walking and evolution

I'm in the process of writing a sermon for tomorrow's evolution sunday and, in order to explain the background to the importance of lucy and selam to the anthropological record, and thus the importance of friday's science article about hominid metatarsals, I quote myself from a 2008 sermon about walking. I am rather proud of this writing and, in the spirit of telling my students they ought to bring attention to the things they do that make them proud, I've reproduced it below.

"That our earliest known progenitor is Lucy, the skeletal remains of Australopithecus afarensis found in Hadar, Ethiopia, in 1974 by Donald Johanson—now joined by another Australopithecus afarensis found in Dikika, Ethiopia, and nicknamed Selam, the so-called Lucy’s Baby, although Selam is approximately 120,000 years older than Lucy and so would be Lucy’s multiply-great grandmother, except that she was killed when 3 years old—these facts are unassailable. When [Rebecca] Solnit [the author of the seminal book Wanderlust: The History of Walking] wrote, only Lucy had been found, and she describes her thus:

"'[Lucy] was apelike in many respects; she had little in the way of a waist or neck, short legs, longish arms, and the funnel-like rib cage of an ape. Her pelvis, however, was wide and shallow, and so she had a stable gait with hip joints far apart tapering to close-together knees like humans and unlike chimps (whose narrow hips and far-apart knees make them lurch from side to side when they walk upright). Some say she would have been a terrible runner and not much of a walker. But she walked. This much is certain, and then come the arguments.'

"The arguments Solnit refers to relate not to the how our progenitors walked, but the why. On this few concur. Solnit, in reporting on the 1991 Paris-based Conference on the Origins of Bipedalism, call these
'a kind of academic stand-up comedy routine. [The] ‘schlepp hypothesis’…explained walking as an adaptation for carrying food, babies, and various other things; ‘the peek-a-boo hypothesis,’ which…connected bipedalism to penile display…to impress females rather than intimidate other males; ‘the all wet hypothesis’…involved wading and swimming during a proposed aquatic phase of evolution; ‘the tagalong hypothesis’…involved following migratory herds across the…savannah; ‘the hot to trot hypothesis,’ which was one of the more seriously reasoned theories, claiming that bipedalism limited solar exposure in the tropical midday sun and thus freed the species up to move into hot, open habitat; and the ‘two feet are better than four’ hypothesis, which proposes that bipedalism was more energy-efficient than quadrupedalism, at least for the primates who would become humans. '

"Whatever the reason we started walking upright, albeit in an ungainly manner, it is important to note that doing so is the single development that made us what we are: separate from the other animal species. It’s what made possible higher-brain development and, ultimately, a sense of entitlement that led to permanent homes, agriculture, the five-day work week, the Humvee and the Mall of America."