BBC NEWS | Science/Nature | Finds test human origins = theory

The broken upper jaw-bone and intact skull from humanlike creatures, = or=20 hominids, are described in Nature.=20

Previously, the hominid Homo habilis was thought to have = evolved into=20 the more advanced Homo erectus , which evolved into us.=20

Now, habilis and erectus are now thought to be sister = species=20 that overlapped in time.=20

The new fossil evidence reveals an overlap of about 500,000 years = during=20 which Homo habilis and Homo erectus must have co-existed = in the=20 Turkana basin area, the region of East Africa where the fossils were = unearthed.=20

"Their co-existence makes it unlikely that Homo erectus evolved from = Homo=20 habilis," said co-author Professor Meave Leakey, palaeontologist and = co-director=20 of the Koobi Fora Research Project.=20

The jaw bone was attributed to Homo habilis because of its = distinctive=20 primitive dental characteristics, and was dated to around 1.44 million = years=20 ago.=20

It is the youngest specimen of this species ever found.=20

The skull was assigned to the species Homo erectus despite = being a=20 similar size to that of a habilis skull. Most other erectus = skulls=20 found have been considerably larger.=20

But it displayed typical features of erectus such as a gentle = ridge=20 called a "keel" running over the top of the jaw joint. Analysis showed = the skull=20 to be about 1.55 million years old.=20

The new dates indicate that the two species must have lived side by = side.=20

Sister species

If Homo erectus had evolved from habilis and stayed = within the=20 same location then both must have been in direct competition for the = same=20 resources.=20

Eventually, one would have out-competed the other.=20

"The fact that they stayed separate as individual = species for a=20 long time suggests that they had their own distinct ecological niches, = thus=20 avoiding direct competition," Professor Leakey explained.=20

Professor Chris Stringer, head of human origins at London's Natural = History=20 Museum, said: "Both were apparently stone tool-makers, but one = possibility is=20 that the larger and perhaps more mobile erectus species was an = active=20 hunter, while habilis scavenged or caught small prey."=20

It is most likely that both species evolved from a common ancestor.=20

Other possibilities

But the linear, ancestor-descendent relationship between the two = species=20 cannot be ruled out altogether.=20

Fred Spoor, professor of developmental biology at University College = London,=20 and co-author of the paper, told the BBC News website: "It's always = possible=20 that Homo habilis lived, let's say, 2.5 million years ago and = then in=20 another part of Africa, away from the Turkana basin, an isolated = population=20 evolved into Homo erectus ."=20

After a sufficient amount of time to allow both species to develop = different=20 adaptations and lifestyles, Homo erectus could have then found = its way to=20 the Turkana basin.=20

With separate "ecological niches", both species could co-exist = without direct=20 competition for resources.=20

"But that is a much more complex proposition," Professor Spoor = explained,=20 "the easiest way to interpret these fossils is that there was an = ancestral=20 species that gave rise to both of them somewhere between two and three = million=20 years ago."=20

Not so similar

The fossil record indicates that modern humans ( Homo sapiens = )=20 evolved from Homo erectus .=20

However, to some researchers, the small size of the erectus = skull=20 suggests that species may not have been as similar to us as we once = thought.=20

On average, modern humans display a low level of "sexual dimorphism", = meaning=20 that males are females do not differ physically as much as they do in = other=20 animals.=20

The scientists compared the small skull to a much larger erectus=20 cranium found previously in Tanzania. If the size difference between = the two=20 is indicative of the larger one being from a male and the smaller being = from a=20 female, it suggests that erectus displayed a high level of sexual = dimorphism - similar to that of modern gorillas.=20

Sexual dimorphism can relate to reproductive strategies and sexual = selection.=20

If erectus was very sexually dimorphic it may have had = multiple mates=20 at a time. This differs from the more monogamous nature of modern = humans,=20 indicating that Homo erectus was not as human-like as once = thought.=20

The researchers dismiss the idea that the small size of the skull = could be a=20 result of it belonging to a youngster.=20

"By studying how the skull bones are fused together we discovered it = belonged=20 to a fully grown young adult rather than a developing juvenile erectus," = said=20 Professor Spoor.