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The last common ancestor of the two Kenyanthropus species has a thickening of the supraorbital, with inflated glabella. The tympanic is tubular, the upper face is broad, there is absolute reduction in subnasal prognathism, and the zygomatic insertion is low. The malar has increased in diagonal length. Finally, incisor heteromorphy is absolutely reduced. The earlier species, K. platyops, has a small cranial capacity, the anterior zygo-matic is inserted anteriorly at the P4, and the upper molars have decreased in size. The later K. rudolfensis has increased cranial capacity and the upper molars have increased in size.

The small cranial capacity observed in the earlier Kenyanthropus species is difficult to resolve. It is hard to accept that there could be any adaptive advantage in the reintroduction of a small brain. The problems in determining how to "objectively" weigh such a character is beyond this study. The most parsimonious way to interpret this difficult question is to accept that Kenyanthropus and Australopithecus originate from a similar base, and whether Homo evolved from Australopithecus or Kenyanthropus remains to be seen. To accept this scheme, we must also believe that the later species of Kenyanthropus, K. rudolfensis, must have evolved a large brain independently from Homo. While these taxa do appear to represent hominins (sensu DWC), the phylogenetic complexities of this group remains obscure.

From these analyses, their interpretation, and our discussion, the origins of the later Pleistocene hominins can be observed from the clade containing Homo habilis and H. ergaster. As we will see in the chapter to follow, it was H. ergaster that was the first hominin to disperse out of Africa around 2 Ma. It was from this group that ultimately a number of hominin species were to later evolve, and in some cases different hominin species would come into contact with each other. Ultimately, however, only one species would survive to continue the hominin lineage into the later Pleistocene and Holocene, H. sapiens.

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Interlude 3 Of Men's Beards and Peacock's Tails

Sex is a big deal. We know, very broadly speaking, why this should be: Without it, the species has no future. But exactly why it takes the form it does in the human species and why there is this huge variety — now those are other questions.

Gibbons don't differ much in size between the sexes, though they differ in their calls and, in some species, in their color. In orangutans, males are bigger than females, but some males are much bigger than others. Mature male gorillas are always hugely bigger than females and have a conspicuous grey back. Male chimpanzees are only slightly bigger than females but have simply enormous testicles. Men are bigger than females, though not vastly so, but have very big penises (all of them — did you know that?); and men and women are just astoundingly different in all sorts of ways. Very odd. Why?

It has to do with social organization, and this in turn has to do with how species make a living. What it's called is sexual selection. It's males getting bigger because they have to compete with each other — but it's much more than that.

Natural selection, which Darwin described in 1859 in On the Origin of Species, is the simplest mechanism of change in evolution. If one individual is ever so slightly better adapted to its environment than another, it has a slightly better chance of leaving offspring. That's essentially it. But in 1872, in The Descent of Man, and Selection in Relation to Sex, Darwin pointed out that sexual preference has similar consequences. Peahens prefer peacocks with bigger tails, so the bigger the tail, the more offspring a peacock has—on average.

Sir Ronald Fisher was one of those who forged the new evolutionary synthesis in the 1930s — the welding together of natural selection and the new science of genetics to produce the first rounded, biologically satisfying model of evolution. Whether it is a complete model has been discussed ever since, but nobody denies that it is a valid model. Fisher was brilliant and (therefore?) opinionated and realized that the course of selection could and should be modeled mathematically. In his 1930 book, The Genetical Theory of Natural Selection, he wrote a great deal not only about natural selection, but also about sexual selection. But this promising start did not last; sexual selection, for some reason, fell into disrepute — for 40 years. It was not until 1972, when Bernard Campbell edited a book called Sexual Selection and the Descent of Man, that it gradually eased its way back into respectability. The slowly emerging field of evolutionary psychology took it up, and sexual selection is now established as a mainstream field for research and experimentation.

As far as reproduction is concerned, there is a basic asymmetry between males and females, and it is this: Females are the ones that produce the young, one or two (or however many it is) at a time, whereas males simply do the fertilizing. This makes the female the indispensable sex; there have to be lots of females, but in theory there need only be one single male in the entire population. Males are therefore in fierce competition to determine which one of them it shall be. The best way to do this is, to put it bluntly, to find out what females want and to be as like that as they can. Whichever male pleases the females most — that's the one who gets the most matings and leaves the most offspring.

