Isotope st ages 1100cover th e past 2.5 myr. As depicted in Fig ure2A, oscillat ion w a s par ticularly extreme overthe pa st 700 ka. In this period, single excur-sions of the 18O curve (about 100 kyr long)encompas sed 1.5or m ore, w hich exceededt h e t ot a l a v er a g e ch a n ge s in ce t h e l a t eMiocene (1.4). Fora m r ecords b a ck to th elat e Oligocene (see Fig. 2B) indicat e tha t t het o t a l ran g e o f 18O v ar ia t io n wi t h in 1 my rs pa n s w a s f a ir ly s t a bl e u p t h r ou gh t h eMiocene. The total variat ion in each spanconsistently ranged between 0.3 and 0.8up until 56 million yea rs a go. At t ha t t ime,

t h e amo u n t o f v ar ia t io n in c re as e d s ig n ifi -cantly and continued to rise to the present(Potts, in press a). As we will see, differenthypotheses about hominin evolution are sup-

2This process leading to isotope enrichment is called fraction-ation, in which the ra t io of s table isotopes is al tered during t hetransfer of an element (in this case, oxygen) from one medium toanother .

318O measurements reflect the relat ive concentrat ion of thetwo isotopes and a re expressed as variat ion from a s tanda rdisotopic reference, measured on a belemnite from t he P ee DeeFormation (Cretaceous a ge), South Car ol ina.

4F o r p e r io ds i n E ar t h s h i s t or y wh e n i ce s h e e t s w e r e n o tpresent , the isotope rat io is primari ly a measure of temperature.



5/44

ported depending on whether one focuses ont h e ov era l l cool in g t re n d (F ig . 1) or t h eoscillat ion (Fig. 2).

St u dy o f t h e f re q u e n c y a n d s t re n g t h o foscillat ion, using a t echniqu e ca lled spectra lan a ly s is , s h ows t h at 18O fluctuation occursat certa in periodicit ies. For the pa st 1 mil-

lion years, the dominant cycles are approxi-mately 100 kyr and 41 kyr long and a dualcycle of 23 kyr and 19 kyr. These cyclicitiesma tch variat ions in Ea rths orbit a round thesun, an idea proposed by Croll (1875) andt h e n e x p an de d by t h e mat h e mat ic ian M i-lan kovitch (1941). A complete eccentr icity

Fig. 1. Oxygen isotope curve (avera ge) based on composite dat a from benthic fora minifera, 70 Ma t opresent (Miller et al., 1987).



6/44

cycle (the circular-to-elongat e shift ing ofEa rths ellipt ical orbit a round the sun) ta kesapproximately 100 kyr. Obliquity (the chang-ing t i l t of the planet s a xis of rota t ion rela-

t ive to t he sun) varies betw een about 21.4an d 24.4 on a cycle 41 kyr long. P recessionof the equinoxes, due to Ea rth s a xial w obble,changes th e timing of the seasons rela tive tothe dista nce from the sun on a cycle last ingabout 21 kyr. Over this cycle, t he northernhemisphere is t i l ted towa rd th e sun a t suc-cessively different points in Ea rth s orbit.

These cycles affect the total am ount ofincoming sola r energy (insola tion), a nd t heyin t erac t t o cre at e comp le x fl u ct u at io n ininsolat ion, which is remarkably registeredi n t h e s h el ls of f or a m s a n d t h e ox yg en -isotope curve. It is assumed, therefore, thatt h e s e orbi t a l v a r ia t ion s h av e a p rev ai l in ginfluence over global climate, including over-all cooling during t he lat e Cenozoic and therat e a nd ra nge of oscillat ion. An importantmilestone of Cenozoic climate change wasthe onset of high-lat itude glaciat ions star t-in g aro u n d 2.8 M a, a n e v en t t h a t h as bee nlinked to events in hominin evolution.

Ocean dust records

Dust plumes arising from continents re-sult from strong seasonality of rainfall and

Fig. 2. A:Oxygen isotope curve 6 Ma to present, ba sedon benthic foraminifera (Shackleton, 1995). B: Range ofvariation in oxygen isotope values in 1 million year inter-vals from la te Oligocene (27 Ma) to present. B ased on da tafrom Miller et al .(1996), Wrigh t a nd Miller (1992), Woodruffet a l . (1981), S ha ckleton (1995), a nd Pr entice and Denton(1988). (Reproduced from Potts, in press a, with permissionof the publisher).



7/44

in the winds tha t carry t he dust to the ocean.Aeolian dust accumulations found in oceans edimen t core s t h u s p rov ide a re cord oflong-term cha nge in seasona lity a nd prevail-

ing wind pa tt erns. These records may r eflectthe history of continental vegetation cover,s in ce la rg e a re a s of op en v eg et a t ion an ddesert enhance the chances of erosion andwind tr an sport of land detritus.

I n a de t a i le d s t u dy of c ore s o ff t h e w e stand east coasts of Africa, deMenocal (1995;deMenocal a nd B loemendal, 1995)h as showntha t African climat e (and vegeta t ion cover)wa s h ig h ly v ar iable f rom e ar ly P l ioce n e t othe present (Fig. 3). Spectra l an a lysis (rightside of Fig. 3) indicates a change in oscilla-t i on a t a r o un d 2. 8 M a , f r om a d om i na n tfreq uency of 23 to 19 kyr (precession) to oneof 41 kyr (obliquity) . Other shifts occur at1.7 and 1.0 Ma, at which point the 100 kyrfrequency begins to dominate. These find-ings suggest that African climate variat ionwa s tuned to orbit-rela ted changes in insola-tion. These shifts represent periods of mark-e dly in c re as e d a r idi t y a t 2 .8 , 1 .7 , an d 1 .0Ma; the origin of certain hominin species inAfrica after 2.8 Ma, therefore, may reflectthe rise of an arid-adapted biota (deMeno-cal, 1995). H owever, a s n oted by deMenoca land Bloemendal (1995), these dates primar-i ly s ig n al s u bs t an t ia l r is e s in t h e o v e ra l lvaria nce, or amplitude of fluctua t ion, rathertha n a permanent shift towa rd a drier, moreopen environmen t (Fig. 3).

Ice cores

Accumulation of ice in glaciers, ice sheets,an d ice shelves results from precipita tion ofwat e r e v ap o ra t e d larg e ly f ro m t h e o c e an .Thus, 18O an a ly s is of core s dr i l led f romma jor ice bodies yields a high-resolut ionrecord of climate, which is the inverse ofdeep-sea 18O . P a r t ic ula r ly in format iv e re -cords int o the middle P leistocene ha ve beenobt a in e d f ro m G re e n la n d a n d An t arc t ica .

One significant finding from the Greenlandcores is evidence of extremely r a pid oscilla-t ions during certain t ime intervals. Knownas D a nsga a rd-Oeschger events, these oscilla-t ions last around 1,000 years; during th esee ve n t s , fl u ct u a t ion s bet w e en in t e rg lac ia lwarmt h an d s e v e re c o ld t ak e p la c e o v e r ace nt u r y or e ve n a d eca d e i n s om e ca s e s

(B roecker a nd Dent on, 1990; Ta ylor et a l.,1993; D a n s g a a r d e t a l., 1993). B ased ont h es e fi n d i n gs , i t h a s b ee n a r g u ed (e .g .,C a l v in , 1 996) t h a t h om i n in a d a p t a b i l it y

e volve d in re sp on s e t o larg e , s h ort -t e rmfluctuations in middle or high latitudes.

Evidence of such abrupt variat ion, how-e ve r, is abs e n t in t h e An t arc t ic ic e c ore s(J ouzel et al ., 1993), s u g ge st in g t h at t h eoscillation events may be largely a northernhemisphere phenomenon. Apartial explana-t ion is that rapid cooling may correspondwith periods of iceberg discha rges (or ra ft-ing) into the North Atla ntic (known a s Hein-r ic h e v e n t s ) ; by addin g f re s h wat e r t o t h esea, these discharges may suddenly reducethe heat -conveying effect of one of the mostimport an t oce an circu la t ion s y s t ems , t h eNorth Atlant ic Deep Wa ter, thereby causin gabrupt cooling of 58C (Bond et al., 1992;St au f fe r e t a l., 1998). Deep-sea da ta fromt h e No rt h At lan t ic s h o w t h at s u c h abru p tcooling-wa rming cycles occurred regula rlyback to at least 1 Ma (Raym o et a l., 1998).

