7. The Evolution of Hominids

7.2 The Australopithecines

Abundant remains of 5 different species of Australopithecines are found in Pliocene rocks all over East Africa, from Lesotho to Ethiopia. They are about 4.3 to 2.5 million years old. Remains of an Australopithecine species found at Hadar in Ethiopia are 3 million years old and was baptized A. afarensis (Lucy).

7.2.1 Australopithecus afarensis

It showed a severe sexual dimorphism, the females being much smaller than males. The animals walked erect and ran as well as modern man. They had brains as small as those of chimpanzees, i.e. about 350 ml, whereas human sapiens sapiens has a liter more, 1,350 ml. The circumvolutions of the brain of these animals were those of a monkey; they probably could not talk. Their teeth were as developed as those of chimps. The females weighed about 25 kg and measured 1.0 meter. The males were bigger. They looked like bipedal chimps and were found all over Africa. No sign of the use of tools could be detected in association with the early Australopithecines. Choppers and flakes (fig. 7.2) are first detected between 2.5 to 2.7 million years and they remained in use, with improvements, at least two million years. However, primitive large bifaces appeared about 1.5 million years ago.

One may conjecture that the erect posture obtained by the Australopithecines provoked a potentially unfavorable obliquity of the pelvis and its orifices. The delivery of babies would then be difficult if these babies were big in regard to the orifices of the basin. It is possible to estimate the birth-weight of the Australopithecines. Assuming a maternal body weight of 22.5 kilos, the newborn Australopithecus would weigh between 1.1 and 1.7 kilos. If the size of the head relative to that of the body of such a new-born compares with the size of the head relative to the body of new-born chimps, then the head of the Australopithecine new-born was smaller than the principal diameters of the pelvic inlet of the mother. This would still have been true if the size of the head had been that of a baby gorilla.

The conclusion that newborn Australopithecines were small in relation to the pelvic inlet suggests that delivery was quick and easy. This was a selective advantage. There were probably no selective forces whatsoever in the early stages of hominid evolution pressuring towards an enlargement of the pelvic canal to ensure easier parturition. In fact, the selective pressures towards effective bipedalism must have been much stronger. This would support the hypothesis that Australopithecus was as well, if not better, adapted to bipedal locomotion than is modern sapiens.

Afbeeldingen17

Figure 7.2. Chopping tools. The chopper and pick are made from pebbles flaked on one side only. The little more sophisticated biface was made by detaching flakes from one end of a cobble but no trouble was taken to work out the “heel”.

The human sapiens foot is fairly small and short for the length of the leg. It denotes an adaptation to erect walking rather than erect posture. For a human, it is less tiring to walk than to stay erect. Prehominians show a distinctive evolution towards the ability to walk in an erect posture long distances in the heat. Whereas most tropical animals, including the carnivores, rest during the heat of the day and are active at dawn and sunset only, the bipedal hunter can sweat the heat off like a horse, continue to walk, harass its prey and exhaust it. An herbivore needs lots of food of a poor nutritive value and needs time to digest it. After the chase of a big herbivore during a day or two, the animal is in such a poor condition that the killing becomes easy. The evolution during the Tertiary era is thus decidedly focused on adaptation to the environment, not on evolving a larger brain, even if a larger brain was the ultimate result of this evolution.

The vocal canal of the Australopithecines is practically identical to that of the chimpanzee. The Australopithecines were thus unable to pronounce the sounds (a), (i) and (u), nor (g) and (k). This must have drastically reduced their ability to communicate through sound alone and, like chimpanzees, they probably supplemented it with gestures. This means that no fully satisfactory communication was possible when the interlocutors were out of sight.

A quadruped’s breathing apparatus makes it impossible to talk and run at the same time. When chimps run, they have to take a breath for every step, which makes it impossible to develop the respiratory control necessary for speech. The acquisition of the erect posture by the Australopithecines allowed the redirection of breathing in the service of sound making. Bipedality was the key event in human evolution necessary for the emergence of speech. The brain structures that regulate bipedal locomotion in humans also regulate speech, indicating that bipedal locomotion was the initial selective pressure for the elaboration of brain structures that are essential for speech and syntax.

The first exploits of Australopithecus must have been to steal their prey away from carnivores; it was a scavenger ready to run away if need be. Gradually its size and skills improved, so as to become a hunter and a self-assertive being itself. This was achieved with Paranthropus. Until 1.9 million years ago, there lived also a small species of Australopithecus africanus.

7.2.2 Australopithecus africanus

It has long been thought that A. africanus was a forest-dweller, at home in trees. A fine examination of his teeth shows that he ate not only fruits and leaves, available in the forest, but also grasses and perhaps animals that ate grasses9. The controversy is open: did forest-dwelling hominids roam woodlands and grasslands for food? Did they consume animal foods before the development of stone tools and the origin of the genus Homo? Their teeth may very well have developed for the consumption of nuts. Chimps use big tree branches to open nuts, and the mothers teach their children how to do it. Nuts have been found to be a staple food for hominids and the teeth and jaws of A. africanus are sturdy enough to crack nuts open without the need of a wooden branch. Nuts need not be consumed immediately: they may be stored and some human populations, e.g. the Corsicans, consumed chestnuts as their main staple diet until about 60 years ago. Nuts contain taxofene, which is an anti-cancer drug, and are extremely rich in fatty acids. There is no need to postulate the consumption of meat and fish by early hominids to account for the development of their brains, which is demanding of fatty acids.

7.2.3 Paranthropus

Paranthropus was an evolutive line that showed a distinct masticatory specialization. He was as big and tall as modern man. He had enormous teeth and his brain was a little more voluminous i.e. about 600 ml. but this may have been due solely to his bigger stature. His diet was restricted to nuts, grains and plants of all sorts, small animals and insects. He apparently used no fire, which makes it difficult to recognize tools that he may have used, if not made. Lack of fire likewise restricted his wanderings to the warmest parts of the planet. He did well there, however, since he could be found all over the Old World, until about 1,750,000 years ago.

References

9. Sponheimer and Lee-Thorp: isotopic evidence for the diet of an early hominid, A. africanus. Science 283: 368-370, 1999

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