David Brin - Neoteny and Two-Way Sexual Selection in Human E

VIP免费

2024-11-20 1 0 64.1KB 19 页 5.9玖币

file:///C|/3226%20Sci-Fi%20and%20Fantasy%20E-books/David%20Bri...nd%20Two-Way%20Sexual%20Selection%20in%20Human%20Evolution.htm

David Brin - Ph.d. © April 1995 (2/89,7/93)

Neoteny and Two-Way Sexual Selection in Human Evolution:

Paleo-Anthropological Speculation on the Origins of Secondary-Sexual Traits,

Male Nurturing and the Child as a Sexual Image

Much progress has been made in tracing the story of human origins, yet mysteries still shroud how we

acquired such unique traits as bipedalism, concealed ovulation, and our prodigious brains. Paleo-

anthropology suffers from both a dearth of hard data and a surfeit of enthusiastic opinions -- for example,

drawing detailed conclusions about evolution from peculiar patterns of fat deposits in male and female

anatomies. Or consider the question of why humans have lost nearly all their hair. It has been suggested

that this adaptation enabled our ancestors to fill a niche unavailable to other predators -- keeping cool

while chasing game under the noonday sun. Alas, this fails to explain why males (the presumed hunters)

retain more ancestral hairiness than females, while children have the least of all. 1

As Herbert Spencer once commented about biological speculation -- there is nothing so tragic as a

beautiful theory, foiled by an inconvenient fact. Especially in the area of human sociobiology, where

evidence is scant and emotions can run high, hypotheses should be offered with good natured humility.

In that spirit I will focus on the trait of neoteny -- or the retention of childlike characteristics in mature

members of a species. This process appears so amplified in humanity that we have been called the

neotenous clan of apes. Humans much more closely resemble chimp or gorilla infants than adults of either

species, e.g. in the smooth, vertical dome of the forehead and the relative ease of bipedality displayed by

very young apes. Furthermore, even aged humans often retain a plasticity of behavior that is typically

found among animals only in the young. Human emphasis on learned, rather than inherited, behavior, has

been widely accepted as a chief driver of this trend, requiring our minds to remain supple and receptive

for ever-longer spans.

This range of physical and mental traits may have a variety of unrelated causes and/or mechanisms,

nevertheless they fall under the same overall theme of retention of childlike characteristics. (More

formally, William Calvin (1991) identifies paedomorphosis ("becoming child-shaped") as juvenilization

of the appearance of the end-product, without implications about the mechanism by which it came about.

Neoteny has been taken by many authors to mean the slowing of some or all aspects of somatic

development.)

Rather than discussing the general neotenization of our species over the last few million years, I wish to

file:///C|/3226%20Sci-Fi%20and%20Fantasy%20E-...xual%20Selection%20in%20Human%20Evolution.htm (1 of 19) [1/3/2005 12:19:29 AM]

file:///C|/3226%20Sci-Fi%20and%20Fantasy%20E-books/David%20Bri...nd%20Two-Way%20Sexual%20Selection%20in%20Human%20Evolution.htm

concentrate on how neoteny may have become enmeshed as part of a powerful selective cycle, going far

beyond its original causes. A complex cycle of sexual selection that may have proved crucial in making

human beings unique among animal species.

Our starting point is a perceived dichotomy between adult men and women -- and thus potentially

hazardous ground. Although evolutionary biology has lately been defended from a feminist perspective

by Patricia Adair Gowaty (1992) and others, caution remains essential when stepping into this arena,

hence I will at times seem to belabor the obvious. Let me also emphasize that Homo sapiens appears less

riven by sexual dimorphism than most species, and exceptions exist to nearly every generalization.

Nevertheless, it seems clear that past and present human dimorphisms are legitimate topics for careful

discussion.

While certain neotenous traits seem to be shared equally among the sexes (e.g. curiosity and plasticity of

behavior), human females certainly do appear more paedomorphic in outward physical appearance than

males. Although they mature at an earlier age, women do not go on to acquire the toughened skin, coarse

body hair, thyroid cartilage, bony eye ridges, or deepened voices which are the common inheritance of

most adult hominoids and other primates. Jones and Hill (1993) have shown that this generalization

remains valid across racial, ethnic and cultural boundaries. Difference in degree of paedomorphism is one

of the few truly decisive human sexual-dichotomies, used by most of us in visually distinguishing women

from men.

How did this dichotomy come about? In exploring one possible explanation, we may come to see the

heritage of human beings as stranger and more poignant than previously thought.

We'll return to the subject of neoteny, but only after first covering some preliminary ground. Often, the

hardest step in speculative paleo-anthropology lies in overcoming assumptions. So let us back up and

begin by asking a very basic question.