Sedimentary environment

B a s ic g eolog ic an a ly s is a t h omin in s i t esincludes field recording a nd ma pping of sedi-ments, describing ma jor deposit ional envi-ronments ( lithofacies), and logging strat i-graphic change. Certain types of deposits ,such as ca liches, an d th e presence of part icu-lar minerals are indicat ive of specific envi-ronmenta l conditions (e.g., ar idity, rainfa ll).Other deposits , such as widely distributedt e ph ra , p ermit re ady corre la t io n bet we e ndista nt locat ions. Correlation of stra tigra phicsections over severa l kilometers ena bles thereconstruct ion of paleogeogra phic set t ingson a basinw ide scale. Such reconstruct ionsh av e p rov ed p owe rfu l in in t erp ret in g t h eadaptive contexts of hominins at Olduvai,Turkana , and other localit ies (Ha y, 1976a;B rown a nd F eibel, 1991).

Lake sediments

Lakes may provide habitats for diatoms orother environmentally sensit ive organisms(e.g., ostracods). Analysis of these organismscan y ield a p recis e h is t ory of lak e de p t h ,s a l in i t y, a n d a l k a l in i t yf a ct or s t h a t a r estrongly influenced by climat ic and tectonicv ar ia t ion s . S t u dy of d ia t o m s p ecies in t h e



8/44

Fig. 3. Contin enta l dust (%) mea sured in the ocean core from Sit e 661, off the west coa st of Africa, 4 Mato present . Percentage aeol ian dust was calculated using magnetic susceptibi l i ty , a measure of theconcentrat ion of ma gnetic part icles typically found in wind-blown dust . The power spectrum (right) is ananalysis of variance in the oscillation time series. This drilling site is one of seven on which deMenocalsanalysis is based (deMenocal and Bloemendal, 1995; deMenocal, 1995).


9/44

O lorg es ai l ie F ormat io n , for e xamp le, h ashelped to establish the history of an ancientlak e ba s in in h abi t ed by h omin in s (O we nand Renaut , 1981; P ot ts , 1994) and to cali-

bra t e ra t e s of e n viron me n t a l ch a n g e fac edby the toolmakers (see below).

Loess sequences

Thick sequences of alt erna ting loess (w ind-b l ow n s a n d ) a n d s oi l (p a l e os ol s ) o ccu rthr oughout vast r egions of Asia a nd E urope.Loess layers represent t imes when vegeta-t ion c ov er wa s s u ff icien t ly low t o a l lowbuildup of aeolian ma teria l. The soils, on t heo t h e r h an d, are o f t e n o rg a n ic -r ic h du e t ot h ic k v e g e t a t io n c o v e r a t t h e t ime o f s o i lformat ion. Loess sequences of north -centr a lChina have been especially well studied andprovide a detailed record of environmentalch an g e . O v er t h e p as t 2 .5 my r , a t leas t 44major shifts from cold to warm condit ionsha ve been documented.

Environmental interpretat ions are basedon the degree of loess a nd soil wea thering,fossil pollen and fauna, carbonate content ,a n d m a g n e t i c s u s ce pt i b i li t y. Th e l a t t e rmethod measures the concentration of mag-netic grains, w hich is especially high in fi nedust blown from distant sources. This finecomponent dominates during warm, wet pe-r io ds , wh i le re la t iv e ly c o a rs e g ra in s f ro mlocal sources tend to be deposited duringcold, dry eras. Thus, high susceptibility mea-s u re me n t s a re fo u n d in t h e s o i ls an d lo won e s in t h e loe ss lay e rs . Th e se me as u re-ments offer a record of oscillat ion in clima tean d v eg et a t ion . F ig ure 4 i llu s t ra t e s a s ec-t ion of t h e L u och u an loe s s s eq u e n ce , inwh ich strong fl uctuat ions between cold, opensteppe and warmer, wooded to forested con-ditions have been documented (Liu, 1988;Kukla an d An, 1989). Because t hey coverhuge continental areas, loess-soil depositsoffer a potent ially powerful m ethod of deter-mining environmenta l sequences in temper-

at e latit udes occupied by ea rly hominins.Fossil pollen sequences

P o ll en i s d e pos i t ed a s p ol l en r a i n , s amp le s o f wh ic h c a n be c h ara c t e r ize d bythe percenta ges of different vegeta tion com-ponents, known as a pollen spectrum. Thebas ic components ar e usually a rboreal (trees

and shrubs) and nonarboreal (herbs) pollen(abbreviated AP and NAP, respectively). Per-centa ge AP refl ects t he density of tree cover,though this provides only a rough measure

in low la t i t u des s in ce t rop ica l t re es h av elower pollen production than grasses andother NAP. Divisions of NAP often includegrasses (Gramineae), aquat ics (such as Ty -ph a and Cyperaceae), and the C/A group(Ch enopodiacea e/Amar a nt ha ceae). The fi rstindicates the relative density of grass cover,e sp ecia l ly wh e n aq u a t ic t ax a are fac t ore dout. The las t group can be a g ood indica tor ofaridity in African settings, since most of theC/A plan ts come from saline soils a nd drys t re ams . E n v iron me n t a l in t erp ret a t ion ofpollen spectra is based on modern a na loguesan d thus r equires careful compa rison of pastan d present sa mples a nd preservation condi-t io n s . T h e lac k o f ac c u rat e an alo g u e s fo rp as t c limat e s , p ar t icu lar ly g lacia l co ndi-tions, can sometimes be offset by incorpora t-ing other evidence (e.g., in E urope, the repre-s en t a t i on of f os si l b eet l es , w h i ch a r eclimat ically sensit ive). The sum of certainAP t ax a is o f t e n u s e d as a c l imat e in de x ,a l t h o ug h t h e v a l id i t y o f t h is ap p roac h h a sbeen questioned. In East Africa, fossil pollenanalysis has yielded a sequence of generalvegetat ion and climate reconstruct ions forlocal i t ies s u ch as O ldu v ai , Tu rk an a , an dH a dar ; wh i le in E u rop e t h e c or in g o f p ea tbogs has ena bled high-resolution a na lysis ofprecipitation and temperature based on pol-len (Bonnefille, 1995; Guiot et al . , 1989,1993; Woillard, 1978). Figure 5 illustrates adetailed series of climat ic fluctua t ions indi-cated by pollen spectra from the sedimentcore at L es Echets, Fra nce.

Stable isotope analysis of soils

Under certain conditions, buried soils, orpaleosols, preserve orga nic residues an d car-bonate deposits that bear a stable-isotopicsignal of past vegetat ion. Interpretat ion of

vegetat ion is based on the photosyntheticc h e m i s t r y o f p l a n t s a n d t h e s i g n a t u r e i tleaves in soils. Most plant species undergoone of tw o distinct photosynthetic pat hw ay s;o n e in v o lv e s t h re e c arbo n a t o ms an d t h eother four (thu s C 3a n d C 4plant s). C 4p la n t sare virtually all grasses adapted to hot , drycon dit ion s an d low CO 2 concentrations in



10/44

Fig. 4. Sequence of loess layers(Ln) and paleosols (S n) from Luo-chuan, north -centra l China , 1.1 Mato present , and environmental in-terpreta tion based on th e degree ofloess/soil weat hering, ma gnetic sus-ceptibil ity , pol len, and carbonateanalysis. 13, weakly, moderately,

and s trongly weathered loess , re-spectively; 4, black loam ; 5, ca rbon-a te cinnam on soil; 6, cinna mon soil;7, leached cinnamon soil; 8, darkcinnamon soil (Liu, 1988).



11/44

t h e a t mo s p h e re . C 3 plants include woodyspecies, herbs, a nd gra sses ada pted to coolgrowing seasons and shade. The two catego-ries ha ve nonoverlapping distributions of13O (rat io of carbon isotopes 13C /12C) . I n

tempera te zone soils, therefore,1 3

C can indi-cate change in climat e or at mospheric CO2.In tropical lowlands, this measure ma y alsoindicat e vegeta t ion structure along the gra -dient from heavily w ooded to open gras slan dhabitat. In addition, the oxygen isotope ratioc an be me as u re d in s o i l c a rbo n a t e an d iscorre la t e d wi t h me an a n n u a l t e mpe rat u re .

