What made us human is one of the great questions. "Climate" provides an interesting possible answer, and there's new evidence to bolster the case

A new paper in Science last week(1) identifies strong climatic variability in eastern Africa during the time hominid evolution was lurching toward humanity.

In the long run, eastern Africa was getting drier. But superimposed on that trend, according to the research by Martin Trauth and his colleagues, were a series of pronounced wet spells. Trauth et al. looked at lake sediments, and were able to identify "humid periods" - periods of on order 100,000 years when the lakes got big.

The idea here - it's called the "variability hypothesis" - is that varying climatic conditions favored more adaptable proto-humans, clever enough perhaps to survive under changing environmental conditions.

More at Scientific American and the BBC. Also, Peter deMenocal, a climate researcher at Lamont Doherty, had a nice review last year in Earth and Planetary Science Letters.

1. M. H. Trauth, M. A. Maslin, A. Deino, M. R. Strecker, Science, 1112964 (August 18, 2005, 2005).