Female peahens like males with big, gaudy tails. Why? Because tails like these handicap the males; they make them more conspicuous to predators, they make it more difficult to fly, and when folded up they drag on the ground and pick up dirt and nasties. If there is a peacock who, despite all this, has a really big and gorgeous tail, it means he has overcome all these problems and thrived. He must be a very fit specimen indeed — go mate with him, all you peahens, be fertilized by his excellent genes and produce wonderfully fit offspring.

The African long-tailed widow bird (Euplectes progne) is like peafowl, but half the size and easier to experiment with. The female is a dowdy mottled brown; the male is red and black with a tail one-and-a-half meters long. The male jumps into the air in front of females, displaying its extraordinary tail to them. Malte Andersson in 1982 caught quite a number of males. He cut portions out of some males' tails, glued them into others' tails, and then released them. For controls, he caught yet other males and either released them again unaltered or cut through their tail feathers and simply reglued them. He found that the number of females who nested in different males' territories was exactly as predicted — most in the territories of the males whose tails had been lengthened, less in the unaltered ones, least in those whose tails had been shortened.

In zebra finches (Poephila guttata), males have redder beaks than females (on average). The redness is due to pigments called carotenoids. These are not genetically controlled, but obtained through the diet — and they are not just pigments; they are also antioxidants that stimulate the immune system. Some British zoologists (in a study published in April 2003) supplied some males with pure water as usual but gave their brothers — literally, their full brothers — water with carotenoids in it. The ones with the carotenoid supplements had redder beaks and enhanced immune systems — and were more attractive to the females.

Think what this means. We infer that peacocks and male widow-birds must be fit if they can overcome the handicaps of their tails, and we suppose that females must subconsciously infer this too. With the zebra finches, we actually know it: Those with red beaks really are the fit ones. In the wild, if you can get a lot of carotenoids into your diet — if you can get and keep a good territory with the right resources — you will get a redder beak and females will know that you are ever so fit. And get this: A female, if she chooses the right male, can get access to his territory and she too can get a red beak and a fantastic immune system.

Sexual selection is based on natural selection; what it does is to take the indicators of fitness and run with them.

The fruit on which gibbons feed in their treetop habitat in the southeast Asian rainforests is clumped, and the clumps are scattered. There is enough to feed just small groups, provided that the groups are well spaced through the forest. So gibbons are monogamous and territorial — meaning that their social groups consist of one male and one female plus offspring, and this pair is the sole occupant of an area of treetops (about 20-40 hectares is usual). It used to be thought that the breeding male and female were faithful unto death. But we now know that there is divorce (one member of the pair may simply swing off and pair up with a different mate). There is also extra-pair copulation (cheating, not to put too fine a point on it) — a bit like human monogamy really.

In the white-handed gibbon, both members of the pair defend their territory. And the males don't actually fight for mates, so males and females are about the same size and both have long, stabbing canines. In fact, there's very little difference between them at all; the male doesn't even have a scrotum (for the benefit of the curious, they're in tiny individual sacs on either side of the penis). A female has to attract a mate and keep him there, and she sings loudly — a soaring, melodious aria, to which the male adds an insignificant little coda. In silvery and dwarf gibbons, the male doesn't even bother to do that. In other species the relationship is more equal, and in some the male has as elaborate a song as the female, and a scrotum too. In a few species the male and female are different colors. Complicated — and we know too little about most species to say exactly what sort of sexual selection is going on, but in general monogamy means equality.

Orangutans live in some of the same southeast Asian rainforests as gibbons do, but because of their huge size, they have to move much more cautiously and to live solitary lives. But they can open enormous fruits that are beyond the capabilities of gibbons. How huge are orangutans? Females weigh about 40 kg, fully mature males more than twice that — 85-95 kg. We might deduce that there's a lot of overt competition between these males, and we would be right — they fight when they meet, and smaller ones hear the deep booming voices of the bigger ones and keep well out of the way. And these huge males have wide, solid flanges on their cheeks, which may sway back and forth as they move, obviously some sexual adornment for the females' admiration.

And there are undersized males with no flanges. The females are not in the least attracted to them, but these males chase after the females and, frankly, force themselves on them. For a long while it was assumed that these unflanged males were not mature, but it now seems that some of them may stay like that all their lives — it's simply an alternative strategy. So there are two ways of doing things if you're a male orangutan: You can develop into a huge, splendid chap and wait for the females to come to you (and they do), or you can stay small and go out and get the females because they won't come to you (and they don't). DNA studies show that the two ways are equally successful at yielding offspring. This balance of two different ways of operating is called an Evolutionary Stable Strategy (ESS).

