"There are no simple linear patterns, so I'd be reluctant to draw a line anywhere.And anyway, if you do that, how many subspecies are you going to end up with?
Using an improved dating method based on the rate of decay of radioactive argon, the researchers put the age of rock just below the fossils at 196,000 years.
The rock layers were formed in rapid bursts, corresponding to wet periods during which huge amounts of organic matter were dumped in the region by the overflowing River Nile, Fleagle says.
Such a move is unnecessary for the Omo specimens, Fleagle says.
Omo I has always been viewed as thoroughly modern in appearance.
And although Omo II, which consists of just a skull with no face, has more primitive features, Fleagle maintains that it is still best assigned to , particularly as both skeletons are now thought to be the same age.
"The only interpretation is that there was a lot of diversity at that time," Fleagle reflects.
And when the researchers, led by Ian Mc Dougall of the Australian National University in Canberra, attempted to visit Kibish on their latest expedition, it was far from plain sailing. We spent days and weeks waiting just to get a boat to go there," recalls Fleagle.
When they finally made it, Mc Dougall's team collected samples of the rock where the Omo fossils were found.
These were unveiled in 2003 and hailed at the time as the oldest humans (see ""Skulls reveal dawn of mankind":
The Herto hominids were christened as a new subspecies, , meaning 'elder'.