Combined analysis of 13C a n d 18O o f a n -cient soils, therefore, ca n a llow quite specificinterpretat ion of the complex relat ionshipbetween soil a nd at mospheric chemistry, veg-etat ion, and temperature (e.g. , Cerling and

Hay, 1986; Cerling, 1992; Cerling et al., 1997;Ambrose a nd Sikes, 1991). I sotopic study ofwidely exposed paleosols has also enableddetailed comparison to modern landscapeanalogues, which may be extended to recon-struct the mosaic of wooded, grassy, and wet-land habitats in strata bearing early humanar tifa cts (Sikes, 1994; Sikes et a l., in press).

F i g. 5. E s t i mat e s of m ean a n -n u al p r ec ip it a t i o n an d t e m pe r a-ture, expressed a s deviat ions frompresent values (800 mm a nd 11 C)plus confidence intervals, 140 ka topresent, based on pollen recoveredfrom the Les Echets pollen core,eastern France (Guiot et al., 1989).



12/44

Faunal analysis

R e la t iv e a bu n dan c e o f d i f fe ren t an imalspecies permits a general characterizat ion

of habita t , assuming tha t certa in taxonomicg rou p s c orre la t e wi t h s p ecifi c h a bi t a t s inboth the past and present (e.g., Vrba, 1975;Potts , 1988; Shipman and Harris, 1988). Adifferent array of methods uses funct ionalmorphology to test for d efinit ive a na tomicalindications of environment. This approachallows a n ecomorphic (a s opposed to a ta xo-n omic) c h arac t e r iza t ion of t h e biot a an das s o c ia t e d h abi t a t s wi t h o u t as s u min g p ar-t icular ta xon-ha bitat correlat ions. Methodsof ecomorphic an alysis have increasinglybeen a pplied t o the faun as of ear ly hominins i t es (e .g . , An drews , 1989; Plu mme r an d

B ishop, 1994; Ka ppelma n et al ., 1997; Reed,1997).

Summary

Development of these a nd other methodsof e n viron me n t a l in t erp ret a t ion h as p ro-gressed impressively over the past severaldecades. The contexts in which these meth-ods can be applied include deep-sea cores,peat bogs, soils , an d m any diverse types ofsediment. The methods can in ma ny cases gob ey on d a g en er a l h is t or y of cl im a t e a n dha bita t to offer a q uite deta iled picture of thead apt ive contexts fa ced by early hominins.

A FRAMEWORK FOR ANALYZINGADAPTATION

The process of a dapta t ion, na tura l selec-t ion, involves the relat ionship between anorg an is m an d i t s s u rrou n ding s . An org an -is ms e n viron me n t (a t leas t as p ect s t h a taffect survival and reproduction) influencesthe traits that persist , increase, and spreadthrough a populat ion. Tra its posit ively a f-fected by t he process of bia sed reproductionare called ada ptat ions. The ana lysis of ada p-t a t io n s t h u s may a f fo rd in s ig h t s in t o bo t ht h e p roce ss of n a t u r a l s el ect i on a n d t h e

environmenta l context in which evolut ionha s occurred. The purpose of this section isto defi ne an a nalyt ical framework that showshow paleoenvironmental data from homininsites may contribute to the study of adaptiveevolution in early humans.

The field of biology is divided between twomain approaches to the concept of adapta-

tion (Amundson, 1996; Endler, 1986). Thefirst is a nonhistorical approach, which de-fi n e s an adap t a t io n s o le ly in t e rms o f t h ecurrent , observed benefits that a part icular

trait offers to an organism (Bock, 1980). Theprocess of origin (na tur a l selection) a nd th ust h e h i st or y of a g iv en t r a it a r e, b y t h isdefinition, less relevant to deciding whethera t r a i t i s a n a d a p t a t i on . A n on h is t or i ca ldefinition of adaptation, then, concerns sim-p ly t h e con t r ibut ion of a t ra i t t o c u rren tfi tn ess (Fisher, 1985).

The second is a historical a pproa ch, whichfocuses not only on fitness but also on theprocess by which adaptat ion occurred. Ac-cordingly, an a dapta t ion is a t rait genera tedby n a t u ra l s elect ion du e t o i t s fi t n e ss ben -

efits. This approach focuses, then, on thehistory of a t ra it . I t allows th e possibility ofstudying and citing the selective causes thatunderlie a pa rticular a da pta tion. With homi-nin evolut ion, w e a re very much interestedin h is t ory. Th u s , t h e s econ d ap p roac h isadopted here. For one thing, current benefit(t h e bas is for de fi n in g a n a dap t a t io n in t h en on h is t or ica l ap p roac h ) h as n o me an in gwhen looking at characters of extinct homi-nins. In an evolutionary perspective, more-over, current benefit even in living humansd oe s n ot a l w a y s g iv e a t r u e p ict u r e o f atrait s original advantage or the condit ions

under w hich it evolved. In a historical an a ly-sis, then, ada ptat ion is the result of nat ura ls elect ion , an d t h e an a ly s is of ada p t a t ionentails an assessment of how the prevalenceof a t r a it i n t h e f os si l r e cor d m a y h a v eresulted from t his process.

B ut how does one recognize nat ura l selec-t i on , or a t l ea s t i t s r es u lt s , i n t h e f os s ilre cord? Th e fi rs t orde r of bu sin es s is t odefine a tra it , feature, phenotype, or char ac-ter (all considered synonyms here) or even acomplex of traits or a definable morphologi-cal or behavioral pa tt ern. The defi nition of afeature or overall phenotype provides the

bas is fo r e x amin in g fu n c t io n , be n e fi t , an devolutionary setting.

I n a h is t o r ic a l ap p ro ac h , an an aly s is o fada p t a t ion mu s t a ls o be bas e d o n t h e p ro-cess of natural select ion itself . As summa-rized by Lewontin (1970), the process hasthree necessary an d sufficient condit ions:v ar ia t io n , h e r i t abi l i t y , an d di f fe re n t ia l fi t -



13/44

ness. For select ion to operate, the units ofevolut ionar y change (e.g., specifi c pheno-t y p ic fea t u re s ) mu s t v ary. Th e v ar ia t io nsmust be inherited (e.g., the phenotypes must

relat e to specific genetic varia tions). And t hevariat ions must differ in their representa-tion in later generations.

These three condit ions furnish a n init ialf rame work , bu t man y ot h e r fa ct o rs e n t e rin t o a n a n a ly s is o f a dap t a t ion . D i ffere n t ia lfi t n e ss oc cu rs p res u mably du e t o ce r t a inbenefi ts th at one var iant offers compared toothers (alternative phenotypes). So it is es-s en t i a l t o d et e rm i ne w h a t t h os e b en efi t swere (or are), especially how a trait s struc-tural design or pat tern of expression pro-vided a select ive adva nta ge. Each beneficial

f e a t u r e m a y a l s o h a v e i t s s h a r e o f c o s t s ,w h i ch ca n n ot ou t w e ig h t h e b en efi t s i f afeature is to increase via natural select ion.The adaptive evolution of a feature is oftenassumed t o ta ke place in a competit ive set-t i ng , a n d i t s i n cr ea s e m a y h a v e s e r iou srepercussions for other individuals withintha t set t ing, which could a ffect the increaseand spread of the feat ure.

Fur therm ore, benefit s, costs, and competi-t ion all take place in relat ion to part icularenvironments. This is one reason the analy-sis of adaptive evolution involves environ-mental reconstruct ion. The st ability or dy-

n amics of e n viron me n t a l ch an g e may a ls oh av e an imp ac t o n n at u ra l s e le c t io n . T h edifferent t ime scales of environmenta l fl uc-tuat ion may affect natural select ion and, aswe w ill see, ma y determine the kind of tra itstha t a re ult imately represented in a popula-tion.

Fina lly, a growing nu mber of evolutionar ybiologists believe tha t l ineages themselvespossess ada ptat ions, referring to chara cter-ist ics (e.g. , geographic range, habitat vari-ety, dietary versat il i ty) that go beyond thema k e u p o f an in div idu al o rg a n is m ( Vrba ,1989; J a blonski, 1987). Since t hese cha ra c-

terist ics ma y impact the survival of popula-t ion s ov er t ime , t h e y ma y a ls o a f fect t h esuccess of par t icular features originated byconventiona l, individua l-level na tur a l selec-tion.

