Detailed studies on two fossils from Kenya suggest that two of our ancient human relatives lived side-by-side, 1.5 million years ago, rather than evolving one after the other.
The fossils were found seven years ago near Lake Turkana in northern Kenya, and now Fred Spoor, from University College London, and his colleagues have studied them in detail.
They both belong to the genus, or group, Homo, the same group modern humans, Homo sapiens, belongs to. One belongs to the species erectus and one to the species habilis .
The first specimen is a well-preserved skull and dating techniques reveal it to be about 1.55 million years old. It is a similar size to the smaller Homo habilis, but shows most of the hallmarks of being Homo erectus.
The second specimen consists of fragments of upper jaw and is dated to 100,000 years later at 1.44 million years old. It has the dental characteristics of H. habilis.
'The two fossils confirm something that had long been suspected,' said Chris Stringer, human origins expert at the Natural History Museum. 'The primitive small-brained species H. habilis (known from East Africa) and the larger-brained and larger-bodied H. erectus (known from various regions of Africa and Asia) overlapped significantly in time, and did not have a simple ancestor-descendant relationship.'
The research suggests H. erectus and H. habilis lived side-by-side in eastern Africa for almost half a million years.
Some scientists have argued that H. habilis evolved gradually into H. erectus but their co-existence makes this unlikely.
'It remains possible that there was evolution from habilis to erectus,' says Chris, 'but as erectus fossils are known from at least 1.75 million years ago, the transition must have been earlier still, and some populations of habilis must have persisted alongside the descendant species Homo erectus for many hundreds of thousands of years.'
The research implies that H. erectus and H. habilis probably had different adaptations and lifestyles. 'Both were apparently stone toolmakers,' adds Chris. 'But one possibility is that the larger and perhaps more mobile erectus species was an active hunter, while habilis scavenged or caught small prey.'
Scientists have debated about the differences between male and female bodies, known as sexual dimorphism, in the species H. erectus .
Some argued H. erectus had similar sexual dimorphism to modern humans, Homo sapiens , where the males on average were only slightly larger than females.
The H. erectus fossil in this new research is the smallest ever found, suggesting this species was not as human-like as once thought.
'The new skull is from an adult, or near adult,' says Chris, 'and yet is much smaller and less strongly reinforced than other similarly-dated erectus skulls. This implies that the species had a high level of dimorphism, more like that of gorillas.'
'In turn, this suggests that male and female erectus may have had very different patterns of behaviour, or that erectus groups had socially dominant male individuals, equivalent to the silverback males found in gorillas.'
This research is published in the journal Nature