104516585 Are Humans Still Evolving Recent and Ongoing Human Evolution

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Are humans still evolving? Recent and ongoing human evolutionLast edited 10 November 2011; Submitted for grade 17 November 2011, submitted to UA 26 April 2012

1 ABSTRACTIt is easy to think that, on or shortly following the appearance of

Homo sapiens, humans stopped evolving. After all, the ability of

humans to discover, refine, and pass on techniques of tool use has

provided an ever-increasing degree of protection from the threats

of the natural world, from the earliest fires ability to ward off

potential predators and lessen the effects of cold weather to mod-

ern medical practices which make disease almost irrelevant to

some segments of human society. However, numerous studies have

shown quite conclusively that this is not the case: the evolution of

mankind has never halted nor slowed, and nothing indicates that it

ever will. Human culture itself has a powerful ability to alter and

even create selection, such as the selection for adult lactase persis-

tence conferred by the development of dairy farming, or the cultur-

al transmission of mate-choice preferences.

Selection since the radiation ofH. sapiens from Africa is easy to

prove by detecting evidence of strong (and ethnically varying)

selective sweeps in the human genome, and even in modern times,

evolution can be demonstrated over only a few generations. In

recent years, human resistance to heart disease has risen, and fe-

cundity has increased markedly due to decreases in maternal age at

first birth, shorter inter-birth interval, and later age at menopause.

This change in fecundity is also associated with changes in growth:

in developed nations, female height is increasing, while in devel-

oping nations it decreases, due to a well-established tradeoff be-

tween growth and reproduction, and varying food availability.

2 INTRODUCTIONAre humans still evolving? Though at one point in time, this was

denied by some of the top scientific minds of the age (Talt 1869),

now the popular consensus is established to such an extent that

even popular news media sources (Harrel 2009; Bootle 2011) are

joining in stating that, yes, Homo sapiens is still subject to the

process of evolution by natural selection despite advances in

medicine and technology that mean most humans are able to

survive and reproduce.

The reason for this is simple: while most humans may survive

and reproduce, some will reproduce more than others, and any

heritable traits that improve reproductive success will be passed on

to the next generation at a disproportionately high rate, causing the

population to evolve.

This paper will provide proof for the claim that humans are

indeed evolving, citing studies demonstrating human evolution on

macro and micro time scales and exploring in humans the

processes of natural and sexual selection with regard to the

characteristically human trait, culture.

3 GENE-CULTURE COEVOLUTIONBefore exploring cases and evidence of human evolution, we

must first define and address the issue of gene-culture coevolution,

a process that has had great impact on human evolutionary change.

Richerson et al define culture as all of the information that

individuals acquire from other individuals in a variety of social

learning processes including teaching and imitation (2010).

Culture serves as a form of information parallel to genetic

information, and unlike genetic information, culture can be

transmitted in multiple ways, horizontally from related or unrelated

individuals or groups as well as vertically from parent to child

(Feldman and Laland 1996).

Gene-culture coevolution is the impact on genetic evolution of

culture, which can create changes in either human preferences or

their environmental conditions. These changes will increase anddecrease the relative fitness of different genotypes, thereby

describing selection and causing evolution.

It should be noted that cultural change does not always increase

selection; culture can also serve to soften selection. For example,

humans moving into a cold environment adapted to this change by

wearing more clothes and building fires and shelters to keep warm

rather than evolving physiological tolerance to the cold (Laland

2008). But selection cannot be eliminated entirely, and even in this

specific example there is evidence for human genetic adaptation to

different environments (Williamson et al 2007), though none

would ague that cultural change is not the crucial factor in human

adaptation to hostile environments. As well, the use of advanced

medicine unquestionably softens selective pressures that might

otherwise favor humans with greater disease resistance, though as

will be discussed later, this merely creates alternative selective

pressures.

Despite this dampening effectwhich is arguably the most

potent adaptation humans possessculture does enact vast

changes on the human genome via niche-construction and sexual

selection. It has been argued by several groups that gene-culture

coevolution may in fact be the dominant form of human evolution

(Laland et al 2010, Richerson et al 2010), likely affecting hominid

evolution since before the divergence from the other great apes

(Richerson et al 2010) and possibly even serving to accelerate

human evolution (Hawks et al 2007).

