Another Branch on the Human Family Tree

by Adam Lee on October 3, 2009

I haven’t written about any new transitional fossils in a while, so it’s a great pleasure for me to mention this one: a hominid skeleton nicknamed “Ardi”, a specimen of Ardipithecus ramidus. This species was known from other fossil fragments, but Ardi is one of the oldest and most complete hominids found so far, and may give us the most insight yet into what the common ancestor of humans and chimpanzees looked like.

Ardi lived about 4.4 million years ago (by comparison, Lucy and her fellow australopithecines are about 3.4 million years old), in the Middle Awash region of modern-day Ethiopia. Today it’s an arid badlands, but in that era, it was a lushly forested woodland, cool and wet but geologically active, with frequent volcanic episodes (a great boon to biologists, since volcanic rock and ash strata are easily dated with radiometric methods and give us good estimates of when a certain fossil lived). Primitive elephants, giraffes, horses, antelope, rhinos and monkeys are well-known from this area, as are other hominid specimens.

The fossil itself is believed to be a female. The bones were so poorly fossilized, according to the Science paper by Tim White and colleagues, that they would crumble if touched. The researchers painstakingly chipped them free of the rock they were encased in with dental picks, bamboo, and porcupine quills (!). From the fossil’s discovery to its publication took nearly 15 years of preparation and study – but from all accounts, it was worth the wait.

In life, Ardi would have stood just under four feet tall and weighed about 110 pounds. The skull was small, about 325 cc, about the same size as a chimp’s. Ardi’s teeth suggest she was an omnivore, and from comparing other A. ramidus teeth and bones found in the region, White and his colleagues found little difference in tooth size or body size between male and female individuals. This suggests that their mating style was relatively peaceful, with little competition for mates (as compared to chimpanzees, who have massive canine teeth which are used to intimidate potential rivals) and possibly more stable pair-bonding and group cohesion.

Ardi’s hands, feet and pelvis tell us a lot about how she got around. Hominids like Lucy show a mosaic of bipedal and arboreal adaptations – as Laelaps puts it, they “had their hands in the trees and their feet on the ground” – and Ardi shows a more primitive version of the same pattern, much as we’d expect from an ancestor of that age.

She stood and walked upright, though not as well as Lucy or as us, and her feet were becoming more rigid like ours, except that she also had an opposable big toe useful for grasping. Her arms were long enough to reach to her knees when standing upright, but her hands were not adapted for knuckle-walking. Nor did they have the specializations for climbing and hanging from trees that we see in modern apes. She still lived in the trees, but would have moved through them more slowly and carefully than chimps or orangutans, and was capable of descending to the ground and walking. This refutes the once-popular belief that bipedalism first developed when human ancestors left the forest for the savanna and adapted to stand upright so as to see over the grass – as shown by species like Ardi, bipedalism evolved before we left the trees.

Another popular but erroneous idea that Ardi refutes is that the common ancestor of humans and chimps looked basically like a chimp, and that humans have changed significantly while modern chimps are little different from our common ancestor. This is probably tied to the misconception of the “great chain of being” that sees humans as the highest or most advanced form of life on Earth. Ardi, who probably lived relatively near the time when our two lineages split, instead shows that both humans and chimpanzees have evolved and specialized since the time of our common ancestor, becoming adapted to two very different ways of life.