Discovering Our Distant Ancestors
About 6.5 million to 5 million years ago, some 100,000 apes, the ancestors of today’s apes and humans, lived in East Africa in a forest shrinking due to climate cooling. There were two kinds of survivors of this climate change: one stayed with what was left of the forest and continued as before and it led to the chimpanzee and bonobo; the other was the ancestor of the human line, they survived by occupying both the trees and the new spaces in between. They were able to live in both terrains because of their critical new ability: to walk upright on two feet.
Called australopithecines, these first walking apes, appeared about 4.4 million years ago. A haunting trace of their presence was found in a trail of footprints they left about 3.5 million years ago at Laetoli in Tanzania. Two individuals walked in the ash from a nearby volcano, leaving behind their human-looking footprints in tracks that extend for 165 feet.
They retained the long arms of the chimpanzee with curved fingers that could move in the trees, and had shorter legs than we do, but their brains were larger than the chimp brain if only slightly, about 400 to 500 cubic centimeters. As with the apes, the sexes were very different in size, the males were about 50% larger than the females. The size difference suggests that australopithecine society was like that of chimpanzees, with strong rivalry between males for females and a separate male and female hierarchy.
The most famous Australopithecus is the 3.5 foot tall skeleton named “Lucy,” discovered in 1974 by paleontologist Donald C. Johanson in Hadar, Ethiopia. She is an Australopithecus Afarensis and lived in Eastern Africa between 3.85 and 2.95 million years ago.
Then in 2000 the skeletal remains of a 3-year-old young female of the same species was found in Dikika, not far from Lucy’s site. Named Selam (Peace) by Zeresenay Alemseged and his team, she was able to walk upright and climb trees. An analysis of her hyoid bone or voice box showed that she would have been unable to talk and most likely made sounds like a chimp.
Most interestingly, unlike a chimp, her brain was still growing at the time of her death. This means that as early as 3.3 million years ago our ancestors lived in groups with some form of complex social organization – adults would have had to create the bonds necessary to take care of their immature young over much longer periods than at any other time in history.
From 3 to 2 million years ago another cool climate change shrunk the forests again, and many species living in them went extinct. But by this time australopithecines were already adapted to living in more open terrain and evolved different solutions for survival. The robust australopithecines, for example, evolved larger cheek teeth for eating coarse leaves.