3.1 Culture can affect genetic selection by changingthe environment

Perhaps the most pervasive way in which gene-culture

coevolution can act is by cultural change enacting a similar change

on the surrounding environment, which then affects genetic

selection.


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To use a favorite example of gene-culture coevolution

proponents, evidence indicates that the spread of the lactase gene,

which allows adult humans to retain an enzyme that allows more

efficient digestion of dairy products, was preceded by the spread of

cattle domestication and therefore dairy consumption (Tishkoff

2007). The implications of this are clear: the cultural trait of cattle

domestication, which provided a reliable food source to early

humans, spread throughout populations in Europe, North Africa,

and the Middle East. At some point, a genetic mutation arose

which allowed possessors to get even more nutrients out of the

presence of domesticated cattle, and it too spread throughout the

region.

The case of the lactase gene is one of great importance to the

question of contemporary human evolution, and will be addressed

further in section 3. Relevant to this section is the fact that the

lactase gene spread as a result of the cultural development of dairy

farming, a clear-cut example of culture affecting changes in the

genome.

3.2 Culture can affect genetic selection by changingmating preferences

In addition to altering the selection based on the environment,

culture may also affect sexual selection in humans. Recent studies

have experimentally demonstrated mate-choice copying in humans

(Little et al 2011), showing that human mate choice is dependent

on social learning. The link between cultural groups and mate

preference is well established (Boyd and Richerson 1985),

showing that any two arbitrarily selected cultures will have

different patterns of mate preference. These studies, and other

modeling done by Laland (1994) indicate that any arbitrary mating

preference for a heritable and phenotypically obvious trait such as

hair or eye color can spread throughout a population via culturaltransmission, producing sexual selection. In addition, Ihara et al

(2003) have shown that according to models of genetic and cultural

transmission, female mate choice and male traits may link and

produce runaway sexual selection that could drastically alter the

phenome of the species.

4 RECENT HUMAN EVOLUTIONOver the past several thousand years, human adaptive change

has not slowed or stopped but accelerated (Hawks et al 2007);

recent studies into the human genome have detected literally

thousands of genes that have been subject to detectable positive

selection sweeps (Nielsen et al 2007). Notable among these arestudies that show increased numbers of selection sweeps among

non-African populations, probably a result of adaptation after the

colonization of new environments (Williamson et al 2007),

increased numbers of complete sweeps in non-African populations

(Coop et al 2009), and studies showing the evidence of powerful

selection sweeps in well-studied genes conferring malaria

resistance and lactose digestion (Sabeti et al 2006). Additionally,

these studies do not provide the whole picture, and much more

investigation is needed to conclusively state how much of the

human genome has been subjected to selection (Wang et al 2006).

In the case of the lactase gene, we see a gene that was not only

subjected to one of the highest rates of positive selection known

(Bersagleri et al 2004), but one that is linked to ethnic background.

The lactase gene is most prominent among peoples of European

ancestry, and occurs at rates approaching zero in East Asian groups

(Bersagleri et al 2004), indicating that the selective pressure for the

lactase gene occurred after the separation of populations that

colonized Europe and East Asia, and after the advent of dairy

farming. This claim is supported by other studies linking the

degree of presence of the lactase gene with cultural use of

fermented and unfermented dairy products (Laland 2010).

Clearly, humans have evolved over the past ten thousand years,

and in patterns that vary with location, as one would expect for any

other organism that has experienced changing climates and

colonized new habitats.

5 ONGOING HUMAN EVOLUTIONAlthough the typical argument stating that humans are not

evolving hinges upon the massive advances in medicine and

technology that have taken place over the past few centuries

(McKie 2002), numerous studies have detected measureable

evolution over even decades, in consistent patterns the world over.

