Geologic Evidence for the Antiquity of Life Readings on the Antiquity and Origin of Life

• :

Nature 409

Nature 417, 782-784.

Nature 416, 76-81.

• :

Nature 418

Nature 416, 73-76.

Science, 2891340.

Nature 410, 77-81. Science, 289

Assigned Reading

1. Stanley (1999), pp. 306-311 & 320-323

2. Nisbet, E.G. and N.H. Sleep (2001) “The habitat and nature of early life.” , 1083-1091.

3. Orgel, L.E. (1994) “The origin of life on Earth.” Scientific American, October 1994, 77-83.

4. Hazen, R.M. (2001) “Life’s rocky start.” Scientific American, April 2001, 77-85. 5. Dalton, R. (2002). “Squaring up over ancient life.” 6. Brasier, M. D., Green, O. R., Jephcoat, A. P., Kleppe, A. K., Van Kranendonk, M. J., Lindsay, J. F.,

Steele, A., and Grassineau, N. V. (2002). “Questioning the evidence for Earth's oldest fossils.”

Suggested Reading

1. van Zuilen, M. A., A. Lepland and G. Arrhenius (2002). “Reassessing the evidence for the earliest traces of life." : 627-630.

2. Chyba and Sagan, 1996, “Comets as a source of Prebiotic Organic Molecules for the Early Earth” in Comets and the Origin and Evolution of Life (Eds Thomas, P.J., Chyba, C.F., McKay, C.P.)

3. Schopf, J. W., Kudryavtsev, A. B., Agresti, D. G., Widowiak, T. J., and Czaja, A. D. (2002). “Laser-Raman imagery of Earth's earliest fossils.”

4. Cody, G.D., Boctor, N.Z., Filley, T.R., Hazen, R.M., Scott, J.H., Sharma, A. & Yoder Jr., H.S. (2000) Primordial carbonylated iron-sulfur compounds and the synthesis of pyruvate. , 1337-

5. Shen Y. and Canfield D. E. (2001) isotopic evidence for microbial sulphate reduction in the early Archaean era.

6. Wächtershäuser, G. (2000) Life as we don't know it. , 1307-1308.

Early Earth History

Antiquity of Life

•The lost record of the origin of Life. It happened >3.5 Ga

–Oldest terrestrial rocks 3.98 Ga (Bowring, MIT) and cooked

Summary of Geologic Evidence for the

–Oldest minerals – zircons 4.2 Ga

–Oldest microfossils – Warrawoona (Pilbara Craton) 3.5 Ga are contentious because of sedimentary relationships –Next oldest known & convincing microfossils from a hydrothermal vent in Western Australia’s Pilbara craton 3.2 Ga –Oldest molecular fossils (“biomarkers”)-2.7 Ga (Brocks et al)

Origin and Early Evolution of Life • The lost record of the origin of Life? Few crustal rocks from >3

Ga and half life of sediments 100-200Ma so most destroyed

d 13

C

Corg

Ccarb

Span of

modern values

autotrophs

sediments

4 0.5

Time Ga

Carbon Isotopic Evidence for Antiquity of Life

2.5

Science

•Re-mapping of Akilia Island & new petrologic & geochemical

(not BIFs).

their formation.

Akilia Island, SW Greenland 13C-depleted graphite

•Rocks interpreted to be sedimentary (Banded Iron Formations--BIFs).

precipitation and settling out of particles from seawater. •Critical indicators of early life b/c they establish existence of liquid hydrosphere in a habitable T range.