A b a s i c f r a m ew o rk f or t h e a n a l y s is ofadaptat ion, then, can be conveyed by a se-r ies of q u e st ion s (Ta ble 1) t h a t t ak e in t o

account phenotype definition, the operationo f n a t u ra l s e le c t io n , an d t h e fac t o rs t h a taffect differential fitness.

The ana lysis of ada pta tion is tricky largelybeca use th e questions posed in Table 1 a red iff icu lt t o a n s w er a n d a n y a n s w e r s p u tfor t h are u s u al ly c on t ro ve rsia l . As de ba t e

T ABL E 1. A 13 par t f r am ewor k for theanal ysis of adaptation

1. How is a pa rticular feat ure (morphological struc-ture, behavior, phenotypic system or complex)ma nifested? Wha t is its structure, or wha t ar e itscharacteristics?

2. What is the function of a par ticular featur e?What is it designed to do? What selective advan-ta ges does the feat ure offer to an organism?

3. Wha t currenciesof adva nta ge or success doesthe featu re provide to the organism? An a dvan -ta ge in survivorship? Ad irect r eproductiveadvantage? Does the feature raise the organismsefficiency or economy (Steudel, 1994)? Ulti-ma tely, how does the feature contribute to thegenetic fi tness of the individual?

4. Besides its benefi cial function, does the featur ehave a ny costs to the organism? What ar e thosecosts?

5. What is known a bout the feat ures heritability?6. What is known a bout the featur es development

(growth and expression during the life cycle)?What are the necessary environmental param-eters for proper development?

7. Does the featu re (e.g., a pa rticular beha vior)have any repercussions (costs or benefits) forother organisms? In other words, what was thecompetitive an d/or social context in w hich thefeatur e evolved or currently functions?

8. In the context of natur al selection, when did thefeatur e originat e? In wha t environmenta l con-text or conditions did it evolve? Wha t a dvant agesdid the featur e offer within tha t original context?Did th e feature represent a response to a specifi cenvironment or to diverse surroundings? Do theoriginal advantages and environmental contextstill apply to the organism t hat possesses thefeature?

9. What were the options or alterna tive phenotypes(e.g., the ancestra l condition) ava ilable to theorganism w hen the specifi c feature evolved?

10. Historically, is the feat ure a by-product of adap-tive change in another t ra it (in wh ich case it is

not an a da ptat ion), or did the feat ure evolve dueto specific benefit s of a t ask it performs or con-tributes to?

11. Does the featu re serve its origina l function, or isit an exaptation (co-opted for some other function[Gould a nd Vrba , 1982])?

12. Over what t ime scale does the feature assist theorganism? Da ily? Seasonally? Does it offer resil-ience to environmenta l varia tions faced over alifetime? Does it offer flexibility to a populationover longer periods of environmental change?

13. Are there lineage-level ada ptat ionstha t is, fea-tures of a lineage (rather tha n of an individual)tha t enha nce its ability to spread, persist, ordiversify when compar ed with other, related lin-eages? If natura l selection can t ake place at thespecies level, wha t kinds of feat ures might beinvolved in this process?



14/44

about the evolution of the human chin dem-onstra tes (Gould, 1977), even the fi rst stepdefining a trait subject to natural select ion(Table 1, questions 13)can create contro-

versy, part ly because tr aits can emerge as aby-product of adaptive modificat ion (ques-t ion 10). B esides these correlated effects,de v elop me n t a l (e .g ., e mbry olog ica l) c on -stra ints a re ubiquitous, w hich place bound-a r i e s o n t r a i t v a r i a t i o n , t h e r a w m a t e r i a lac t e d o n by n a t u ra l s e lect ion , a n d t h u s o nada ptat ion itself. Structura l design, the ben-efits, and the costs of morphological t raitscan be con s ide red wi t h s ome s u cce ss byfunctional a nat omists. B ut specialists inter-e st e d in p as t beh av ior (e .g . , a u s t ra lop it hbipedality, late P liocene st one fl aking) usu-al ly h a v e g re a t e r d i f f ic u l t y e v a lu at in g de -sign propert ies, frequency of occurrence,heritability, and the adva nta ge over alterna-t ive behaviors. In fact , very li t t le is knownabout t he herita bility of even simple struc-tures a nd behaviors unique to human s. Theheritability of chara cters (e.g. , megadontia)t h a t d is t in g u is h e xt in ct h omin in s , more -over, is largely inaccessible except perhapsby analogy to l iving mammals that possesssimilar t raits .

G iven these difficulties, the study of ear lyhominin adaptat ion is largely left with thea na lysis of context (Ta ble 1, quest ion 8). Thequestions of time, place, and environmentals et t i n g a r e e ss en t i a l t o a l l e vol u t ion a r yan a lyses of ada pta tion. The selective historyof a t ra it is reflected in part by w hen, where,and the condit ions or surroundings underwhich that t ra it first becam e apparent in thefo s s i l re c o rd a n d be g an t o c h ara c t e r ize apar ticular lineage or clade. The precise defi-nit ion of hypotheses relat ing past environ-ments to th e emergence of unique hominintraits, therefore, helps to galvanize researchon early human evolution.

ENVIRONMENTAL HYPOTHESESOF HOMININ EVOLUTION

A concise history

C h a l l e n g es of t h e sa v a n n a . Over muchof the t wentieth century, scientists th oughttha t huma n evolut ion entailed a simple tra -jectory from a pelike to huma nlike and tha tthis process wa s promoted by t he challengesof an open sa vanna . In h is bookTh e Evolu-

t i on of M a n , Elliot Smith (1924:40) de-scribed the terrestrial domain of early hu-m a n a n c es t or s a s t h e u n kn ow n w o r ldbeyond the treesa place of novel food

s ou rce s, q u i t e d ist in ct f rom t h e lan d ofplenty of the apes. Dart (1925) took thisidea t o drama t ic h e ig ht s . To c on v ey t h eselection pressures tha t drove huma n evolu-t ion , D a r t dre w an an a log y t o t h e s o ut h e rnAfrican veldt . The early australopiths, heargued, confronted many dangerous preda-tors, a relat ive scarcity of water, and fiercecompetit ion for food, all of which he ult i-mately encapsulated in the killer-ape sce-nar io (Da rt , 1953). La ter views, such a s th ed i et a r y h y p ot h e s es of R ob i n s on (1 95 4,1961a ,b) a nd J olly (1970), a lso explicitlyrelated hominin origin to a shift from for-ested/w ooded apelike ha bita t to a n opens av an n a s et t in g .

I n t r i n si c a n d ex t r i n si c a p p r o a c h es. I findeed huma n evolution followed a progres-sive path from apelike to huma nlike, it couldalso be reconstructed by extrapolat ing upfrom the an at omical, beha viora l, and ecologi-cal characteristics of apes and other nonhu-man primates and extrapolat ing down fromethnological data on human foragers. Thepaper Ecology and the Protohominids byBa rtholomew and B irdsell (1953) ma de ex-plicit th is approach t o modeling huma n evo-

l ut i on . Th i s p a p er w a s t h e fi r s t t o d r a w at tent ion to key t heoret ical aspects of earlyhuma n ecology, including populat ion den-s i t y an d dy n amics , t e rr it o r ia l i t y, forag in g ,energet ics, body size, and sociality . Yet noenvironment wa s reconstructed, an d no ideaswere offered about the interact ion of earlyh u man s wi t h s p e c ifi c e n v iro n me n t a l v ar i-ables. Implicit in their discussion was thatthe environment of hominin evolut ion wasalre ady g en e ra l ly k n own . I t in volve d, a tfirst , increasingly arid, savanna condit ionsin Africa, follow ed by increasingly cool, gla-cial conditions in nort hern la titu des.

