"(BBC Sept. 14, 2012) What explains the extraordinarily fast rate of evolution in the human lineage over the past two million years?
A leading human origins researcher has come up with an idea that involves aggression between groups and the boom-bust cycles that have punctuated our spread into new environments.
Prof Ian Tattersall said there were few examples to rival the accelerated evolution that led to our species.
He was speaking at this year's Calpe conference in Gibraltar.
"However you slice it, evolution within this [human family] has been very rapid indeed," Prof Tattersall, from the American Museum of Natural History (AMNH) in New York, told the conference.
"I think it's fair to say that our species Homo sapiens and its antecedents have come much farther, much faster than any other mammalian group that has been documented in this very tight time-frame."
This phenomenon of accelerated evolution is known as "tachytely".
Among our ancestors, brain size doubled between two million and one million years ago. Then it has almost doubled again between one million years and the present day.
Along with the increase in brain size came a reduction in the size of the teeth and face along with other changes in the skull.
The increase in brain size seems to have coincided with a modern physique characterised by a linear shape, long legs and relatively narrow hips. These features can already be seen in the skeleton of the "Turkana boy" from Kenya, who lived about two million years ago.
This contrasts sharply with the short legs and long arms of the Turkana boy's antecedent "Lucy" (Australopithecus afarensis), who lived in Ethiopia about one million years earlier.
Such fast change is not seen among apes, and while Prof Tattersall acknowledges the importance of the move our ancestors made from a tree-dwelling, to a ground-dwelling existence - something which has not affected our primate cousins - he says it is not enough to explain what is observed.
"Clearly the definitive abandonment of dependence on trees... has to count as one of the most radical shifts in adaptive zone ever made by any vertebrate since the very first tetrapod heaved itself out of water and on to terra firma," he said..."