(Left to right: Gibbon, Human, Chimpanzee, Gorilla, Orangutan.) Five members of the primate family: Homo sapiens and four kinds of apes. The similar anatomy of human and ape skeletons had been recognized for centuries, but DNA studies confirmed that chimpanzees are our closest relatives, and we are theirs.
WHEN, WHY, AND HOW DID WE STOP BEING JUST another species of big mammal? Clues come from three types of evidence, all explored in the next two chapters. Fossil bones and preserved tools are traditional evidence from archaeology, the study of the past through physical remains. A newer kind of evidence comes from the science of molecular biology, which examines our genetic heritage and traces our descent from an apelike ancestor.
One basic question concerns the differences between us and chimpanzees. Just looking at humans and chimps and counting visible differences doesn’t help, because many genetic changes have effects that can’t be seen, while other changes have very obvious effects. A Great Dane and a Chihuahua look much more different from each other than a chimp and a human being do. Yet all dogs belong to the same species, but chimps and humans are different species.
So how can we tell our genetic distance from chimps? The problem has been solved by molecular biologists. They have discovered that the gene difference between us and chimps is greater than the difference between any two living human populations or any two breeds of dogs. But the gene difference between us and chimps is small compared with differences between many other pairs of related species. This means that only a small change in the chimpanzee genes led to enormous changes in humans’ behavior.
Next we’ll consider what we can learn from the bones and tools left by creatures along the way between our apelike ancestor and modern humans. Fossil bones show the switch from our walking on all fours to walking upright, and our increase in brain size. Our large brain was surely necessary for the development of human language and inventiveness. In fact, we might expect the fossil record to show our tools getting better as our brains got bigger. But the greatest surprise and puzzle of human evolution is that stone tools remained very crude for hundreds of thousands of years after our brains had expanded almost to their present size.
Sixty thousand years ago, Neanderthals had brains even larger than those of modern humans, yet their tools show no signs of inventiveness or art. Neanderthals were still just another species of big mammal. Even for tens of thousands of years after some other human populations had evolved skeletons like those of modern people, their tools remained as boring as Neanderthals’ tools.
Within the small percentage of difference between our genes and chimpanzee genes, there must have been an even smaller percentage that was not involved in the shapes of our bones but that gave us the human traits of inventiveness, artistic creativity, and the use of complex tools. In Europe, at least, those traits appeared suddenly at the time Neanderthals were replaced by the early modern humans known as Cro-Magnons. That’s when we finally stopped being just another species of big mammal. At the end of part 1, I’ll talk about what triggered our steep rise to human status.