Despite th eir emphasis on ecology, B a rth o-lomew a nd B irdsell (1953) provided an out-l in e for an in t r in s ic ac cou n t of h omin inevolution whereby adaptive change resultedlargely from responses within populat ionsover time w ithout rega rd to change in exter-n al c o n di t io n s . All t h e y re q u ire d was t h ebas ic scene, a r elat ively open sava nna . Their



15/44

argum ent followed Da rw ins (1871) ba sicscena rio: bipedality (ground living) an d sma llcanine teeth implied tha t t he earliest homi-n in s u s e d t o ols , wh ic h p ermit t e d a n e x-

panded diet , including animal foods; thus,h u n t in g in ban ds an d c h an g e s in s o c ia l i t yoccurred a s one ada ptive cha nge led to oth-ers. Washburn (1960) refi ned this type ofexplan at ion, a nd placed tools at t he center ofan e volv in g s y s t em in wh ich e a ch n ov elada p t a t io n fo s t e re d o t h e rs in an in t r ic a t eseries of feedback loops. Tools, hunt ing,fire, complex social life, speech, the humanway and the brain evolved together to pro-duce a ncient m a n of the genusH om o (Wash-burn, 1960:63). Again, Washburns intrinsicfeedback model w as not w ithout context . I fb ip eda l t o ol u s e in it ia t e d t h e ca u s e -an d-effect process of hominin evolution, life inthe open plains provided the crucial, initialsett ing (Wa shbu rn, 1960).

Washburns influence and interest in in-t r in s ic a c cou n t s (as op pos ed t o e xt r in s ica c cou n t s , w h i c h i n t e r pr e t e v ol u t i on a r yevents in relation to environmental change)ma y e xp la in wh y man y p aleoan t h ro polo-gists in the 1950s to 1970s paid little atten-tion to environmenta l context. In tw o classicvolumes on hominin evolution (Washburn,1961, 1963), none of the 35 papers focusedp rima ri ly on e n viron me n t a l s et t in g s . a n donly seven considered the subject .5 Thisapproach w as echoed in t he influentia l fourvolume set Perspectives on Human Evolu-t i o n p u b li s h ed b et w e e n 1 96 8 a n d 1 97 8(Washburn an d J ay, 1968; Washburn an dDolhinow, 1972; Isa ac and McCown, 1976;Washburn an d McCown, 1978). The onlyar ticles on paleoenvironment a l context werein Volume 3 (six of i ts 21 chapters). Twodealt with Miocene faunas and floras (VanCouvering and Van Couvering, 1976; An-drews and Walker, 1976), two others consid-ered environmental evidence from specifich om i n i n -b ea r i n g b a s i n s (O l d uv a i , O m o)(Hay, 1976b; Howell, 1976), and two chap-ters highlighted the general context of the

East African rift (Isaac, 1976; Bishop, 1976).I t is interest ing to note tha t Bishops paperdemolished the myth of a Pliocene drought(p . 40) ch a m p ion ed b y D a r t , w h i ch h a d

fueled the view tha t ea rly huma n evolut ionwas a response to a sharp shift from moistforest to a rid sa va nna . Moreover, the pa persconcerning Miocene sites showed that earlyapes lived in a complex mosaic of environ-ments, including closed and relatively openvegetat ion. These papers were among thefi rs t t o h in t a t t h e in t r ica t e e n viron me n t a lhistory related to ape a nd huma n evolut ion.Isa a c (1976:133) ended his a rticle by notingthe paucity of contextual studies and com-pared the exist ing information base aboutEa st African paleoenvironments t o a n ex-posed tip of an iceberg.

While most an thr opologists t ook a n int rin-sic a pproa ch, geologists a nd vertebra te pale-ontologists showed strong interest in paleo-environmental data and the reconstruct ionof early hominin set t ings. Thus, in Howella nd B ourl ier es (1963) volum e Afr ican Ecol- ogy and Human Evolut ion, a lmost a ll of the19 papers discussed the importance of con-textual dat a ( the main exception wa s tha t byDeVore a nd Wa shbu rn). The pa pers (e.g., byButzer) included cautions about simplist iclinkages between higher lat it ude glacial se-quences and lower lat itude environmental

h is t ory. O t h er p ap ers s u rmis ed t h at com-plex tectonic a nd climat ic events w ere inter-woven in creatin g the environmenta l historyof Africa (e.g. , papers by Bishop and by deH e i n z el in ). S e v er a l p a p er s s h ow e d e vi -denced that Pleistocene Africa underwent acomplex series of habitat changes betweenm o i s t a n d d r y a n d b e t w e e n f o r e s t e d a n dopen (e.g., pa pers by Morea u, Monod, G roveand Pullan, and de Heinzelin). Art icles byB i b e r s o n a n d b y H o w e l l a n d C l a r k w e r ep a r t i cu l a r l y s u cce ss f ul i n p la c i n g P l ei s -tocene stone technologies and behavior inspecific sedimentary, vegetat ional, and cli-

matic contexts.La ter volumes by B ishop and C la rk (1967),Butzer (1971), and Bishop (1978) continuedto emphasize t he importa nce of environmen-tal (sedimentary, geomorphological, tapho-nomic, a nd ecological) set t ings for under-sta nding hominin evolut ion. More than halfof the a rticles published in B utzer a nd Isa acs

5Four papers (by Cha nce, Oakley, Hal lowell, and Sa uer in the1961 vo lu m e ) r e it e r a t e d t h e g e n er a l v i ew t h at t h e e ar l ie s tancestors inhabited the African savanna while later homininslived in an ice-age context. Three others (by Mayr, Dobzhansky,and Harrison and Weiner in the 1963 volume)discussed environ-m e n t s as a t h e or e t ica l ly i m po r t an t de t e r m i nan t o f n a t u r a lselection, adaptive change, and speciation in hominins.



16/44

(1975) A f t er t h e A u s t r a l op i t h ec i n es a n dnear ly one-third of the contributions t o J ol-lys (1978)E a r l y H om i n i d s of A f r i ca focusedspecifi cally on t he climat ic, geologic, and

paleoecological contexts of hominin sites.B y t he early 1980s, a w ide range of envi-

ronmenta l data wa s becoming ava ilable duet o a s u rg e in p ale ocl ima t e s t u die s a n d de -tailed work by field geologists at fossil andarc h eolog ica l s i t es . L a p ort e a n d Zih lman(1983) made a strong case for the idea t ha tenvironment was a major driving force inearly huma n evolut ion. In their view, ada p-tive change in apes, including Pliocene homi-nins, represented responses to either newenvironments or physical access to existingenvironments (e.g., via land bridge). They

argued that global cooling reduced tree coverover large regions, and African uplif t andrifting created a rain shadow effect (drying)in the eastern part of the continent . Thesefac t o rs c au s e d t h e s p re ad o f t h e s av an n amosaic, marked by strongly seasonal rain-fall, discontinuous distribution of trees, andheat-ada pted, drought-resista nt vegeta t ion.Echoing earlier authors, Laporte and Zihl-man (1983:106107) posited that the chal-l en g es of t h e s a v a n n a -m os a i c ca u s edground-dwelling in a prehominin a pe andbipedality in the earliest hominins. Two-legged wa lking enabled mobility a nd carry-

ing of food and water over long distances asresources becam e more w idely (patchily)d is t r ib u t ed . F i na l l y, t h ey n ot e d t h a t t h eorigin of hominids [sensu st ricto] is not a nis ola t e d e volu t ion ary e ve n t , bu t ra t h e r ispar t of the overa ll rad iat ion of African homi-noids; nor is it an unusual ecological occur-rence. The hominids a re only one of severalm a m m a l i a n g r ou ps t h a t i n cl ud e t h e p ig sa n d b o v i d s t h a t w e r e a b l e t o e x p l o i t t h es a v a n n a -m os a i c h a b i t a t t h a t h a d b ecom ew i d es pr e a d b y t h e e nd of t h e M ioce ne (Laporte and Zihlman, 1983:108).

