t all seems so simple on the evolution diagrams we are used to seeing.
The ape gradually stops dragging its arms along the ground, becomes upright and evolves into modern man - the homo sapien.
But the analysis of two fossils from Kenya is throwing all that into question.
Research published in the journal Nature today has thrown up a serious challenge to the widely accepted view on human evolution.
An international team of researchers, including a geologist from the Australian National University (ANU), has found that two different species of early man lived side by side in the same place for almost half a million years.
Susan Anton is an associate professor of anthropology at New York University and co-author of the research.
"The co-existence of the two species suggests that they were more like sister species, as opposed to homo habilus being the mother to homo erectus," she said.
Assumptions in doubt
ANU geologist Ian MacDougall was part of the research team that travelled to the Koobi Fora Formation in Kenya.
He says the work throws up a whole host of questions.
"It would suggest that the two species lived in the same area but must have different ecological niches, as it were," he said.
"They weren't seriously competing, otherwise one would have expected the one species to completely dominate, and in fact wipe out a less adaptive species.
"It really does throw into doubt a whole series of assumptions that have previously been made on the basis of the fossils."
Mr McDougall is cautious about rushing to conclusions that history needs to be re-written.
"I don't think it throws into question the linear model from [homo] erectus to [homo] sapien," he said.
"But who knows what will turn up tomorrow?
"It makes it just a little more complicated, and that's been the story of the discussions on prominent evolution and particularly these early fossils found in Africa.
"The evolutionary tree is much more bushy than was thought even 10 years ago."
Disagreement
Colin Groves, professor of biological anthropology at the ANU, has long argued that the linear model of evolution is far too simple. He welcomes this new work.
"The general public seems to have the idea that evolution sort of progresses onward and upward," he said.
"But for a long time it's become clearer and clearer that - just like any other species - human evolution consists of divergent species coming off and new species arising.
"The idea of human evolution as a ladder, I think, is impossible to maintain anymore."
There is some disagreement about the implications of the study.
The researchers say it indicates that homo erectus and homo habilis both descended from another species.
Other scientists say it merely shows that an ancestral being can survive for a long time alongside its descendant.
The debate continues.
