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.

The death of the "Savannah Hypothesis"?

There are many competing ideas about what caused our lineage to diverge from that leading to our closest relatives, the chimpanzees and bonobos, but the savannah hypothesis is perhaps still the best-known. It was proposed for the first time in 1925, when Raymond Dart wrote that:

"For the production of man a different apprenticeship was needed to sharpen the wits and quicken the higher manifestations of intellect - a more open veldt country where competition was keener between swiftness and stealth, and where adroitness of thinking and movement played a preponderating role in the preservation of the species."

At the time, the first australopithecine (a member of Australopithecus africanus), had just been discovered in South Africa but almost no other record of human evolution existed; Dart's ideas about the environments associated with the emergence of humans were some of the first to be articulated. Since then, although the savannah hypothesis has not disappeared from palaeoanthropology, it has been challenged by a variety of other ideas. These new ideas increasingly emphasise the importance of increased environmental variability, rather than simple shifts in external conditions like a change from wooded to savannah environments.

In addition, recent discoveries of the earliest hominins have suggested that savannah environments were not important to our evolution after all. Ardipithecus ramidus, for example, the most complete early hominin known to science (and one of the biggest news stories of 2009 when it was published in a special edition of Science) is fairly conclusively associated with woodland and forest patches. Sahelanthropus tchadensis, in contrast, is found in gallery forest near - but not inside - an area of savannah, but whether this environment represented the habitat in which Sahelanthropus lived or just that in which it died is not clear.

In light of these recent finds and changing theoretical perspectives, the savannah hypothesis seems to be persisting primarily through disciplinary inertia. So, I would like to know: does the savannah model still, implicitly or otherwise, inspire palaeoanthropological research, or do we retain the hypothesis in our papers as a mark of respect for the ideas of past masters?

References:

Dart, R. 1925. Australopithecus africanus: the man-ape of South Africa. Nature volume 115, pages 195-199.

White, T. D., Asfaw, B., Beyene, Y., Haile-Selassie, Y., Lovejoy, C. O., Suwa, G. & WoldeGabriel, G. 2009. Ardipithecus ramidus and the paleobiology of early hominids. Science, volume 326, pages 64-86.

Vignaud, P., Duringer, P., Mackaye, H. T., Likius, A., Blondel, C., Boisserie, J.-R., de Bonis, L., Eisenmann, V., Etienne, M.-E., Geraads, D., Guy, F., Lehmann, T., Lihoreau, F., Lopez-Martinez, N., Mourer-Chauvire, C., Otero, O., Rage, J.-C., Schuster, M., Viriot, L., Zazzo, A. & Brunet, M. 2002. Geology and palaeontology of the Upper Miocene Toros-Menalla hominid locality, Chad. Nature, volume 418, pages 152-155.

About me and my research

Hello! My name is Isabelle Winder, and I am a PhD student at the University of York, in the department of Archaeology. I am originally from Cambridge, and have a BSc in physical geography and an MSc in palaeoanthropology, both from the University of Sheffield. I find both subjects fascinating, and have chosen to continue to a research career in order to continue learning about them. This blog is my way of sharing those interests with any interested parties; it will contain comments on the big stories (new and old), reviews of articles and books that I read, and my thoughts about new finds and theories.

I am currently working on a project that develops a new approach to the "Neanderthal Problem" through a holistic analysis of Neanderthal and modern human morphological differentiation, niche overlap and landscape exploitation. This project is supervised by Prof. Geoff Bailey, who recently won the Antiquity prize for a joint paper on the importance of dynamic landscapes to hominin evolution. More generally, though, I am interested in hominin evolution and ecology, landscape reconstruction and exploitation by various species, human and comparative anatomy and primate evolution and adaptation, as well as physical geography and earth science.

I am a student member of the Paleoanthropology society and the Primate Society of Great Britain.