No, After You, Afarensis
Kathy A. Svitil
THE HUMAN FAMILY tree is untidy at best, and determining just who descended from whom is often uncommonly difficult. But paleoanthropologists at least understand how we descended from our apelike forebears, the australopithecines–or so they believed until this past year. Now even that bit of certainty is gone.
The oldest australopithecine, Australopithecus anamensis, lived from 4.1 to 3.5 million years ago and had more primitive teeth and skull features than the next species, A. afarensis (best known from the skeleton called Lucy), which roamed eastern Africa from 3.6 to 3 million years ago. A. africanus came next–it lived in southern Africa from 3 to 2.6 million years ago and had a bigger brain, smaller canine teeth, and a shorter muzzle than A. afarensis.
One might be forgiven, then, for assuming the three are linked in a straight line. But that’s not so likely anymore. In July researchers reported that A. africanus actually had more apelike body proportions than its presumed ancestor, A. afarensis. Paleoanthropologists Henry McHenry of the University of California at Davis and Lee Berger of the University of the Witwatersrand in South Africa looked at the arm and leg joints of more than 100 australopithecine specimens. From those joints, the researchers estimated the size of the once-attached limbs. “The pattern suddenly jumped out: africanus had small hind limbs,” says McHenry. Translated into modern human equivalents, the size of the hind limbs corresponded to an average human body weight of just 75 pounds. But the forelimbs, while they varied in size (probably, McHenry says, because of a pronounced difference between males and females), on average matched a human body weight of 112 pounds.
“The larger forelimbs of africanus might be an adaptation to a more arboreal life,” McHenry says. “But the hind limb is adapted to bipedality, and so there’s no doubt that they were terrestrial. They probably spent a lot of time in trees–feeding, sleeping, or for protection.”
In afarensis, the apish proportions were not so pronounced–in all probability the presumed ancestor spent less time in the trees than the purported descendant. So did africanus actually evolve from afarensis? Berger does not believe it did. And because africanus shares so many other features with later species of Homo, he’d like to bump afarensis off the human branch of the hominid family tree.
McHenry is not convinced. “Maybe he’s right, but I don’t feel we can be that sure yet,” McHenry says. “Afarensis is more primitive in its skull and teeth, and it forms a reasonable ancestor for africanus. All the formal analyses using computer algorithms seem to show that afarensis is ancestral to africanus, which is on the branch with Homo. We’ll need a much more thorough study of the whole body of africanus to say anything with certainty.”
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