Geology Matters: 1

Fedo & Whitehouse (2002) , Vol. 296:1448-1452.

analyses do not support sedimentary origin for these rocks. •They appear instead to be metasomatized ultramafic igneous rocks

•Therefore highly improbable that they hosted life at the time of

•Evidence for life >3.85 Gyr ago from

•BIFs formed early in Earth’s history, supposedly by chemical

Science

•Quartz-pyroxene outcrop originated as igneous rock, compositionally modified during repeated episodes of

pyroxene, amphibole) •Deformational petrologic features inconsistent with BIF

Geology Matters: 2

See the images by Fedo & Whitehouse , Vol. 296 (2002): 1448-1452.

metasomatism & metamorphism (lt = quartz; dk =

Geology Matters: 3

Science,

•REE pattern, elemental ratios & mineralogy all consistent with Akilia

See the figures by Fedo & Whitehouse Vol. 296 (2002): 1448-1452.

igneaous rocks, not BIFs

•Geochemical evidence against Akilia rocks being BIFs

protolith secondary, metasomatic isotopic systematics

Ga Isua Supracrustal most metasomatic rocks

metacarbonate

by Schidlowski Mojzsis Ga .

d13

2 or CH4

AGU, 82(Lepland, A., van Zuilen, M., Arrhenius, G) The principal method for studying the earliest traces of life in the metamorphosed, oldest (> 3.5 Ga) terrestrial rocks involves determination of isotopic composition of carbon, mainly prevailing as graphite. It is generally believed that this measure can distinguish biogenic graphite from abiogenic varieties. However, the interpretation of life from carbon isotope ratios has to be assessed within the context of specific geologic circumstances requiring (i) reliable interpretation (ii) control of

processes, and (iii) understanding of different graphite producing mechanisms and related carbon . We have carried out a systematic study of abundance, isotopic composition and petrographic associations

of graphite in rocks from the ca. 3.8 Belt (ISB) in southern West Greenland. Our study indicates that of the graphite in ISB occurs in carbonate-rich (metacarbonates) while sedimentary units, including banded iron formations (BIFs) and metacherts, have exceedingly low graphite concentrations. Regardless of isotopic composition of graphite in rocks, their secondary origin disqualifies them from providing evidence for traces of life stemming from 3.8 Ga. Recognition of the secondary origin of Isua metacarbonates thus calls for reevaluation of biologic interpretations

et al. (1979) and et al. (1996) that suggested the occurrence of 3.8 biogenic graphite in these rocksThe origin of minute quantities of reduced carbon, released from sedimentary BIFs and metacherts at combustion steps > 700°C remains to be clarified. Its isotopic composition ( C = -18 to -25‰) may hint at a biogenic origin. However, such isotopically light carbon was also found in Proterozoic mafic dykes cross-cutting the metasedimentary units in the ISB. The occurrence of isotopically light, reduced carbon in biologically irrelevant dykes may indicate secondary graphite crystallization from CO

- containing fluids that in turn may derive from bioorganic sources. If this were the case, trace amounts of isotopically light secondary graphite can also be expected in metasediments, complicating the usage of light graphite as primary biomarker. The possibility of recent organic contamination, particularly important in low graphite samples, needs also to be considered; it appears as a ubiquitous component released at combustion in the 400-500°C range. A potential use of the apatite-graphite association as a biomarker has been proposed in the study by Mojzsis et al. (1996). Close inspection of several hundred apatite crystals from Isua BIFs and metacherts did, however, not show an association between these two minerals, moreover graphite is practically absent in these metasediments. In contrast, apatite crystals in the non-sedimentary metacarbonate rocks were found commonly to have invaginations, coatings and inclusions of abundant graphite. Considering that such graphite inclusions in apatite are restricted to the secondary metasomatic carbonate rocks in the ISB this association can not be considered as a primary biomarker in the Isua Supracrustal Belt References: Mojzsis,S.J, .Arrhenius,G., McKeegan, K.D.,.Harrison, T.M.,.Nutman, A.P \& C.R.L.Friend.,1996. Nature 384: 55 Schidlowski, M., Appel, P.W.U., Eichmann, R. & Junge, C.E., 1979. Geochim. Cosmochim. Acta 43: 189-190.

Tracing Life in the Earliest Terrestrial Rock Record, Eos Trans. (47), Fall Meet. Suppl., Abstract P22B-0545 , 2001

Know Thy Rock: 1

Nature (2002): 627-630.

•Carbonate in occurs in 3 distinct phases •Likely formed during multiple injections of fluid

and their host rocks.