T u r n o v e r h y p o t h es es . At about the sametime, Vrba (1980, 1985, 1988; also Br ain,1981) began to present an important idea,k n own as t h e t u rn ov er p u ls e h y p ot h e s is ,wh ich e xp an ded t h e s e s ame t h e mes cli-ma tic forcing of evolution, a P liocene a riditys h i ft , a n d coin ciden t ch an g e in h omin in san d ot h e r mamma lian l in e ag es . Tu rn ov er

pulse refers to a concentra tion of speciat ionand ext inct ion events ( i .e. , turnover) in abrief period of time as a result of environmen-tal change. According to Vrbas original hy-

p o t h e s is , man y di f fe re n t Afr ic an mamma-l ia n cl a d es w e r e i n vol ve d i n a t u r n ov erepisode at 2.5 Ma. Synchronous change inmult iple groups w as caused by a shift fromwa rm, m oist conditions to a cooler, drier, andmore open habit a t. This environment a l eventwa s forced by globa l cooling, ma nifested a s as h a r p t r a n s it i on (18O e nr i ch m en t ) i n t h edeep-sea isotope record. The hypothesis t husposits a strong causal link between homininevolution and global climatic events. Accord-ingly, t he origins of H om o and of the robustaustralopiths at about 2.5 Ma were consid-e red p ar t of t h e t u rn ov er p u ls e. S in ce aspeciation pulse in grazing bovids occurredat this same t ime (e.g., Vrba 1985, 1988),H o m o a n d Paranthropusw ere a lso deemedp art of t h e Pl ioce n e a r id , o p en -g ras s lan dbiota of Africa. Vrba et al.(1989) posited t w oother turnover pulses, about 5 Ma and 0.9M a, wh ic h a ls o ap p e are d t o c o in c ide wi t hperiods of global cooling and continenta ldrying. The origin of the Hominini, on theone ha nd a nd the extinction of robust a ustr a-lop it h s , cou p le d wi t h t h e dis pe rsa l of H .erectus in t o E u r a s ia , on t h e ot h er w e rethought to be correlated with these environ-

mental shifts .Th e or i g in a l h y p ot h e s is h a s b ee n r e -v amp e d in t wo main way s . S in c e t h e mid-1980s, paleoclimatologists have broadenedthe age estimate of Pliocene global environ-mental change. Ba sed on the deep-sea oxy-gen isotope record (Fig. 2A), for example, amajo r s h i f t in g lo bal ic e s h e e t s be g an a se ar ly a s 2 .83.0 M a, wi t h t h e mo s t s ig n ifi -cant change between 2.8 and 2.4 Ma (e.g. ,Prentice and Denton, 1988). The deep-seadu s t re cord (s ee F ig . 3) a ls o in dica t e s amajor change in cyclicity beginning around2.8 Ma. Thus, the pulse in the late Pliocene

turnover pulse hypothesis has been broad-ened; the ta rget interval ha s been extendedfrom a point in t ime (2.5 Ma) to a span aswide as 400 kyr. According to this broaderpulse model, key events in hominin evolu-t ion and concerted turnover in other mam-mals should be evident from 2.8 to 2.5 (or2.4) M a ra t h e r t h an ran do mly dis t r ibu t e d



17/44

t h rou g h ou t t h e e n t ire 3 .5 my r s pa n of t h eP liocene (P rentice an d Denton, 1988). InVrbas (1995b) view, the turnover pulse inAfrica began 2.8 Ma but became appa rent in

the fossil record only between 2.7 and 2.5M a . Th is is t h e fi r s t w a y in w h ich t h ehypothesis has been altered.

The second way involved refining the testimplicat ions of the turnover pulse modeland broadening its theoret ical basis. Build-ing on a syn thet ic view called habit a t th eory,Vrba (1992, 1995a ) posits th a t t he record of aturnover pulse should depend on the breadtho f re s o u rc e s an d h abi t a t s u s e d by o rg an -i sm s . F i r s t a n d l a s t a p pe a r a n ce s w i l l b esynchronized or spread out according to theecologica l t olera nces of different lineages. Aglobal cooling event causing the spread ofsava nna between 2.8 to 2.5 Ma t hus shouldhave caused turnover in specialized organ-is ms fi rs t an d in mo re g e n e ra l ize d o rg an -isms later. Furthermore, i t should cause alarger number of first appearances in arid-ada p t ed t a x a (e .g . , h omin in s a n d g ra zin gbovids) and more last appeara nces in wa rm,woodland -a da pted species.

Vrba h as bee n a s s idu ou s in t e s t in g t h eturnover pulse idea. Man y a ppeara nces a nddisa ppea ra nces of bovid lineages in t he Afri-can fossil record are clustered around 2.5Ma (Vrba, 1985, 1988, 1992, 1995b). Grazing

bov ids (a lc ela p h in es s u ch as wi ldebe es t ,har tebeest , a nd topi and a nt ilopines such asg aze lles ) c on t r ibut e d s t ron g ly t o t h e fi rs tap p ea ra n ce s. Th is s u pp ort s t h e idea t h a tar idi t y an d g ra s s lan d e x p an s io n we re t h eenvironmental causes of bovid lineage turn-over. Vrba (1985, 1988) ha s a lso empha sizedWesselmans (1984) conclusion (based on astudy of micromammals from Omo, Ethio-pia) that forest taxa prevalent at 3 Ma gavewa y t o xeric-ada pted, open vegetat ion t axab et w e en 2. 4 a n d 2. 5 M a . I n t h e v ol um ePaleocl imat e and E volut ion, wi t h E mphasison H u m a n E v ol u t i on (Vrba et a l., 1995),

papers concerning paleoclimate and vegeta-t ion tend to confirm tha t la rge environmen-ta l cha nge occurred between 2.8 a nd 2.5 Ma .Many of these papers, however, stress theevidence for strong fluctuat ion at this t imerather than a simple cooling or aridity shift .Conclusions dra wn from African bovids, mi-cromam ma ls, and hominins are sa id to sup-

port the turnover pulse idea (Vrba, 1995b;Wesselman, 1995), though an ana lysis ofsuids and hominins argues strongly a gainstth e hy pothesis (Whit e, 1995).

The pulse hypothesis ha s been q uestionedon other grounds, however, and counteredby what may be called the prolonged turn-over h ypothesis (B ehrensmeyer et a l., 1997;McKee, 1996). Building on earlier observa-tions (Feibel et al., 1991), a recent study byBehrensmeyer et al. (1997) focused on theTurkana -Omo ba sin since th is a rea offerst h e r ich e st an d bes t ca l ibra t e d re cord ofAfrican fossil mamm als in the crucial spanbet we e n 3 a n d 1 .8 M a . Tak in g s amp lin gvariat ion (rises and falls in fossil samplesover time) into account, this study found nostat ist ically significant pulse in either firstor la st a ppea ra nces a t 2.5 Ma or between 2.8and 2.5 Ma. Instead, turnover in this inter-v al wa s u n remark able , an d mamma lian l in -eages displayed a prolonged period of turn-ov er e sp ecia l ly bet we e n 2.5 a n d 1 .8 M a.Moreover, a combination of xeric and mesicspecies wa s found to persist from 3 to 2 Ma.B e h re n s me y e r e t a l. (1997) argued that acomplex vegeta tion mosa ic coupled wit h w idefl uctua tion over time would explain both thepersistence of species of diverse ecologicaltolerances a nd t he prolonged period of turn-over. Whether simila r results m ight a pply tothe w hole of Africa ha s yet t o be determined.

Adaptive evolution in hominins

Although the turnover pulse idea ha s a t-tracted much at tent ion, environmental hy-potheses pertaining to adaptive change re-main poorly developed. Hypotheses of thistype differ from turnover models. They at -tempt to describe how environmental con-text impinged on survival and natural selec-tion and in t urn infl uenced the emergence ofkey adapta t ions. Since ada ptive change maybe only loosely coupled to speciat ion (El-dredge, 1989), th ese hypotheses d o not neces-

sa rily relat e directly t o the patt ern of speciesturnover.

H a b i t a t - sp e c i fic h y p o t h es es . The mostp romin en t n arra t iv es of h omin in ada p t iv eevolution are habitat-specific. Each of thesescena rios points t o a par ticular type of envi-ro n me n t an d s e t s fo r t h t h e re as o n c e r t a in



18/44

ada p t a t io n s aro s e t o me e t t h e s p e c ia l de -mands of that set t ing. Some scenarios sim-p ly n ame t h e p ar t ic u lar t y p e o f h abi t a t inwhich hominins w ere a ssumed to live (e.g. ,

t h e s a v a n n a) , wh i le o t h e rs ma y s p e c i fy apart icular direction of environmenta l cha nge(e.g., the onset of dry or cold conditions). Inboth cases, i t is implicit tha t nat ura l selec-tion had a consistent effect over time (i.e.,directional selection). This process served toimprove adaptive opportunities and favoredce rt a in in n o vat io n s a s wa y s of s olv in g t h echallenges posed by the special habitat ortrend.

