How Important is Temperature?

I was reading the Editorial of Climatic Change this morning, and am intruiged by the hypothesis that it was temperature which drove the encephalisation of hominins (as well as several other key evolutionary events like the emergence of phototrophs, eucaryotes and Metazoa). They note that lower temperatures encourage the evolution of larger brains by improving the efficiency of thermoregulation and removing constraints on the size of energy-intensive organs. During mammalian evolution, moreover, suitable temperatures (below the threshhold for brain growth but above the minimum required for survival) appear only during the last 500 million years, incidentally also the period in which encephalisation is obvious in hominins, birds and toothed mammals.

Apparently, this hypothesis - and variants which emphasise temperature variability rather than specific shifts - have been around for some years, but the current version differs in attempting to use temperature drops to explain specific saltations in hominin evolution rather than just the general increase observable over the past few million years. Specifically, they identify both geographic regions and temporal periods which might be expected to lift the thermal constraints on encephalisation and allow hominin brain expansion, including South Africa and the higher elevations in the Rift Valley at all times and glacial periods in the late Pliocene and early Pleistocene.

The most intruiging of these suggestions for me is that the emergence of Homo erectus, itself the first hominin of relatively "modern" appearance, might have coincided with a glacial period in which temperatures fell dramatically. The emergence of Homo erectus remains one of the least well-explained major events in hominin evolution, and although temperature change is a possibility, it would be interesting to see more hypotheses. That Homo habilis might also have emerged as a result of cooling is interesting, but less so, as that species is much less markedly different from earlier taxa.

I would like to see more evidence in support (or otherwise) of this argument, because it is fascinating to consider the possibility that modern human cognitive capacity emerged as a result of releasing factors which remove earlier constraints on encephalisation rather than directly in response to new selective pressures, potentially invalidating many extant hypotheses. Currently, the evidence seems to be based primarily on the correspondence between climatic and fossil records and some energetic modelling, which is interesting but remains inconclusive. I'm not sure how these hypotheses can be demonstrated to be correct, but their exploration should prove interesting on several fronts in palaeoanthropology and cognitive science.

I suppose this may mean that in the future advocates of reducing anthropogenic carbon dioxide can cite the risk of affecting the intelligence of future generations to support their arguments...

References:

Schwartzman, D., Middendorf, G. and Armour-Chelu, M. 2009. Was climate the prime releaser for encephalization? An editorial comment. Climatic Change volume 95, pages 439-447.