Why is it that a human female generally has to compete with other women to get a mate?

May we stipulate that women do often vie over men? In one contemporary society, the United States,

nearly all of the most popular magazines for women trumpet articles advising their readers how to stay

competitive in what is portrayed as a desperate struggle to find and keep a mate. American women spend

many times more each year on cosmetics than the nation appropriates for space research. (If we add

fashion, diet food, plastic surgery, and related activities, costs compare to the defense budget.2) Granted,

contemporary America is an extreme case, and even women in secure marriages work on their appearance

for a complex of other cultural reasons. Still, no one can reasonably dispute that female humans often do

engage in zero-sum contention over an apparently limited supply of suitable males.

Now of course men compete over women, too. But among animals this is only normal. Except for some

spermatophore-donating insects, and a few fish and birds, competition between males for sexual

opportunity seems almost universal.

file:///C|/3226%20Sci-Fi%20and%20Fantasy%20E-...xual%20Selection%20in%20Human%20Evolution.htm (2 of 19) [1/3/2005 12:19:29 AM]

file:///C|/3226%20Sci-Fi%20and%20Fantasy%20E-books/David%20Bri...nd%20Two-Way%20Sexual%20Selection%20in%20Human%20Evolution.htm

Also nearly universal is the far calmer mate-selection process engaged in by females of most species,

either accepting the victor in male-male struggles or actively choosing among candidates. This is not to

say that females don't compete in nature! The struggle to raise successful offspring is deadly serious.

Ethologist Sarah Blaffer Hrdy (1981) has shown that, among our primate cousins, inter-female

competition for status and access to resources may seem quieter than the flashy violence of males, but it is

also generally more relentless and complex. Darwin's image of females as demure, passive watchers-and-

choosers greatly oversimplified a vast domain of intricate and assertive behaviors, with rivalry as much a

feature of the female sex as its vaunted propensity for cooperation. 3

Still, we are discussing a particular type of competition... rivalry to win a mate. And in this narrow area

Darwin retains his original authority. Nature's story is nearly always about two sexes with markedly

different agendas. For a male, each time he prevents one of his rivals from copulating with a female, that

is one more womb which might be induced to carry forward his genetic heritage. 4 The same is not

normally true for a female, looking at males. Once engaged in gestation, her reproductive success is

unaffected by copulations taking place nearby. When there is an abundance of food, one female gets little

or no direct benefit by denying any other female a chance to reproduce, or to be inseminated by the same

male. 5

So we return to our central question -- why do human females engage in rivalry over access to suitable

mates?

A leading hypothesis holds that humans became paragons of adaptability by emphasizing general, species-

wide behavioral and mental neoteny. Further, our offspring are born nearly unformed, or altricial,

replacing reflex instinct with lessons drawn from experience and the accumulated wisdom of the tribe,

channeled by only the most general of innate predispositions. This process takes a long time, during

which our children are helpless as no others in the history of life on Earth.

The presumption goes that human mothers need long-term, dependable partnership to help them carry big-

brained, dependent children across the hazardous, exhausting stretch from embryo to maturity. And while

some human societies have used brother-sister alliances to fill this need, or communal role-sharing, the

majority have left mothers primarily dependent on continued loyalty and aid from the fathers of their

children.

To put this in perspective with nature at large, consider the extreme case of the elephant seal.

During each annual mating season, females congregate onshore. If food is plentiful and the beach roomy

enough, there is small cause for struggle between females, so most behaviorists used to be drawn to the

noisy, extravagant displays of competing males. Known as a "beach master," each bull elephant seal

outweighs any female many times over. By threat, bluster, and frequent bloody fights, he drives off all

male interlopers to secure a local monopoly over insemination. Females acquiesce to this situation.

Indeed, should the bull be away at the far end of his territory, and a rogue male attempt mating on the sly,

file:///C|/3226%20Sci-Fi%20and%20Fantasy%20E-...xual%20Selection%20in%20Human%20Evolution.htm (3 of 19) [1/3/2005 12:19:29 AM]

file:///C|/3226%20Sci-Fi%20and%20Fantasy%20E-books/David%20Bri...nd%20Two-Way%20Sexual%20Selection%20in%20Human%20Evolution.htm

females will often squall for the beach master to come drive the invader out.

