The human being of today has, of course, evolved many characteristics that are now considered human, so many that it is difficult to point to any one of them and consider it the key. What we must do, then, is go back in time, watching humanity become more and more primitive, less human, more apelike.
Yet we must stop at some point where our ancestors are still more nearly human than they are apish. Any organism that is more human than ape is called a hominid (from a Latin word for "man"). Any organism that is more ape than human is called a pongid (from a Congolese world for "ape").
Therefore, we might recast the first sentence of this section to read "The first hominid advance was biological. It consisted of becoming hominid."
As we go back in time, studying the bones and teeth (all that remain) of earlier forms of hominid life, we come to an organism that was perhaps the size of a modern chimpanzee or even a little smaller and with a brain that was no larger. Yet in one crucial respect, it was much closer to the human than to the ape. This human characteristic is so obvious that, if we were to see the organism in real life, we would at once say, "This is no ape."
It was the first hominid, and what made it so was its bipedality. It walked on two legs, as we can tell from the shape of its spinal column, its pelvic girdle, and its thighbones.
The fact that human beings walk on two legs strikes us as characteristically human. We are bipeds (from Latin words for "two legs"), while other mammals are quadrupeds (four legs).
Of course birds walk, run, or hop on two legs, and the Greek philosopher Plato (ca. 427-ca. 347 B.C.) therefore defined a human being as a "featherless biped." That is insufficient, however, for there are bipeds with fur (kangaroos and jerboas) and bipeds with scales (various dinosaurs), which Plato knew nothing about.
Let's consider bipedality, then, to see what makes human bipedality different from other types.
Animals that are bipedal are often restricted to two legs because two others have been devoted to some other (and preferred) form of locomotion. Most birds are designed to be flyers, and the forelegs have become wings for that purpose. Penguins are swimmers, and the forelegs have become flippers. In either case, walking, running, or hopping is secondary.
There are, of course, nonflying birds like ostriches, for which walking or running on two legs is the only means of locomotion. In such cases, the body is designed for it and is essentially horizontal, so that there are roughly equal amounts of it at front and rear. With the two legs centered, bipedality is a mechanically easy thing to maintain. This is also true of bipedal reptiles and mammals like the tyrannosaur and the kangaroo. Long tails provide balance, and the body remains essentially horizontal.
Suppose, though, a quadruped's body ends at the hips and there is no tail to act as a balance. In that case, the only way the body's center of gravity can be brought above the hind legs is to tip the entire body into a vertical position.
Some tailless animals actually do this. Bears and chimpanzees can stand up-right on their hind legs and even walk about in this fashion, but they are clearly uncomfortable doing so and prefer to let the forelegs share the work.
Penguins, too, hold their bodies upright, but they are swimmers and clumsy on land. Though they can walk long distances if they must, they prefer to belly-whop on ice when they can.
The human being, then, is a tailless, habitual, and comfortable biped. But what is it that makes the human a comfortable walker on two legs? It is that the spinal column, just above the pelvis, bends backward in human beings, assuming a shallow S-shape and adding a spring to the human walk that makes it comfortable. No other organism has that backward bend of the spine in the small of the back. Bipedality also produces problems. There are slipped disks, inflamed sinuses, and accidental falls, for instance. None of these would be likely were it not that human beings are still not entirely adapted to walking upright.) The earliest hominids were first identified by an Australian-born South African anthropologist, Raymond Arthur Dart (1893-1988), to whom a skull, rather human-looking except for its extraordinarily small size, was brought from a South African limestone quarry in 1924. Dart, in 1925, named the type of organism to which the skull belonged Australopithecus (from Greek words meaning "southern ape"). Further finds made it clear that it was not an ape but a hominid, and at least four different species have now been identified, which are lumped together as australopithecines.
In 1974 an American anthropologist, Donald Johanson, unearthed an unprecedentedly complete and ancient skeleton of an australopithecine female, who was given the name Lucy. (You can tell a female from a male by the shape of its pelvis.) It was possibly as much as four million years old judging from the age of the rocks in which it was found.
Lucy is an example of Australopithecus afarensis, because Afars is the name of the region in east-central Africa where her remains were found.
Australopithecines existed only in eastern and southern Africa, so east-central Africa may have been the cradle of humanity.
Lucy was the size of a chimpanzee and slighter in build. Her australopithecine relatives seem to have ranged between 3 and 4 feet in height and to have weighted perhaps 65 pounds. Their brains were no larger than a chimpanzee's and about a quarter the size of our own.
The early australopithecines probably lived much as chimpanzees do, may have spent their time partly in trees, must have been very largely vegetarian, and undoubtedly could not speak. However, they were as bipedal as we are and walked as easily and as comfortably on their hind legs as we do.
Why did the australopithecines develop the backward bend in the spine? Why did the processes of evolution invent the hominid, in other words? Four million years ago the Earth had been warm for quite a while, and large tropical animals, such as elephants, rhinoceroses, and hippopotamuses, tended to lose their hair because such insulation kept them too warm. For some reason, the hominids also lost their hair though they were much smaller than other hairless mammals. We don't know at what stage hair was lost.
The Earth of the australopithecines was turning cooler, however. The forecasts shrank and were replaced by grasslands. Those organisms whose habitat was the forest and who did not give up the trees naturally retreated with the forest.
Some arboreal pre-hominids managed to adapt to the grasslands in east-central Africa and to spend more and more time out of the trees. It must have been a difficult transition. As they spent more time on the ground, they had an increasing tendency to rise to their hind legs and peer about over the tall grass, searching for food or watching out for predators. Those who could stand upright more easily and could do so for longer intervals were better able to survive.
Even a slight crook in the spine, which made standing upright slightly easier, gave those who had it a better chance of surviving, and of having children to inherit that crook. What we call natural selection would thus drive the pre-hominids toward bipedality and a true hominid character.
Bipedality had side effects that were also beneficial and that reinforced the natural-selection drive. The forelimbs were freed for duty as something other than support. The freed hands could more easily manipulate portions of the environment, feel them, and bring them close to the eyes, ears, and nose, so that the brain was continually flooded with sensations.
Any change that made the brain a bit larger or more complex made it possible for the brain to handle the flood of sensation more efficiently, and this led to an improved chance of survival. Thus, natural selection introduced a drive for bigger and better brains.
The early australopithecines, with a brain as large as that of a chimpanzee but with a body that was slighter, already had a larger brain-to-body ratio than any pongid then or since. Since the brain-to-body ratio is a crucial factor in what we call intelligence (provided the brain is reasonably large), the australopithecines were probably the most intelligent land animals existing in their time.
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|Publication:||Asimov's Chronology of Science & Discovery, Updated ed.|
|Article Type:||Reference Source|
|Date:||Jan 1, 1994|
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