The discovery of human fossils and art i-fac t s wi t h ice -a g e fa u n a an d s t ra t a , for e x -a m p le , l ed t o t h e i de a t h a t a d v a n ce s i nhuman evolut ion were forged in the harshset t ings of P leistocene E urope. This viewmay be termed the ice-age hypothesis. Anupdated version of this view is as follows:cool steppe and cold periglacial conditions inmid-latitude Eurasia posed a very challeng-in g e n v iron me n t t o e a r ly h u ma n p op ula-t ion s , a n d adv a n c es in h o min id s ocia l an dcognit ive abilit ies helped to solve these prob-lems of survival. Accordingly, the onset ofice-age ha bitat s (due to global cooling) re-sulted in the evolution of key huma n ad a pta -tions.

As alrea dy noted, the savanna hypothesis

is another habitat -specifi c sta nda rd and ha sdominated thinking about the earliest phasesof hominin evolution. According to this idea,ada p t a t io n t o dr ie r an d in c re as in g ly o p e nenvironments w as the ha llmark of homininevolution from lat e Miocene thr ough at leastthe early P leistocene. Sa vanna wa s the con-text tha t incited the emergence of funda men-t a l h u m a n t r a i t s t e r r es t r i a l b i pe d a l it y,larger brains, stone toolmaking, meat-eat-ing, an d a ssociat ed foraging behaviors suchas hun ting. Essent ially a ll textbooks prior to1996 explained the origin of th ese tra its int e rms o f a da p t a t io n t o re la t iv e ly o p e n s a-

v a n n a , d efi n e d b y t h e p re se nce of g r a s s,discontin uous trees, overall a ridity, and sea -sonal ra infall (e.g., Klein, 1989; Wolpoff ,1980).

Recent support for the savanna hypoth-esis has risen from the ranks of paleoclima-tologists a nd paleontologists (Ta ble 2). Theshared view is th at emergence of dry, open

sava nna wa s caused by st eplike episodes ofglobal cooling sta rt ing in t he la te Miocene.Vrba (1995b:406), for example, refers to theonset of the ma ssive overall cooling trend of the late P liocene. In tense global coolingled to pulses of aridifi cation in Africa, w hich

caused savanna expansion. The shift fromd en s e w ood la n d t o s a v a n n a h a d a s t r on geffect on adaptive characteristics, includingbipedality in ea rly Australopithecus, hyper-trophy of the chewing apparatus in Paran- thropus,a nd va rious developments in H omo,such as encephalization and dependence ontoolma king (St a nley, 1992; deMenocal, 1995;P rent ice a nd Den ton, 1988; Vrba et a l., 1989;Vrb a , 1988).

Al t h ou g h t h e s a v a n n a h y pot h e si s h a sg a i n ed r ece nt s u pp or t , i t h a s a l s o b e enstrongly contested. Some authorit ies advo-cate a contradictory modelthe woodland/

forest hypothesiswhich argues for the im-p or t a n c e of cl os ed v eg et a t i on i n e a r l yhominin evolut ion. E arly aust ra lopiths, ac-cording to some interpreta tions, w ere closelyassociated with wooded environments, exhib-ited significant ar boreal a ct ivity, a nd shouldbe c on s ide red ada p t ed t o clos ed h abi t a t s(Clar ke an d Tobias, 1995; Berger an d To-bias, 1996). Fossil pollen from Makapans-gat , for example, suggests tha t A. afr icanusinhabited conditions approximating a tropi-cal forest (Rayner et al., 1993). At Aramis,t h e oldes t k n own h omin in , Ardipi thecusr a m i d u s , i s as s o c ia t e d wi t h e v ide n c e o f a

rela tively tree-domina ted sett ing (WoldeGa b-r iel e t a l . , 1994). St a ble is ot o pe s t u die s,mo re o v e r , s h o w t h at t h e s h i f t t o C 4-domi-n at e d e n viron me n t s (op en , g ras s y, h e at -adapted vegetation) occurred later than pre-viously thought, as late as 1.7 Ma, with littleevidence of consistently open savanna untilafter 1.0 Ma (Cerling et al . , 1991; Cerling,

TAB L E 2. Recent adv ocates of the savanna hypoth esis

Who Wha t a nd w hen Result

VrbaS. Sta nleyG. D entonM. Prentice

Pulses of savannaexpansion =

> 2Aridification

>Global cooling (5.0,

3.0, 2.8 to 2.4, 1.7,and 1.0 Ma)

Tur nover (inH o m o and robust austra -lopiths)

Novel adaptations toopen conditions(e.g., terrestri-ality, stone tool-making)



19/44

1992; Kingston et al., 1994; Sikes et al ., inpress). This means that open grassland, akey ingredient of the savanna hypothesis,w a s b y n o m e a n s a r eg u la r f ea t u r e of t h eEa st African landscape by th e late P lioceneor even th e early P leistocene. Comparison offorest and open-habitat chimpanzees offersanother interest ing perspect ive. Tool use,hunt ing, food-sha ring, grea ter socia l coopera-t i on , a n d ot h er t r a i t s u s ua l l y a s s oci a t edwit h h u ma n e v o lu t io n ap p e ar t o be mo restrongly displa yed by closed-ha bita t chimps.On t his ba sis, B oesch-Acherma nn an d B oe-sch (1994) have suggested t ha t w oodla nd orforest ma y ha ve provided th e init ial set t ingin wh ic h h omin in s e volv ed. Th e s av an n ahypothesis has thus been crit icized by pri-

mate researchers, paleontologists, and envi-ronmental scientists , many of whom favorreplacing it wit h a w oodlan d/forest-ad apt a-tion model.

A re la t e d ideat h e r ip ar ia n -wo odlan dscavenging modelalso underlines the im-porta nce of closed habita t t o ear ly huma ns.In the modern Serengeti , animal carcassesa r e f ou n d t o b e s ea s on a ll y a b u nd a n t i nwooded areas near water. This observat ionled Blumenschine (1986, 1987; Cavallo andB lumenschine, 1989) to suggest th at a sca v-enger niche could exist in riparian wood-land s. Thus, during Oldowa n times, mar row-e at in g t o o lmak e rs may h av e o c c u p ie d t h ewooded zones nea r w at er, w here they couldexploit scavenging opportunities, which werefar less a va ilable in open sett ings. The ripar-ian-woodland model, like other versions ofthe woodland /forest hypothesis, supposesthat woodlands offered a vital challenge orop port u n i t y in t h is cas e , carc a s s es t h a tcould be exploited with the help of stonetoolswhich grasslands did not.

S h o r t - t er m v a r i a b i l i t y h y p o t h e si s . Th eideas considered thus far see ada pta tion as a

re sp on s e t o t h e con s ist e n t ch a l len g es ortendencies posed by a specific type of habi-ta t. According to a different t ype of expla na -t ion, important adaptive changes may arisein response to environmenta l var iat ion. Va ri-ability hypotheses ca n be divided into short-term (e.g., seasonal variation)and long-term(i.e., variability selection).

The short-term perspective pay s a tt entiont o v ar ia t io n s e x p e rie n c e d by an o rg an is mover its lifetime, especially sea sonal fl uctua -t ion, disease, and change from year t o year.

This view, wh ich we may call the seasona lityhypothesis, places emphasis on how an or-ganism a djusts to a lternat ive environmentsdu rin g i t s l i fe t ime , s u c h as warm-c o ld o rrainy-dry periods. S hort-term varia t ion isespecially marked in certain regions, particu-larly savannas, higher-lat itude biomes, orany other habita t prone to lar ge moisture ort e mpe rat u re s eas o n al it y. S t ro n g s eas o n al-ity , furthermore, brings about interannualvaria t ion. The ra iny season ma y be extremein one yea r but fa il in another; t he differencebe t we e n win t e r an d s u mme r t e mp e rat u re

ma y be large in some year s but much less inothers.The influence of short-term habitat varia-

t ion on sa vanna organisms, including earlyhumans, is discussed by Foley (1987). Foleynotes tha t a lthough ear ly hominins occupiedwooded-to-open savanna, the most signifi-ca n t a s p ect of t h is t y pe o f h a b i t a t i s i t sma rked seasonality. Dr iven by monthly var i-a n c e i n r a i n f a l l, s ig n ifi c a n t s h if t s occu rw i t hi n ea ch y ea r i n t h e a b un d a nce a n dav ai labi li t y of re s ou rce s . Sa v an n a org a n -isms are a dapted not to the varia t ion per seb u t t o t h e p a t t e r n o f v a r i a t i o n . I t i s t h e

expected cyclicity of r esources and theiravailability that is important (Foley, 1987).Short -term h ypotheses of a da ptive evolutionthus underline repeat able var iat ions, whichrepresent regularities in the annual (or life-t ime) ada ptive set t ing of an orga nism.

