The Origin of Bipedalism
Somehow, by events and processes which were probably similar to those described, a species of primates appeared which (unlike all other placental mammals) was bipedal. This animal walked and ran on the back legs alone, leaving the forelimbs free for other purposes. How did the process of speciation happen? Surely at a half-way stage, the semi-bipedal animal would have been too vulnerable to predators? We must assume that unusual circumstances conspired to make the new species possible.
We know that three million years ago, in southern Africa, there were bipedal primates. We do not know how they got there. The best we can do is to fill in the gap in our past with imagination, allowing our imagination to be guided by what we know of the process of speciation, what we know of the new bipedal primates and what we know of the stock from which they apparently emerged. Once upon a time, then..
Once upon a time, a group of apes made their home on a peninsula along the coast of Africa. They were related to present-day chimpanzees and probably lived a rather similar life. Most of the time, they gathered food and ate it as they gathered - fruit was their favourite, but they ate shoots and young leaves, insects, grubs and small mammals and even birds when they could catch them. Since this particular group of apes lived on the coast, they varied their diet with shellfish which they found on the rocks. They were clever toolmakers and users and they had worked out how to prise open shells with a sharp stone. They spent much of the daylight time searching for food on the ground or the seashore. But like all apes they were superb climbers; each night, they retired to the comparative safety of a convenient and suitable tree. If the leopard climbed their tree, they might not see him, but the tree would move and they would awake and be ready to defend their young. They were fierce fighters with terrible canine teeth, not easy opponents even for a great cat.
Their lives had been much the same for millions of years and they might have continued the same for many more millions. But one night there was a great earthquake, followed by a tsunami. Many of them were drowned, but the survivors picked up the thread of their lives and their numbers grew again. As male chimpanzees do still, the young ape warriors patrolled the boundaries of the group territory, preventing apes from other groups from trespassing - and perhaps seizing the opportunity to push their own frontier a little further inland.
Except that after the earthquake, they became aware that the landward territorial boundary had disappeared. They were on an island. If their numbers grew too great, there was no room for expansion inland. And their numbers did grow too great. As it happened, there had been few serious predators on the peninsula when it became an island, and now those of the mainland could no longer come over. More young apes survived, food became short. There were still plenty of shellfish on the rocks, but most of them were on inaccessible rocks or under water even at low tide. The apes were not well adapted to swimming and generally avoided getting wet if they could. But hunger drove them into the water willy-nilly; increasingly, they learned to swim out to offshore rocks and to gather shellfish standing upright in shallow water. They were natural quadrupeds, normally seldom standing long on their back legs alone, but they found that water up to their chests or waists supported them and it became easier. Their forelimbs, less needed now much of the time for locomotion, changed and their hands became more skilful as they learned how to prise off and open difficult shells.
With these adaptations, the food supply improved, but their numbers still increased and the tribe split into two and later three groups; each group, led by a dominant male, occupying an area of the coast and defending its resources from members of the other groups. Hand-to-hand fighting in the water was difficult, but they found a better way. Like chimps, they had always known how to wield natural clubs and throw stones; but like chimps, they had never really been very good at it. Standing on their back legs in the water, they could now repel trespassers from a distance by throwing pebbles at them. If the enemy came closer, he could be attacked with a heavy stick and prevented from getting close enough to use his teeth. Gradually they learned how to keep upright even in shallower water and finally even on land, leaving their hands and arms free to fight and carry their young, their weapons and whatever else they needed. And necessity gradually made them much better throwers.
Perhaps a million years passed. Separated from the members of their species on mainland Africa, the ape population of the island had adapted to the new circumstances of the island and changed. They had become a new species, very similar in many ways to their continental relatives, but mostly bipedal, even on land. Their feet had become less able to grip branches but their arms were still longer and stronger than ours and they were still excellent climbers. They were the dominant species on their island; but of course island environments were always unusual. Mostly, unusual animals developing on islands failed to survive if they ever became exposed to the predators and competition of the mainland.
Sea levels fell and the island became reconnected. Suddenly, there were lions again, as well as sabretooths, leopards, big snakes and all the wild carnivorous life of pre-human Africa. But somehow, the new apes survived. Predators like an easy victory. They need to keep at the peak of fitness to get their food; any animal which is likely to cause them serious injuries will be avoided. An animal which greets the predator's appearance from a distance with a volley of sharp stones is not easy prey. The skill at throwing and the use of natural or roughly fashioned clubs which the apes had learned in conflict with the members of their own species turned out to be even more useful when their enemies were quadrupeds who could only attack with teeth and claws at close range. The new apes not only survived - they flourished on mainland Africa; they increased in numbers, spreading to cover large areas of the continent. Even now, millions of years later, their bones establish their distant identity. We call them australopithecines.
The origin of the australopithecines is not really known; this is an imaginative reconstruction. But perhaps something like that happened. Modern evolutionary theory suggests that new species - and particularly, perhaps, species which are really new and different - typically arise first in small populations in isolated places; often on islands. So perhaps it was with the australopithecines.
The Australopithecines (next section)
Speciation: the Origin of Species (previous section)