Steve Connor: The story of humans unravels

The story of human origins is a messy affair and it seems to get more complicated with every new discovery. It is now clear that the tidy idea of the first "out of Africa" migration about one million years ago is wrong – some of our human ancestors must have emerged from our ancestral homeland much earlier than that.

The amazing discoveries at Dmanisi in Georgia have now been firmly dated to 1.8 million years ago, almost twice as old as the postulated first migration of Homo erectus, a highly successful ancestral species found both in Africa and across Asia, and which spanned more than 1.5 million years of our evolutionary history.

One of the implications of the Dmanisi fossils is that there was a very early migration out of Africa of a group of human-like "hominins", who may have spent a long interlude in Eurasia before migrating back into Africa, where they contributed to the further evolution of the Homo genus – the family of man.

The "out of Africa" hypothesis has mutated into the "out of Africa again and again" hypothesis over the years. It was originally thought that there were two great movements, the first, of Homo erectus, some one million years ago, and the second, of our own species, Homo sapiens, about 100,000 years ago.

It was this latter migration that led to modern man colonising the globe and replacing any other ancestral species, such as Homo erectus or the Neanderthals, that may have already inhabited these far-off places.

It is now clear that there may well have been more than just two migratory movements, and that some of them could well have involved movements back into Africa.

The discussion about the migratory movements of our ancestral relatives has implications that go beyond telling the simple story of global colonisation. It could also tell us about how and when we evolved the traits that make us human – such as the large brain and bipedal gait.

The prevailing view had been that brain enlargement was connected with the movement out of Africa because that was when our ancestors started to be carnivores, therefore providing more fuel for the brain and allowing it to grow.

But the discovery of small-brained humans living in Georgia 1.8 million years ago also casts doubt on such a simple view of human evolution.