The difference between seasonality andother hypotheses, then, can be subtle. Habi-t a t -s pe cifi c v ie ws lay s t re ss on t h e e ve r-present, or consistent , a da ptive problems ina given set t ing. The short-term variabilityide a e mp h as ize s h abi t a t v ar ia t io n t h at anorgan ism is likely to experience a nd a da pt toin every genera tion.

L o n g - t er m ( v a r i a b i l i t y sel ec t i o n ) h y p o t h -

esi s. In contra st to seasonality and habitat-s pe cifi c ideas is t h e v ar iabi li t y s elect ionhypothesis. According to t his view, environ-me n t a l v a r ia t ion is a cce n t u at e d o ve r t h elo n g t e rm an d p lay s a c r i t ic a l ro le in t h eada ptive process. E nvironmenta l change in-



20/44

volves large, episodic shifts in adaptive set-t ings over many hundreds of thousands ofy e a rs . As a re s u l t , a l in e a g e o f o rg an is msma y face mult iple, substant ia l disparit ies inselective environment over time. This ideafollow s from the previously coined term v a r i - abil i t y selecti on (Potts, 1996a,b, in press a).The varia bility select ion hypothesis thussta tes that certa in ada ptat ions have evolveddue to la rge environmenta lly caused incon-sistencies in s elective conditions. This d ispar -i t y i n t h e D a r w i n i a n o p t i m u m f a v o r s t h ebuildup of complex mechanisms for dealingwit h unexpected, episodic chan ge. This pro-ce s s of a da p t ive e v olu t ion t h e re fo re e n -hances an organisms capacity to thrive innovel conditions.

Var iability select ion (VS) calls at tent ionto the lar ge remodeling of landscapes, vegeta-t ion, anima l communit ies, and regiona l hy-drology over long t ime frames. The long-term sum of habitat variability far exceedsseasona l or other types of va ria tions encoun-tered by individuals within their l i fet ime.This recurrent, wide-scale revision of envi-ron me n t c an t h u s h a v e a n u n u s u a l imp ac ton the adaptive properties of a lineage. Thes ea s on a l i t y h y pot h e si s i m pl ie s t h a t s ea -sonal (or, more broadly, genera t ion-scale)change can explain the emergence of ada p-tive flexibility. The VS hypothesis, by con-

trast, seeks the evolutionary cause of versa-t i l it y in lon g er in t erv a ls o f more dra mat icchange in an organisms survival regime.

The VS hypothesis requir es tha t differentindividuals in different generat ions of thesa me lineage experience ma rked inconsist en-cies in adaptive set t ing. As it is magnifiedover t ime, the disparity in adaptive milieusha pes an overa ll selective environment t ha tcannot be predicted from the more consis-tent selective environments of shorter peri-ods . Se lect iv e disp a r i t y u l t imat e ly fav orsg e n e s t h a t bu i ld me c h a n is ms o f adap t iv e

versat il i ty. I t promotes a process of ad apta -tion to novelty, including settin gs never pre-viously encountered. The result ing a dapta -t ions do not ant icipate futur e condit ions (aviolat ion of na tur al selection th eory); rat her,their evolution stems fr om a prior history ofmajor, periodic revision of the adaptive set-t ing.

In VS, t herefore, the select ive effect isintegra ted over time. This sets up the situa -t ion in which habitat-specific traits end upeventual losers to features that in the past

proved successful in novel surroundings.This unusua l process thus improves ada pta -t ion t o long-term environmental dyna mics.(See Potts ( in press a) for a more detaileddiscussion a nd r esponse to challenges.)

B y contra st , ha bitat -specifi c and seasonal-ity expla na tions of evolution usually a ssumet h e op erat io n of d irect ion al s elect ion , inwhich the fitness results tend to be consis-t e n t ov er t h e lon g t e rm. Th e y fo cu s on aspecifi c sett ing, tr end, or properties of var ia-t ion that recur from generat ion to genera-tion. The VS hypothesis is thus ea sily distin-g u i sh e d f r om b ot h h a b i t a t -s p ec ifi c a n dseasonality hypotheses.

Test implications

E a c h of t h es e h y pot h es es h a s t e st a b l eimplicat ionstha t is , a set of expecta t ionsregarding

the general paleoenvironmental record ofthe la te Cenozoic;

the precise associat ion of hominins andh abi t a t s ;

change in the breadt h of habita ts in whichhominins lived;

the context in which hominin adaptations

fi rs t be c ame ap p are n t an d c o n t in u e d t othrive; an d

the functions or utility of particular adap-tive features and trait complexes.

H a b i t a t - sp e c i fic h y p o t h es es . G lo bal an dregional environmenta l records sh ould showtha t a part icular type of habita t (e.g. , wood-land, sava nna , glacial cold) wa s either main-tained over a prolonged period or developedov er t ime a s a t re n d. H o min in fo ss i ls orar t i fac t s , more ov er , s h ou ld be as s o cia t e dcon s is t en t l y w i t h t h a t p a r t icu la r t y pe ofp as t e n viron me n t o r a t leas t a re la t iv e ly

n arro w h abi t a t ran g e . Th e in flu e n ce o f ap ar t ic u lar h abi t a t t y p e o n adap t a t io n c anbest be demonstra ted in a situa tion in wh ichalternat ive habitats were also present (andin which hominins were ra re or absent ).

Th e main e xp ect a t ion t h e refore is t h a th o min in s l iv e d in an d we re e s p e c ia l ly a t -tra cted to a part icular set of environmenta l



21/44

condit ions. Accordingly, ada ptive featuresshould be well suited to those conditions an ds h o u ld h av e fi rs t a p p e a re d in t h a t s p e c ifi ctype of habitat . Functional analysis should

therefore show tha t those fea tur es had prop-erties (e.g., shape, size, biomechanical, be-havioral, or ecological properties) that werepart icularly beneficial in the environments p e c ifi e d by t h e h y p o t h e s is ( e .g . , a r id s a-vanna, woodlands, glacial cold).

S h o r t -t er m v a r i a t i o n ( sea s on a l i t y ) h y -

p o t h e s i s . The ma in expectat ions of thishypothesis, dist inguishing it from the oth-ers, are that 1) hominins lived in settings ofhigh seasonality , 2) key a dapta t ions reflectthe scale of ha bitat varia t ion typically fa cedover a l i fet ime, and 3) large-scale fluctua-

tions ha ve occurred on seasona l, deca de, andcentury t ime scales during spans of majorada ptive change.

S h or t -t e rm v a r ia t i on m a y b e q u it e e x-treme. According to climatologists Crowleyan d North (1991:11), the sea sonal cycle isthe la rgest clima te change w e know ofthechange in t emperat ures over North Americafrom w in t er t o s u mmer is fa r g re at e r t h anglacial-interglacial cha nges in mean a nnua lt e mpe rat u re s of t h e Ple ist o ce n e. B y t h isreasoning, seasona l cycles represent th e wid-est ra nge of environmenta l change to w hichorganisms adapt in both temperate (Calvin

1996) a nd t ropical la tit udes (Foley 1987).Greenland ice-core records further indi-

c a t e t h a t h u g e de v ia t io n s in c l imat e mayoccur abruptly on a decade-to-century timescalethus within the lifet imes of huma nsand other long-lived organisms. I t is pos-sible that such rapid oscillat ionand dra-ma tic shifts on an even shorter cycle, such a sE l Nin ocha ra cter ized t he period of hum a nevolution (Calvin, 1996).6 Thus, short-termvariat ion can account for the range of set-t in g s t o w h ich h omin in s h a d t o

Documents

Evolución y Clima