"If the dates are true, they are very interesting and tantalizing," said Leslie Aiello, who teaches human evolution at University College London.
The, cranium and femur were found near Kenya's Lake Rudolf, a rich source of hominid fossils. By use of gamma-ray spectrometry, Brauer's group dated them to 270,000 years old for the skull and 300,000 for the femur.
This fitted in with the age of the deposits they were found in, the scientists said. But Chris Stringer of London's Natural History Museum said the gamma-ray technique is still experimental.
The skull is fairly complete, with all the teeth in the upper jaw. "Our observations indicate that the hominid might represent an archaic Homo sapiens or a transitional specimen very closely related to modern humans," Brauer's group wrote.
Stringer said it would have stood like a human, but with primitive features. "They don't look exactly like us," he said.
The brow ridge is still very strong.... It would have a big face and flat forehead," he added. "They'd still stand out as being pretty strongly built by modern standards."
It had been generally accepted that modern humans, Homo sapiens sapiens, emerged about 40,000 years ago. Archaic Homo sapiens, more modern than Neanderthals but not quite like us, were believed to have emerged 90,000 years ago.
Neanderthals, or Homo neanderthalensis, lived anywhere between 125,000 and 35,000 years ago, although this is being debated. Before Neanderthals came Homo erectus, which lived between 1.8 million and 100,000 years ago.
All this is hotly controversial, and Brauer's group proposes moving the era of origin of early archaic humans back to 500,000 to 700,000 years ago.
"Early and late archaic Homo sapiens and also the earliest modern humans seem to have existed considerably earlier than has been assumed," they wrote.
Last month German scientists said they had found perfectly preserved 400,000-year-old wooden spears in Europe, much older than any such tools found before. It had previously been assumed that humans this old scavenged and were incapable of the organized hunting that such a find suggested.
They said Homo erectus probably made the spears, but Aiello said something like Brauer's early man may have. "I think it would have to be something like this that threw the spears," she said.