THE discovery of two fossils has challenged the belief our ancestor homo erectus evolved from homo habilis, according to an article in the British magazine Nature.
The finds, on the eastern bank of Lake Turkana in Kenya, suggested the species may have co-existed for some 500,000 years in East Africa.
The team that found the remains was led by mother-daughter team Mary and Meave Leakey of the famed Kenyan anthropological family who have uncovered a host of critical human and hominid remains in east Africa.
One of the fossils was an upper jaw bone of homo habilis that dated back 1.44 million years.
It was nevertheless more recent than any of the previously found fossils of its kind.
The second was a remarkably well preserved skull of homo erectus, which, paradoxically dates back even further, to some 1.55 million years ago.
"What is truly striking about this fossil is its size," said Fred Spoor of London's University College, one of the paper's lead authors. "It's the smallest homo erectus found anywhere in the world."
This suggests male and female skulls were different sizes -- challenging current thinking.
The discoveries have created a stir among academics tracing humankind's roots, because it challenges the presumed evolutionary timeline of the species: homo habilis to homo erectus to homo sapiens.
"Their co-existence makes it unlikely that homo erectus evolved from homo habilis," said Meave Leakey, one of the lead authors of the paper.
"The fact that they stayed separate as individual species for a long time suggests that they had their own ecological niche, thus avoiding direct competition."
Homo habilis is thought to have lived from about 2.5 million to 1.8 million years ago.
Homo erectus is important because it is believed to be the first hominid to leave Africa.