See Figure 1 by Van Zuilen et al. Vol. 418

3.8 Ga Isua (SW Greenland) rocks

across contacts between igneous ultramafic rocks

Know Thy Rock: 2

Nature Vol. 418 (2002): 627-630.

rocks.

6FeCO3 --> 2Fe3O4 2 + C

introduction of elements into rock by circulating fluids

See Figure 3 by Van Zuilen et al.

•Graphite is associated primarily with the metacarbonate rocks, NOT with metasedimentary

•This suggests the reduced carbon formed by thermal disproportionation of the carbonates. E.g.,

+ 5CO

Metasomatism:

Know Thy Rock: 3

Nature Vol. 418 (2002): 627-630.

•Most of the reduced C does not have the large 13C-depletion expected from biological materials.

See Figure 2 by Van Zuilen et al

•Most of the reduced C (graphite) in the 3.8 Ga Isua rocks is in the metacarbonate phases and not the metasedimentary phases & likely formed by thermal disproportionation of the carbonate minerals at a later time.

•The isotopically-depleted C is only found in the metasedimentary rocks, where it’s concentration is very low & it may be contamination….

Know Thy Rock: 4

Nature Vol. 418 (2002): 627-630.

(of presumed biological origin) combusts at low T,

material (I.e., contamination)

See Figure 4 by Van Zuilen et al.

•The isotopically-depleted C in this 3.8 Ga Isua sample

suggesting it is unmetamorphosed recent organic

Biogenic Origin of Reduced Carbon in

Nature Vol. 418:627-630.

Bottom Line: No evidence for a

3.8 Ga Isua (SW Greenland) Rocks

Van Zuilen et al (2002)

autotrophs

d 13

C

Corg

Ccarb

Span of

modern values

sediments

4 0.5

Time Ga 2.5

Revised C Isotope Evidence for Life’s Antiquity

With the carbon isotopic evidence for life >/= 3.8 Ga now seriously challenged….

It’s time to look at some fossil evidence for early life….

But don’t be surprised to find plenty of controversy there too!

So jump ahead 300 Myr to 3.5 Ga…

See Figure 1 by Schopf et al. Nature, vol. 416 (2002): 73-76.

•Photo-montages of inferred microfossils from rocks ranging in age from 0.7-3.5 Ga.

Schopf’s Apex ‘microfossils’ #1

See the image by Gee Nature, 416 (2002): 28, and Nature, 416 (2002): 76-81.

Non-biologic Origin of 3.5 Gyr

Brasier et al.

•Schopf’s “microfossils” seem to have formed hydrothermally (hot water + rock)

“Microfossils”?

Questioning the authenticity of 3.465 Ga Apex fossils: 1

See Figure 1 by Brasier et al. Nature, Vol. 416 (2002): 76-81.

•Rather than emanating from a sedimentary rock,

hydrothermal rock vein created by the interaction of hot rock + H2O

the Schopf ‘microfossils’ came from a

Questioning the authenticity of 3.465 Ga Apex fossils: 2

See Figure 2 by Brasier et al. Nature, Vol. 416 (20020: 76-81.

“Many of these filamentous structures [from

ways not shown in the original descriptions because of the choice of focal depth and/or

the apex chert] are branched or formed in

illustrated field of view.”

Questioning the authenticity of 3.465 Ga Apex fossils: 3

See Figure 2 by Brasier et al. Nature, Vol. 416 (2002): 76-81.

•It would appear as though Schopf (1993) “left out” some essential morphological features of his ‘microfossils’…

See Figure 3 by Schopf et al. Nature(2002): 73-76.

•Indicates presence of reduced carbon (graphite)

Schopf’s ‘microfossils’ #2: Raman Spectroscopy to the rescue?

, vol. 416

•Raman spectra & spectral maps (G band) of 0.7-3.5 Ga ‘microfossils’

associated with ‘microfossils’.

Questioning the authenticity of 3.465 Ga Apex fossils: 4

See Figure 4 by Brasier et al. Nature, Vol. 416 (2002): 76-81.