Why do female elephant seals prefer to share one male rather than get individual attention? Turn the

question around and consider -- what does the female really need from a male? The answer is sperm, and

little else. Female elephant seals, like those of most species, are generally capable of rearing their pups

alone. So choice of a mate is determined solely by factors which might reflect the quality of his genes --

his heritable fitness. Is he a healthy specimen, likely to father quality offspring? Will the males he sires

likely become beach masters themselves? (Of course these questions are never posed, per se. But natural

selection serves up appropriate answers, just as if they had been asked.) It matters little if the bull she has

chosen also impregnates scores of other females. That he is able to drive off all comers and defend a

beach is testimony to potency he might pass on in his genes. Having secured impregnation, the cows

depart with no apparent sentimentality. They got what they came for.

In her book, The Woman That Never Evolved, Sarah Hrdy (1981) shows that harem systems differ

dramatically. Some, such as the gray langur monkey, can be much more stressful than that of elephant

seals. Langur mothers don't cycle through well-timed mating seasons, but re-enter estrus when their latest

child either weans or dies. Also, while a mother langur doesn't need provisioning by a mated male, she

does require the security of her troop. For these reasons, the bull langur has no single rutting season. To

maximize reproduction, he must "police" his harem year-round. And, since his prime period averages

only a few years, it is in his Darwinian interest to see that all local females serve his reproductive needs.

One bloody consequence is that a new bull, on taking over a langur troop, often kills unweaned infants so

that their mothers will resume ovulating sooner.

So while female elephant seals, gorillas and reindeer can be relatively complacent with their males,

females in yet other polygynous species must look on their mates warily. 6 Nevertheless, in all of these

species the purely sexual aspects of selection are classically Darwinian... featuring inter-male struggle and

various degrees of female choice. Inter-female competition, while pervasive, seldom extends to jealousy

over copulation itself.

Let us assign reindeer, langurs and elephant seals to one end of a spectrum labelled harem size -- the

number of "wives" a prime male in a species impregnates during his lifetime. Along the vertical axis we

then chart ratio of size between adult males and adult females for each mammalian species. By plotting

this chart, R.D. Alexander and others (1979) discovered a significant correlation. Species like elephant

seals, where solitary bulls struggle to hold herds of breeding females, show exaggerated size differentials

between the sexes. Clearly this is not in order for male to dominate female, or else females would

presumably have also grown, to compensate. Rather, it is simply because a big male is better at driving

off competing would-be inseminators. Successful bulls pass on the trait of largeness to their male

offspring.

At the other end of the spectrum are species whose male/female size ratio is near unity, and where harem

size is reduced effectively to one. Roughly four percent of mammalian species form "monogamous" pair

bonds, with the rate a bit higher among primates, such as gibbons. (It is virtually the rule for birds. Chicks

must grow fast to achieve flight before the seasons change. This, plus a high metabolism, means few

file:///C|/3226%20Sci-Fi%20and%20Fantasy%20E-...xual%20Selection%20in%20Human%20Evolution.htm (4 of 19) [1/3/2005 12:19:29 AM]

file:///C|/3226%20Sci-Fi%20and%20Fantasy%20E-books/David%20Bri...nd%20Two-Way%20Sexual%20Selection%20in%20Human%20Evolution.htm

avian young survive on the labor of one parent alone.)

Now by definition, monogamous species have approximately equal numbers of successful male and

female breeders, so one might expect both to behave similarly, competing the same amount with others of

the same sex. Each should be as choosy in selecting a mate, and exhibit the same degree of jealousy about

copulation. But this is not the case, because most "monogamous" males are not purely monogamous in

every sense of the word. Generally, these males do not give their mates so much absolute fidelity as

devotion... meaning they will do anything and everything to serve and protect the nest and their offspring.

But, given an opportunity to engage in outside sex without risk or harm, they will often take advantage.

Such opportunistic philandering by so-called "monogamous" males was until recently hardly discussed.

Now, however, we know that it plays a distinct role in the behavior patterns of most such species.

For example, the females of many bird species force prospective mates to engage in lengthy, exhausting

courtship "dances" and other displays, before becoming sexually receptive. For years this was thought to

involve species identification -- preventing hybrid insemination by a related species. But plumage, scent,

and a thousand other simpler markers are available to accomplish the same end. Now it is thought that

mating dances serve more directly pragmatic role, by culling out philanderers. Few already-mated males

can afford the time and energy -- exhausting themselves in an effort at wooing -- if they already have a

mate and nest elsewhere. To male birds, monogamy may not mean absolute fidelity, but it does mean

having priorities. 7

Thus, even monogamous species retain dimorphisms of sexual motivation and behavior. Monogamous

females must remain careful and choosy, and even "monogamous" males must still prove themselves in

order to win fatherhood.

