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 emphasized 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 is being reported today in the journal Nature. The lead author is Fred Spoor, an evolutionary anatomist at University College London. Other authors include Meave G. Leakey and her daughter Louise Leakey, the Kenyan paleontologists who are co-directors of the Koobi Fora Research Project that made the discovery. The fieldwork was supported by the National Geographic Society.

The fossils were found east of Lake Turkana in Kenya in 2000. It took years to prepare the specimens, encased in hardened sediment, for study and to be sure of the identification of the species, the scientists said. University of Utah geologists determined the dates of the fossils from volcanic ash deposits.

The most recent fossils of the habilis species known before now were 1.65 million years old or older. Some fragments of fossils with apparent habilis attributes have been dated as early as 2.33 million years old.

In recent years, scientists not involved in the project said, discoveries were hinting at possible overlap between the habilis and erectus species. But the implications were considered so profound that little was said about these dates, pending more conclusive evidence.