Gorillas are even larger than orangutans, and males and females are just as different in size (and all males are big — there are no sneaky runts). But they live in Africa, mainly on the ground, and in social groups. They can do this because, although they prefer fruit, if there isn't any fruit available they can make out just fine on the ground herbs that are all around them. There may be one male and several females, and their offspring, in a group. And lo, gorillas, like orangutans, have an ESS: A male, when he matures, may stay in his father's troop if there are spare females in it (and take it over when father becomes senile), or he may leave it and try to steal females from other individuals' troops. Steal them? If a male looks stronger and fitter than the one they're with, the females will join him in a flash.

Chimpanzees weigh 30-60 kg and live in communities where males and females mingle and separate at will. Why are males not much bigger than females — don't the males compete? Yes and no; when a female is in her fertile period she mates with them all, one by one, and it's not they who compete — it's their sperm! Those huge testicles pour out vast quantities of sperm, and the winner takes all. Oh, there are dominant males in the community, and sometimes one will sequester a female and be her sole consort for a while, but sperm competition is the rule.

And us? We are bizarre. Men are bigger than women, but not too much. But men have beards and moustaches, a lot more body hair than women, and broad shoulders and narrow hips. Women have breasts and a buildup of fat on the buttocks, hips, and thighs. Men and women alike are hairless compared even to chimpanzees and have more body fat. Chimpanzee and gorilla mothers develop breasts, but the breasts shrink again when the babies have been weaned; they are not permanent like women's. We live much longer, too. If a female chimpanzee lives into her late forties, she ceases to breed, and she may be dead before then anyway; women ostentatiously, almost ceremoniously, stop being able to breed at about 50, but live on well after that. (Life, we are told, was nasty, brutish, and short in premedical times. But this is a statistical shortness and is just because so many babies died in their first couple of years that it brings down average life expectancy — if people survived early childhood, most of them lived on into their sixties and seventies and more, like we do today.)

So many people have put forward so many ideas for why we are hairless. To cool down and to swim better are two that we often hear. Recently, Mark Pagel and Walter Bodmer suggested it was to rid ourselves of parasites: Ticks and biting flies can be seen, can't hide away, and can be got rid of, and this became attractive to the opposite sex. Hair was retained on the scalp as a sort of sun hat, in the armpits to waft underarm pheromones (sexual odors) into the world, and on the groin — why? Maybe for the same reason, they suggested.

Pagel and Bodmer haven't gone far enough, have they? On a hairless skin, not only can you be parasite free, you can be seen to be parasite free. Scabies and ulcers and wounds show up brightly on naked skin. Unblemished skin advertises in the starkest possible way that you are fit.

Different body shapes and different hair patterns signal sexual fitness from a great distance. Excuse us for asking, dear reader, but have you ever been to a nude beach? People are walking way over at the other end of the beach, and you can tell their sex because of their body shape. As they come nearer, you can begin to discern something of the hair distribution. Women have a dark patch there and a separate one there, but men have a continuous swath from one to other, parting company on either side of the face, reuniting on the chin, straggling a bit as it goes down the chest and belly, but pretty much continuous all the same — or was in preshaving days. But even so, the five o'clock shadow is discernable well before the sun has slipped below the yardarm.

A Cambridge zoologist, C.B. Goodhardt, said all this long ago, in the early 1960s. He gave lectures in which he would explain that sexual differences in hairiness and hair distribution were deliberately exaggerated, as supernormal stimuli, by "living savage races," and would cause a roar of laughter from (most of) the audience by then showing a slide of a Scottish soldier in bearskin and sporran. But he thought that, for this to work, the ancestral skin color would have to be light. Actually, it doesn't. Compared to Caucasoids (both pink Europeans and brown Arabs or Indians), black Africans are exceptionally hairless except on those same places, and the matte hair stands out against the shiny skin. It works anyhow.

Geoffrey Miller (2000), in The Mating Mind, has gone on to show how mental characteristics, too, have been sexually selected; and evolutionary psychologists have done experiments to test whether it's true (it is).

We say, "Vive la difference, mate!"

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