Penn State Professor of Anthropology Nina Jablonski was unavailable to lecture herself (a colleague stepped in to read through her presentation), so her review of hominid extinction was reiterative and fit nicely into the series on extinction (AAAS Symposion, Extinction I and II).
Jablonski argument could be summed up in a word: Flexibility. Human behavior driven by a big brain and great mobility has brought us to where we are today, though this very expansion of our niche might cause our extinction, wrote Jablonski.
Like our ape-like ancestors, we are long lived and complex, maturing to reproductive age slowly and nurturing our young with similar patience. Jablonski called the summation of these factors our "slow life history".
Slow life histories began to develop in our ancestors about 10 to 20 million years ago in the consistent warmth of tropical forests, where fruits and nuts could be gathered, providing the energy rich sugars and fatty acids needed to build bigger and bigger brains. Bigger brains meant more flexibility.
Jablonski attributes much of our intellectual success to both the food availability in ancient forests and our capacity to procure the food. Humans are different now. We have cultivated a "rain forest in our heads," wrote Jablonski, in the form of the ingenious techniques of cooking and storing food (not to mention techniques for caring and nurturing children).
Jablonski described the two major extinctions of hominids, that of the "robust australopithicenes" (Paranthropus) and the early members of the genus Homo, H. erectus and H. neanderthalis (a good chronology of human evolution can be found here). Basically, Homo was better suited than Paranthropus to range widely for food and generally not limited to a particular area (with the exception of H. neanderthalis; it is generally accepted that they became trapped in a refugium of southern France and ran out of food). It wasn't just better bipedal movement that assisted Homo in this feat. The naked skin of Homo and the subsequent ability to efficiently dissipate heat also allowed for the capacity of our closest ancestors to move to their food.
Jablonski was recently featured in an interview with the NY Times about her latest book, Skin: A Natural History.
I'll finish up the series in a fourth part hopefully tomorrow, and moving on to one more symposium about the evolution movement at the grassroots.