Many people think science is about facts that can be proved, and that don’t change. But what is accepted as fact in science is changing all the time. The one characteristic of science is that it is not a resevoir of immutable truth, but an ongoing exploration that is never going to end.
An example of a fact – or at least a semi-fact that most paleontologists accepted as a working hypothesis – has just been changed with the discovery of a skull and a jaw bone in Kenya. Until now, most scientists thought that a species called Homo habilis living in Tanzania, Africa about three million years ago was the ancestor of a species called Homo erectus, who was the ancestor of Homo sapiens.
This theory of human origins is now in grave doubt because the recently discovered skull belongs to H. erectus, and the jaw bone, from a hominid living at the same time, belongs to H. habilis. It looks, as if the two Homo species lived at the same time, for about half a million years. So H. erectus probably did not evolve from H. habilis.
Most probably – but this is subject to change – they both have another common ancestor. For the time being, at least, H. habilis and H. erectus are thought to be sister species living at the same time for half a million years.