Stand and deliver: why did early hominids begin to walk on two feet?

Oddly, in view of what Stanford has to say later on in his .book, he also takes time to trash the idea that bipedalism was driven by environmental change. More significant, he argues, was that from the beginning hominids appear to have been ecological generalists. The key to their success was, and is, their ability to thrive in diverse environments. Yet despite his emphasis on environmental adaptability, he is still convinced that the hominids' unusual and implication-ridden form of locomotion was a response to something, and he is clearly concerned to discover a single underlying explanation for it. He finds it in meat eating.

Between 7 million and 8 million years ago, at the beginning of the scenario he reconstructs, some very early hominids "shuffled across the ground a bit between fruit trees." But as the climate became increasingly seasonal, and the grasslands expanded at the expense of the forest, natural selection would have favored those individuals who shuffled most efficiently across the enlarging open areas. That would have laid the groundwork for the success of the archaic bipedal hominids. They were the animals that could most effectively scavenge meat from carcasses they encountered in increasingly open areas, even as they hunted smaller game in forests and woodlands--much as some chimpanzees do today. Thus, despite Stanford's earlier insistence that hominids succeeded be cause they were generalists, he eventually falls back on environmental change as at least the initial external impetus for the multistage sequence of events that led to bipedalism.

Once a taste for meat had been acquired, everything else followed. "By three million years ago," he writes, "the whole equation of foraging energetics and diet had begun a fundamental shift." A "virtuous circle" had been established. More efficient upright walking fed back into increasing intelligence and social complexity, and those attributes led to ever more effective hunting. The last part of Stanford's short book is devoted to a once-over-lightly of the later horn inid fossil record, illustrating how that dynamic has played out over the past couple of million years.

Two very different books, then, presenting radically different scenarios for the origin of bipedalism in our lineage. But, significantly, what both books have in common is a firm belief in the gradual environmental molding of lineages, generation by generation, through natural selection. Indeed, both authors see natural selection as a driving force in human evolution--though Stanford correctly emphasizes that natural selection promotes the diversity of species, and stoutly denies that evolution is toward anything.

Yet natural selection can only work on novelties presented to it spontaneously; it cannot call anatomical innovations into being, however desirable they might appear. In nature, form has to precede function, for without form there can be no function. Yes, in retrospect bipedalism opened up a huge range of radical new possibilities for hominids. But evolutionists can hardly invoke those possibilities and their exploitation as explanations for the appearance of the new behavior.