5.1 Improving cardiovascular healthDespite advances in medicine, cardiovascular disease kills an

estimated 17.3 million people per year, most of who live in

developing nations (World Health Organization 2011). Like any

other common fatal health risk, this creates selective pressure on

the species, favoring those individuals who are less prone tocardiovascular diseases. Though any overall improvement in

cardiovascular health could be attributed to improving public

knowledge and healthcare, studies taking into account these

confounding factors indicate that systolic blood pressure and LDL

cholesterol levels (two predictors of cardiovascular disease) have

dropped over the past 60 years (Byars et al 2010, Stearns et al

2010). It is unclear to what degree this improvement in

cardiovascular health is truly due to genetic evolution rather than

scientific advancement, but sources indicate that at least part of the

improvement in health has a genetic basis (Stearns et al 2010).

5.2 Changes in heightAdult height is associated in humans with levels of nutrition

(Frisch 1978), and with reproductive success, though differently in

males and females. In male humans, height is a common sexually

selected trait (Hensley 1994) that correlates with male reproductive

success: the men with the peak reproductive success are above the

populations mean height (Nettle 2002a). The inverse is true in

females, where there is a tradeoff between high adult height and

early sexual maturity, so females of a short stature (and more


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Are humans still evolving? Recent and ongoing human evolution

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importantly, early maturation) have greater lifetime reproductive

success (Helle 2008), though the directional selection on height is

less pronounced for women than it is for men (Nettle 2002b).

Studies in the developing world yield conflicting results: height is

positively correlated with female reproductive success (Allal 2004;

Sear et al 2004), likely due to varying levels in nutrition. In these

nations, there is a higher infant mortality rate for the offspring of

short women. This variation in selection by location implies that

changes in height are implicitly tied to levels of nutrition, and that

selection will only act to increase male height and decrease female

height if the population has the resources to do so.

5.3 Widening reproductive windowBy far the most consistent finding among all studies into human

evolution is that the female reproductive window is widening.

Short-period investigations into changes from generation to

generation in populations in Quebec (Milot et al 2011), Finland

(Helle et al 2005), Gambia (Allal et al 2004), the USA (Byars et al

2010), Australia (Kirk et al 2001), Kenya (Borgehoff 1989), Nepal(Gubhaju 1983), and Sweden (Low 1991) have all yielded

consistent results: over the past century low maternal age at first

birth correlates strongly with lifetime reproductive success. This

not only affects women: a 1996 study by Kr et al showed that in

monogamous societies the age of a mans wife was by far the

greatest indicator of his lifetime reproductive success.

As well as early reproduction, studies have linked shorter inter-

birth interval (Stearns et al 2010) and later age at menopause with

lifetime reproductive success (Kirk et al 2001; Byars et al 2010),

though the correlation between these traits and fitness is far lower

than that between first reproduction and fitness (Stearns et al

2010).

As with selection on height, reproductive ability is intrinsicallylinked to food availability (Frisch 1978), which has increased

tremendously in recent generations. Any organism with unlimited

access to resources will reproduce as rapidly as possible, and over

the past centuries humans have not only been reproducing at a

maximum rate, but that maximum rate increases as humans evolve

to reproduce more often and from an earlier age to a later age.

6 CONCLUSIONThe idea that humankind has somehow removed itself from the

effects of natural selection seems rooted in the vain idea that we

are better than other species, and above the effects that drive

changes in lesser animals. And yet, investigating the questionillustrates quite clearly that we are subject to the same processes

that govern the evolution of every other organism: natural selection

causes genes that allow digestion of a readily available food source

or increase resistance to common diseases to spread amongst a

population, sexual selection causes the proliferation of arbitrary

male-trait and female-choice genes, and when food is available in

excess and the likelihood of death by disease is at an all-time low,

the humans which reproduce fastest win themselves a greater stake

in the genetic composition of the next generation. Even culture, a

feature of the species lauded as something that somehow separates

us from the animals, has a tremendous effect on the selective

pressures that drive human evolution. Numerous studies have

proven that humans have continued to evolve over the past five

thousand years, and that we will continue to do so with no sign or

indication that that will ever change.

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104516585 Are Humans Still Evolving Recent and Ongoing Human Evolution