Once again, the human lineage changes as new findings overturn the "accepted wisdom"
As noted in prior posts, one of the most intellectually interesting and satisfying aspects of science is the constant growth of knowledge. I recall that, when I was in college, one of the major sociological/psychological/anthropological debates was about whether humans were inherently violent. Later, the debate metamorphicized into a debate between chimpanzees and bonobos (the hunters vs. the lovers – both gross stereotypes). Now, once again, the human lineage has been shaken by a discovery that overthrows the “accepted wisdom”.
Analysis of a near-complete skeleton of a human ancestor found in Ethiopia has changed scientists' thinking about the appearance and behavior of our distant forebears. The “centerpiece” of the diverse collection of primate, animal, and plant fossils (afterall, it is all about us) judged to be 4.4 million years old is the skeleton of a human ancestor that demonstrates, according to the researchers, our earliest forebears looked nothing like a chimpanzee or other large primate, as is now commonly believed. Instead, the findings suggest that the last common ancestor of humans and primates, which existed nearly 2 million years earlier, was a primitive creature that shared few traits with modern-day members of either group. The find, researchers believe, demonstrates that our ancestors began walking upright in woodlands, not on grassy savannas as prior generations of researchers had speculated.
The specimen has been named “Ardipithecus ramidus", or ARDI (as the media has been calling her). The fossils were originally discovered 15 years ago in the Afar Triangle of Ethiopia. The fossils were found in a layer of sediment sandwiched between two layers of volcanic ash, each dating from 4.4 million years ago (which indicates that the fossils are also of that age). The team also found more than 100 fossils from 36 other members of the same species. Additionally, fossils of 29 species of birds, primarily small ones like doves, lovebirds, mousebirds, passerines and swifts, were found, as well as several that were previously unknown. Animal fossils included 20 new species of small mammals, including shrews, bats, rodents, hares, and small carnivores, as well as larger animals, including baboons, colobus monkeys, and spiral-horned antelopes. Fossilized wood, seed, and other plant remains indicate the presence of hackberry, fig, and palm trees. Collectively, these finds indicate that the environment was more humid and cooler than it is today, and contained grassy woodland with forest patches.
The result is that the ecosystem can be reconstructed. The previously held savanna theory is no longer an explanation for upright walking, as is the thesis that humans evolved from an animal akin to a chimpanzee. [As has been noted previously, when one deals with small numbers, as is often the case with fossils, ascertaining the common characteristics of a species is difficult, let alone calculating interrelationships amongst species.]
Ardi stood about 47 inches tall and probably weighed 110 pounds. Many researchers previously believed that such an early ancestor would, like modern chimps, be a knuckle-walker, using the knuckles for support while moving on all fours. Instead, Ardi appears to have climbed on all fours on branches, but walked upright on the ground. Her feet, like those of monkeys but not chimps, were designed more for propulsion than for grasping. Her face had a projecting muzzle, giving her an ape-like appearance, but many features of her skull, such as the ridge above the eye socket, are quite different than those of chimpanzees. Her brain is about the same size as Lucy's. Her hands lacked many of the specializations that allow modern-day African apes to swing, hang, and easily move through trees. Those specializations apparently evolved in large primates after they separated from the last common ancestor with humans more than 6 million years ago. (Few fossils of such primates are available because they lived primarily in forests, which are not conducive to preservation of bone.)
The most controversial aspects of the researchers’ assessments involves interpretations of what the fossils indicate about behavior. Of particular importance, it was noted, is that the sizes of males and females were about the same, and that the specimens do not have large, sharp canine teeth. Both findings suggest that the fierce, often violent competition among males for females in heat that characterizes gorillas and chimpanzees was absent in Ardipithecus. One researcher concluded that this implies that males were beginning to enter into monogamous relationships with females, and devoted more time to caring for their young than did earlier ancestors. Some commentators view such conclusions as somewhere between questionable and nonsense.
The findings are reflected in 11 papers published by 47 authors in the Journal Science. The various articles can be found at http://www.sciencemag.org/. The Discovery Channel is going to run a 2-hr. special on the findings on Sunday, Oct. 11, 2009, at 9PM PDT/EDT.