•The spectroscopic results therefore provide no support

•Unfortunately for Schopf et al., Raman spectra of dark specks within surrounding host (quartz) rock of Apex ‘microfossils’ give same Raman spectrum.

for the “biogenicity” of Schopf’s ‘fossils’.

structures #1

•Morphology is at best an ambiguous indicator of

•Evidenced here by inorganic aggregates precipitated from a simple solution of BaCl2, Na2SiO3, NaOH

See the images by Garcia Ruiz et al. Astrobiology, Vol. 2(3) (2002): 353-369.

Abiotic origin of microfossil-like

biogenicity.

So… morphology can be be a poor indicator of biogenicity.

As can Raman spectrospcopy.

And carbon isotopes.

Yet our quest for for evidence of life 3.5 Ga does not end here.

We need to take a look at… Stromatolites.

Stromatolites-1

Stromatolites are fossils which show the life processes of

domes. Ozarkcollenia, a distinctive type of layered

cyanobacteria (fomerly called blue-green algae). The primitive cells (Prokaryotic type), lived in huge masses that could form floating mats or extensive reefs. Masses of cyanobacteria on the sea floor deposited calcium carbonate in layers or domes. These layered deposits, which have a distinctive "signature" are called laminar stromatolites. This is an example of a layered stromatolite from the Ozark Precambrian. Most often, stromatolites appear as variously-sized arches, spheres, or

Precambrian stromatolite, pushes the appearance of life in the Ozarks to well over 1.5 Ga.

Stromatolites-2

does today.

Stomatolites are colonial structures formed by photosynthesizing cyanobacteria and other microbes. Stromatolites are prokaryotes (primitive organisms lacking a cellular nucleus) that thrived in warm aquatic environments and built reefs much the same way as coral

stromatolites?

Nature, 382, 423-425, October 3, 1996.

•Seems statistically feasible that the morphology of

processes.

A possible abiotic origin for

Grotzinger, J. and Rothman, D.H., “An abiotic model for stromatolite morphogenesis,”

stromatolites can occur through non-biological

Modern Living StromatolitesAustralia

between microbes, other biological influences and the physical and chemical environment.

mucus that each cell secretes, then bind the sediment grains

light, their growth keeps pace with the accumulating sediment.

: Shark Bay,

•Hamelin Pool’s stromatolites result from the interaction

•The cyanobacteria trap fine sediment with a sticky film of

together with calcium carbonate which is separated from the water in which they grow. Because the cyanobacteria need sunlight to grow and they have the ability to move towards

The majority view seems to be that

life, placing its origin in the vicinity of 3.5 Ga.

depleted sulfur minerals….

stromatolites are the first good evidence for

By 3.47 Ga there is additional evidence for microbial life in the form of isotopically-

Microbial Activity ~3.47 Ga Suggested by Sulfur Isotopes

See the image by Shen et al. Nature, Vol. 410 (2001): 77-81)

SO4 2- + 2CH2O = S2- + 2CO2 + 2H2O

Microbial sulphate reduction?

Australia).

Only this time it did not form at the

Rather microbial life seems to have evolved in a submarine thermal spring system…

By 3.5 Ga then there is evidence for life from stromatolites (Warrawoona, NW Australia) & isotopically-depleted sulfur in barite (N. Pole,

By 3.2 Ga there is new and different evidence for life…

surface….

excellent evidence for both microbial life,

eukaryotes & oxygenic

molecular fossils.

By 2.7 Ga there is

photosynthesis from

Warrie

Mem

ber

Roy H

ill Sh

ale M

emb

erJeerin

ah

Form

atio

nM

arra

Mam

ba F

orm

atio

nM

ad

din

aF

m.

min20 30 40 50

min20 30 40 50

Ham

ersley G

rou

p

Fortescu

e Gro

up

Roy4

Pyritic Sandstone

Roy6

Fibrous Quartz

War1

Black Chert TOC = 0.5%

Mad1

Basalt

Mam1

Black Shale TOC = 2.6%

Roy1

Black Shale TOC = 7.4%

Roy2

Black Shale TOC = 8.6%

Roy5

Black Shale

Roy3

Black Shale TOC = 9.0%

Low TOC High TOC

nC20nC14

660

670

680

690

700

710

720

776

780

Summons, Brocks, et al.

