Ardi Is A Big Step Closer To Evolutionary Split

It just got easier to believe in human evolution. Lucy has moved out of the limelight. Ardi has stepped into our lives.

Lucy used to be to be oldest well-described ancestor on the evolutionary line leading toward humans. She was discovered in Ethiopia's Afar Triangle in 1974 and nicknamed for the Beatle's popular song "Lucy in the Sky with Diamonds." This small-brained, upright-walking creature (Australopithecus afarensis, which lived about 3 million years ago,has fascinated scientists for years, but was too historically recent to say much about our common ancestor with our closest living relatives, the chimps and bonobos.

Ardi is the latest evolutionary debutante. Her coming-out party was Oct. 2, when 11 research articles were published in a special issue of the journal Science. Her nickname derives not from a song, but from her scientific name, Ardipithecus ramidus. This species is based on skeletal reconstruction of bones from more than a hundred specimens dating to 4.4 million years old. This collection, also from the Afar region, includes most of a skull and a female pelvis. Though discovered in 1994, scientists spent 15 additional years of research and collecting before presenting her.

Based on the geological context and anatomical evidence, Ardi lived in a woodland environment, rather than on the savanna. When up in the trees, she used four limbs to get around, which accounts for her dexterous hands and large feet with opposable toes. When on the ground, she walked upright while foraging for a wide variety of foods. Though her body and brain were both chimp-sized, she lacked traits associated with the chimpanzee line such as large canines, a short back and a hand adapted for knuckle-walking.

In addition to improving our understanding of human origins, Ardi's appearance should also help unite the religious and political divide between those who believe in organic evolution and those who do not.

Ever since the days of Charles Darwin and his bulldog-like defender, Henry Huxley, critics of human evolution have ridiculed natural scientists by promulgating the erroneous idea that the common ancestor between humans and chimps must have looked like the latter. We now know that both chimps and humans are highly specialized, and very different versions of a more generalized common ancestor.

Before Ardi, opponents of evolution could get away with the "chimp-as-grandfather" distortion because there was no well-documented species to occupy the bifurcation point between the human line and the chimp line. Though we still don't have that common ancestor, we now know that it must have resembled Ardi more closely than any modern ape. My hope is that those who had trouble accepting the image of a chimp as distant ancestor will find it easier to accept the image of Ardi, who I think was much better-looking.

To help understand this evolutionary shake-up, it might be helpful to go to the woodworking shop for an analogy. Think of chimps as screws and humans as nails. Both fasteners share the same overall body plan, being elongated cylinders with an enlarged head at one end. But there are salient differences at closer inspection: the screw having a threaded spiral rather than a smooth shaft, and having a slotted top rather than an indented one. Did the common ancestor of these fasteners more closely resemble the nail or the screw? Did the wire nail evolve by losing its spiral? Or did the screw evolve by gaining one?

The answers to these questions are neither and neither. The common ancestor of nails and screws looked like neither. The first screws were large, made of wood, and were used to press fluids such as oil, wine, ink or water. The first nails were prismatic in shape (flat-sided shafts), made from wrought iron and hammered into shape. The common ancestor between screws and nails would have looked like neither, being some sort of primitive fastening tool.

Likewise, the common ancestor between chimps and humans looked like neither. It probably looked a lot like Ardi.

•Robert M. Thorson is a professor of geology at the University of Connecticut's College of Liberal Arts and Sciences and a member of The Courant's Place Board of Contributors. His column appears every other Thursday. He can be reached at profthorson@yahoo.com.

What's your opinion? Make it count with a Letter to the Editor. Visit www.courant.com/letters and scroll down.