Two fossils found in Kenya have shaken the human family tree, possibly rearranging major branches thought to be in a straight ancestral line to Homo sapiens.

Scientists who dated and analysed the specimens — a 1.44 million-year-old Homo habilis and a 1.55 million-year old Homo erectus — said their findings challenged the conventional view that these species evolved one after the other. Instead, they apparently lived side by side in eastern Africa for almost half a million years.

If this interpretation is correct, the early evolution of the genus Homo is left even more shrouded in mystery than before. It means that both habilis and erectus must have originated from a common ancestor between 2 million and 3 million years ago, a time when fossil hunters had drawn a virtual blank.

Although the findings do not change the relationship of Homo erectus as a direct ancestor of Homo sapiens, scientists said, the surprisingly diminutive erectus skull implies that this species was not as humanlike as once thought.

Other paleontologists and experts in human evolution said the discovery strongly suggested that the early transition from more apelike to more humanlike ancestors was still poorly understood. They also said that this emphasised the need to search more widely for fossils from the critical period at the still unknown dawn of our own genus, Homo.

The challenge to the idea of a more linear succession of the three Homo species was reported on Wednesday in the journal Nature. The lead author is Fred Spoor, an evolutionary anatomist at University College London.

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