A Missing Link?

		The incomplete skeleton of our most famous forebear, Lucy

In effect, Lucy is one of the "missing links" anthropologists have been searching for since the turn of the 20th Century. She is essentially part ape, part human; the surprising thing about Lucy, however, is which part is which. Before Lucy's discovery, scientists reasoned that the main trait separating humans from the apes was intelligence, anatomically represented by the cranial capacity of the skull. The prevalent hypothesis held that intelligence was a precursor to, if not the impetus for, bipedalism, and anthropologists expected to one day find the remains of a sort of big-brained chimp. Upright, tiny-brained Lucy flew in the face of that line of reasoning. Below her neck, her locking knee-joint and short, narrow pelvis show she could walk upright, and her hands were human-like with slightly more curved fingers. Above her neck, her skull fragments, although not entirely conclusive, point to a cranial capacity of about 450cc (on par with that of a modern chimp) and her jaw and skull were ape-like as well. Lucy's discovery forced anthropologists to reexamine the evolutionary pressures that drove bipedalism.

Why Walk?

"Why adapt an upright stance? 'No one knows,' is the short answer," says Kimbel.

Numerous hypotheses attempt to explain why bipedalism arose at all. Some anthropologists propose that standing upright helped our ancestors see over tall grasses or appear larger to predators. Some say standing upright helped our ancestors stay cool by reducing the amount of surface area exposed to direct sunlight. Other researchers argue that bipedal locomotion is less energetically costly than walking on all fours, or that running on two legs helped us escape four-legged predators.

"If you've ever chased a dog," says Kimbel, "then you know that's not very likely."

Kimbel discards the above hypotheses because they all assume hominid evolution took place on the grassy African plains. And for that, says Kimbel, there is just no evidence. In fact, the remains of other possibly bipedal apes have been found in prehistoric woodland habitats as well. And Lucy, comfortable as she would have been standing in line at the bank, would have been equally at home in the trees, where her long arms and curved hands would have served her well.

Kimbel favors the hypothesis proposed by Dr. Owen Lovejoy in the early 1980's; that the need for males to provide food for dependant offspring favored bipedalism. Walking upright might make us more vulnerable to slipped spinal discs and creaky knees, but it does increase our endurance, allowing us to walk for long periods of time. And, of course, it frees up our hands for foraging and carrying the goods back home.

Science has proven that the ability to provide for offspring is a powerfully selective trait. But Kimbel acknowledges this scenario has its detractors as well, mostly because the hypothesis requires that these early hominids were monogamous. What evidence exists of such a complex social system?

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Photo: Institute of Human Origins