•GEOCHEMISTRY vs STRATIGRAPHY

Steranes Hopanes

C27 100%

C28 26%

C29 33%

C30 5%

Diasteranes

C27 50%

Me-C31 12%

C31 26%

C30 55%

C29 100%

Ts

Tm

22S 22R

2a-Methyl-

Time (min)54 58 62 646056

•

Summons, Brocks, et al.

Regular Steranes

ab

ab

ab

BIOMARKERS BY GC-MS-MS

•

potentially useful for studying the early biological record.

• Hydrocarbons often abundant in Sometimes accumulate massively ie petroleum reservoir.

•

Hydrocarbons

Hydrocarbons, the remains of lipids from once living organisms, are rich in information and

sediments.

Hydrocarbons are mobile. Need to establish

indigeneity / syngeneity ?

OHOH

2HO

HO

OH

OHOH

H

H

H

H

H

skeleton

and 13C content

•

R. Summons

Absolute requirement for O in biosynthesis

preservation of

Recalcitrance of Hydrocarbons

• Are recalcitrant, have distinctive

structures that are diagnostic for:

– type of microbe

– their physiological processes &

– environments they inhabit

Hydrocarbons

bacteria

cyanobacteria MM

M

H

M

M

1 2 3

4 M

BACTERIA

EUCARYA

ARCHAEA

Thermocrinis

Aquifex

Thermotoga

OH

OH

OH

OH

O

O

OH

HO

>2.7 Ga

>2.7 Ga

R

R. Summons (data)

man animals

fungi flagellates

diplomonads

micros poridia plants

ciliates slime molds

green sulfur

gram positives

protcobacteria

avobacteria

Sulfolobus

Des ulfurococcus

Pyrodictium

Thermofilum Thermoproteus

Pyrobaculum Pyrococcus

ethanobacterium ethano-thermus

ethanoplanus

Archaeoglobus alococcus

Halobacterium

ethanosarcina

Methanospirillum ethanopyrus

ethanococcus 1 jannaschii

4 vanniellii

2 igneus 3 thermolithotrophicus

Parallel Molecular Signatures

Black Chert Breccia

Black Chert vein

A Typical Banded Iron Stone (BIF)

Complexity of Extant LifeSpecies Type Approx. Gene Number

Prokaryotes

E. Coli typical bacterium 4,000

Protists

O. Similis

S. Cerevisiae

Distyostelium discoideum

protozoan

yeast

slime mould

12,000-15,000

7,000

12,500

Metazoan

C. Elegans Nematode 17,800

D. melanogaster Insect 12,000-16,000

S. Purpuratas Echinoderm <25,000

Fugu rubripes Fish 50,000-10,0000

Mus musculus Mammal 80,000

Homo sapiens mammal 60,000-80,000

After Maynard-Smith and Szathmary, 1999

Major Transitions in Origin/Evolution of Lifereplicating molecules populations of molecules in protocells

independent replicators chromosomes

RNA as a gene and enzyme DNA genes, protein enzymes

prokaryotic cells Cells with nuclei & organelles ie eukaryotes

asexual clones sexual populations

single bodied organisms fungi, metazoans and metaphytes

solitary individuals colonies with non-reproductive castes

primate societies human societies with language

After Maynard-Smith and Szathmary, 1999

*

Q: A: N = Ng fp ne fl fi fc fL ~ 1,000

Ng=# of stars in our galaxy ~ 4 x 1011 (good) fp 0.1 (v. poor) ne 0.1 (poor) fl=fraction of habitable planets on which life evolves fi=probability that life will evolve to an intelligent state fc

long distances fl fi fc ~ 1/300 ( ) fL

technological civilization ~ 1 x 10-4 (v. poor)

*

which we might one day establish radio communication.

The Drake EquationWhat is the possibility that life exists elsewhere?

=fraction of stars with planets ~ =# of Earth-like planets per planetary system ~

=probability that life will develop capacity to communicate over C. Sagan guess!

=fraction of a planet’s lifetime during which it supports a

An estimate of the # of intelligent civilizations in our galaxy with