So where do human beings fit in this spectrum? Few comparative ethologists call humanity a truly

monogamous species, even by bird standards. Indeed, many men, in both behavior and avowed fantasies,

lean toward the attitude of male gorillas, if not elephant seals! Our position on the male-female size ratio

chart would appear to suggest that humans have a modest "natural harem size" -- between one point one

and one point four -- yet some men spend their lives aiming to achieve the milestone of their bedded

"hundred," or even "thousand".

Nevertheless, we also share traits with pair-bonding species. Many men and women are capable of

forming tight, long-lasting and devoted associations. Moreover, our offspring are altricial, helpless, nearly

impossible for a mother to rear successfully in the wild without at least some outside aid. For a very long

time any woman who chose a loyal, dependable mate almost certainly had advantages over one who

failed to do so.

In summary, then --

1) It is reasonable to suggest a selected tendency in human females to prefer mating with males who offer

effective, committed support, along with their sperm.

file:///C|/3226%20Sci-Fi%20and%20Fantasy%20E-...xual%20Selection%20in%20Human%20Evolution.htm (5 of 19) [1/3/2005 12:19:29 AM]

file:///C|/3226%20Sci-Fi%20and%20Fantasy%20E-books/David%20Bri...nd%20Two-Way%20Sexual%20Selection%20in%20Human%20Evolution.htm

2) Given the nurturing demands to be placed on the male she chooses, one can expect female humans to

prefer not to share their mates with many other women.

So far we may seem to be belaboring the obvious, but we are discussing matters all-too often associated

with strong opinion and emotion, so it's best to move in careful steps.

Now ideally, given desiderata 1) and 2) above, men ought to behave like male birds, and indeed, the

"best" of them do seem to follow that pattern. While such men may stray on rare occasions, they seldom

do so if it seems home or family might be jeopardized. But human males show an incredible range of

motivation and behavior 8 One does not have to reach speculatively back into the Pleistocene to illustrate

the difference between mating with "bird-like" or "elk-like" men. Contemporary American society shows

the calamitous consequences when women bear children fathered by the latter type, who promise

anything, then depart when it's convenient. 8b Hence we have driver number three.

3) A large fraction of human males are not (from a solemn female point of view) suitable for pair-bonding

or fatherhood. High male variability probably meant that choice remained an important, even crucial,

activity for our female ancestors.

Are quality males a scarce commodity? That's no problem in polygynous species, where females simply

share the alphas. But such a scarcity presents severe, even desperate difficulties where females prefer

pairing! Adding factor three poses the problem starkly. Human females began competing for mates

because they needed the kind of competent, collaborative devotion received by female birds -- but which

only a fraction of human males seem inclined or capable of delivering. Hence it is a combination of

limited supply and high demand which has created the unusual situation of competition among women for

successful mating.

Put in this way, it seems a prosaic, not particularly surprising conclusion to reach after so many

paragraphs. And yet, the quandary of human females, and their contention for quality mates, goes far

beyond the clichéd plaint of the woman nightclub comic, who bemoans (to fervent feminine applause) the

scarcity of "decent men". I contend, in fact, that this dilemma has already radically shaped the flow of

human development.

Sexual Selection in Humans

Departing from the traditional view since Darwin, recent biological theory perceives evolution as a

sequence of fairly rapid state changes that punctuate lengthy periods of relative equilibrium. There are

several ways species can launch into rapid change.

-- Geographic barriers separate sub-populations, isolating divergent gene pools. Long separations result in

speciation. If groups are re-united before that point, a sudden influx of stockpiled genes from the isolated

reservoir can speed change within the parent stock. 9--

file:///C|/3226%20Sci-Fi%20and%20Fantasy%20E-...xual%20Selection%20in%20Human%20Evolution.htm (6 of 19) [1/3/2005 12:19:29 AM]

文档加载中……请稍候！
如果长时间未打开，您也可以点击刷新试试。

下载文档到电脑，查找使用更方便

5.9 玖币 0人已下载

立即下载 VIP免费下载

摘要：

file:///C|/3226%20Sci-Fi%20and%20Fantasy%20E-books/David%20Bri...nd%20Two-Way%20Sexual%20Selection%...

收起<<

David Brin - Neoteny and Two-Way Sexual Selection in Human E.pdf

共19页,预览6页

还剩页未读，继续阅读

声明：本站为文档C2C交易模式，即用户上传的文档直接被用户下载，本站只是中间服务平台，本站所有文档下载所得的收益归上传人(含作者)所有。玖贝云文库仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对上载内容本身不做任何修改或编辑。若文档所含内容侵犯了您的版权或隐私，请立即通知玖贝云文库，我们立即给予删除！

David Brin - Neoteny and Two-Way Sexual Selection in Human E

相关推荐

开通VIP享超值会员特权

作者详情

相关内容

热门标签

举报选择: