Three million years ago, there were primates which we now call australopithecines. The australopithecines had brains no bigger than chimpanzees, they were shorter than us, with very long arms, quite probably (though we do not know) they were covered in fur, but they were bipedal, as we are. We have even discovered their footprints and they look like human footprints.
We are now finding more and more australopithecine remains all over sub-Saharan Africa. They appear to have been one of nature's success stories, diversifying into at least two species, including a relatively heavyweight animal with very powerful jaw muscles, adapted to eating tough vegetation and a "gracile", more lightweight species. But various as they were, all the australopithecines differed from their primate predecessors in two ways. They walked on two legs; and their canine teeth became progressively smaller. Their bipedalism is an obvious point of difference from their ape predecessors; their smaller canine teeth are a less obvious difference, but a very significant one. Long, sharp canine teeth are weapons. Without those weapons, the australopithecines will have needed - must have had - other means of defence.
Animals must be prepared to defend themselves and (particularly) their young against predators. So nature provides them with weapons. Male mammals often have particularly well-developed weapons, which they also use in competing for females and territory; but females too are usually armed for defence against predators (see link). Most mammals which lack horns or antlers rely mainly on biting. They often have canine teeth specially adapted for use as weapons (this even applies to some deer which lack antlers - see link). Quadrupedal primates (including chimpanzees) typically have well-developed canines. But bipedal primates, including the australopithecines, our Homo predecessors and ourselves, do not have canine weapons. We bipeds scarcely ever defend ourselves with our teeth.
The reduction in the size of the canines would not have surprised Darwin:
The free use of the arms and hands, partly the cause and partly the result of man's erect position, appears to have led in an indirect manner to other modifications of structure. The early male forefathers of man were, as previously stated, probably furnished with great canine teeth; but as they gradually acquired the habit of using stones, clubs, or other weapons, for fighting with their enemies or rivals, they would use their jaws less and less.
(Darwin: The Descent of Man)
Was Darwin right? Chimpanzees do not rely entirely upon their teeth when fighting: they also use sticks and throw stones. There is a remarkable Youtube video clip (see link) which shows chimpanzees attacking an artificial leopard with sticks. Other sources also confirm that chimpanzees often throw sticks and stones, but tend to suggest that they are not very good at it. The forelimbs of a chimpanzee are general purpose organs, adapted for locomotion - and needed for locomotion whenever the animal wants to move quickly and nimbly. A habitual quadruped is likely to be slow and liable to lose its balance when fighting on the back legs alone. And chimpanzees appear always to throw underarm - a very weak and ineffective method compared with the human throwing action.
But the australopithecines, although related to chimpanzees, were bipeds, perhaps as quick on their two feet as an active modern man. They had hands and arms which (not being required for locomotion on the ground) could afford to specialise in manipulation - throwing and handling weapons and tools. They probably became better at throwing stones; they would have been better able to carry a supply of stones around with them, and they would have been better at fighting at close quarters with clubs. As bipeds, able to keep predators at a safe distance much of the time with missiles and clubs, they perhaps had less and less need to bite or threaten their enemies with their teeth. It was more useful for the canines to be adapted for eating.
Many other suggestions have been made as to the reasons for the evolutionary success of bipedal apes. But surely Darwin was right (see link): bipedalism made it possible for the australopithecines to "use stones, clubs and other weapons" instead of fighting with their teeth.
It is worth looking again at the Youtube video. Notice how the attacking chimpanzee loses his grip on the stick at the moment of impact and leaves it behind - partly perhaps because his grasp is not firm and flexible enough to withstand the shock of impact and partly because he wants to make use of all four limbs to ensure a rapid retreat. In fact he does not make a clear distinction between a missile and a hand-held weapon which is retained to repeat the blow.
There is no evidence that australopithecines made artificial stone weapons (though we do not know for certain that they didn't - like chimpanzees, they undoubtedly made tools from other materials (see link). But unworked stones, well chosen, make very good missiles anyway. The ability to throw stones (and to carry around with them a stock of suitable natural ammunition) must have given these bipeds a competitive advantage which no other animal had. They could drive away their enemies by inflicting injury at a distance whilst remaining out of range of hostile and predatory teeth and claws.
From the beginning, bipeds probably became expert at throwing stones. A shower of stones can kill any animal which cannot run away (people have been executed by stoning) and well-aimed sharp stones might well drive away a leopard or a lion. Against an animal which can run away, the value of stoning is primarily defensive; but in conflict with a predator, defensive action is all that is required. Predators are careful to avoid injuries because even a minor injury can diminish their chances of killing prey and can rapidly lead to starvation. Stones - or better still, sharpened stones - can be an effective defence, in the right hands, against predators.
The human throwing action is a complex one (see link), in which great power and accuracy are achieved by coordinating all the muscles of the arm, wrist, fingers and back - even of the legs. We do not know whether the australopithecines could throw missiles as well as we can. But if, as it seems, the ability to throw missiles effectively was a fundamental factor in bipedal survival, then the evolution of those muscles (and of the brain which controlled them) must have been influenced by the need to develop that ability. We bipeds are perhaps (among many other things) animals evolutionarily designed to survive by throwing missiles.
The evidence suggests that the long struggle for bipedal survival and ultimately human dominance over all other species began when the australopithecines differentiated themselves as animals which walked, ran and fought on their back legs only, and could use their hands and arms to wield weapons and to throw missiles, thus uniquely becoming able to drive away predators and competitors whilst avoiding combat at close quarters.
Animals in the wild have learned from hundreds of thousands of years of experience of us and of our bipedal predecessors that our missiles - first stones, later spears, arrows and bullets - can kill or maim quite unexpectedly from a distance without harm to ourselves. They have learned that even to be seen, at a distance, by a biped, can be dangerous or even fatal. It is best to keep out of our way, at least during daylight. That is why we can often walk through a wood which is full of wild animals of all kinds and yet hear and see almost nothing. Modern animals have learned the need to keep out of our way and have passed the knowledge on genetically or by example to their offspring. Those who failed to learn this essential lesson have left no descendants.
The australopithecines were certainly not people, by any reckoning. They were bipedal apes. But in one respect at least they were a step in the direction of people. Unlike other animals, people have learned to transcend the limitations of their own bodies. What we cannot achieve with our own bodily apparatus, we have learned to achieve with the aid of external tools, equipment and weapons. Without wings, we have nevertheless learned to fly. Without sharp teeth, horns or hooves, first the australopithecines and then people learned to defend ourselves with missiles and to keep our enemies at a distance. The stone missiles of those bipedal apes were the first step in the direction of the more sophisticated missiles of the twenty-first century. And the long story of pre-human and human development which started with those bipedal apes - a story in which we bipeds gradually became complete masters of our environment (if not of ourselves) - began with them.
Implications of Bipedalism for Females
Bipedalism must have created new problems for females which quadrupedal females had not faced. A quadrupedal mammal divides its weight between four feet and its abdomen is a bag which hangs from the spine, below the body, providing space for the digestive system, other internal organs and (in the case of females) for the womb and its contents. When the female is pregnant, the effect is simply to add additional weight and bulk to be carried in the abdominal bag. By contrast a bipedal mammal carries the abdomen in front of the body and it contains muscles necessary (among other things) to maintain balance. When the female is pregnant, the abdomen is in a more exposed position and the extra bulk and weight which it contains disturbs balance and makes the female less agile and more vulnerable.
Perhaps even more importantly, bipedalism would have affected brain development. The australopithecines had brains no bigger than chimpanzees. Later brain development was dependent on further modifications of another part of the bipedal body. The story of brain development is also the story of the development of the pelvis. When the australopithecines adopted the upright posture, the pelvis changed, to allow for the new centre of gravity of the body and for the necessary bone and muscle modifications required for walking vertically on the back legs alone.
But the pelvis is also a girdle through which the infant head must pass. The picture above, taken from the Cambridge Encyclopedia of Human Evolution (M H Day, P88), shows (left) the pelvis of a female chimpanzee, (centre) that of "Lucy" - a fossil australopithecine - and (right) that of a modern woman. In each case, the infant's head is shown emerging at the time of birth. It shows how relatively more difficult it was for the infant australopithecine's head to pass through his mother's pelvic girdle. The change in pelvis shape needed for upright walking, running and self-defence had made it difficult to give birth to a big-headed child. In the australopithecines, brain size was of the same order as in chimpanzees. Any increase in brain and therefore infant head size would have made births even more difficult.
Lucy's bigger-brained successors (of the new genus Homo) probably evolved two solutions to this problem. One was the solution the semi-bipedal marsupials had adopted, though to a much more extreme degree - for the infant to be born at an earlier and more helpless stage of foetal development, when the head was smaller. Jennifer Worth, in her autobiographical Call the Midwife, comments on this:
The helplessness of the newborn human infant has always made an impression on me. All other animals have a certain amount of autonomy at birth. Many animals , within an hour or two of birth, are up on their feet and running. Others, at the very least, can find the nipple and suck. But the human baby can't even do that. If the nipple or teat is not actually placed in the baby's mouth and sucking encouraged, the baby would die of starvation. I have a theory that all human babies are born prematurely. Given the human life span - three score years and ten - to be comparable with other animals of the similar longevity, human gestation should be about two years. But the human head is so large at two that no woman could deliver it. So our babies are born prematurely, in a state of utter helplessness.
Jennifer Worth: Call the Midwife
The other solution to the problem was a modification in the shape of the female pelvis, allowing for the need for a wider birth canal, but at the expense of bipedal speed and agility. This modification must have had implications for walking, running, fighting and climbing trees.
Reduced female speed and agility (particularly but not only during advanced pregnancy) must have meant a greater dependence on the protection of the male. In most other animals, females (whilst often not as aggressive as males) are well able to defend themselves an their young against predators. By contrast, human females have historically tended to depend on male protection at times of physical danger. Perhaps greater brain size may have been balanced by a progressive tendency for the infant to need more protection from the parents and the mother more from the father.
And unlike other animals, hominid mothers (like modern women) may now have had difficult births, for which they needed a midwife (see link). Thus they were also more dependent on the other more experienced females of their group.
Homo, Fire and Flint (Next section)
The Origin of Bipedalism (Previous section)