Tuesday, July 20, 2010

Fossil Skulls From Africa and Europe

Two fossil skulls, one from central Africa and the other from the Black Sea republic of Georgia, Europe, have shaken the human family tree to its roots, sending scientists scrambling to see if their favorite theories are among the fallen fruit. Probably so, according to paleontologists, who may have to make major revisions in the human genealogy and rethink some of their ideas about the first migrations out of Africa by human relatives.

Although the skulls have caused confusion and uncertainty, scientists speak in superlatives of their potential for revealing crucial insights in the evidence-disadvantaged field of human evolution. The African skull dates from nearly 7 million years ago, close to the fateful moment when the human and chimpanzee lineages went their separate ways. The 1.75-million-year-old Georgian skull could answer questions about the first human ancestors to leave Africa, and why they ventured forth. Still, it was a shock, something of a one-two punch, for two such momentous discoveries to be reported independently in a single week, as happened in July.

Every decade or two, a fossil discovery upsets conventional wisdom. One more possible "missing link" emerges. An even older member of the hominid group, those human ancestors and their close relatives (but not apes), comes to light. Some fossils also show up with attributes so puzzling that scientists cannot decide where they belong, if at all, in the human lineage. At each turn, the family tree, once drawn straight as a ponderosa pine, has had to be reconfigured with more branches leading here and there and, in some cases, apparently nowhere.

The skull of Africa was uncovered in the desert of Chad by a French-led team under the direction of Dr. Michel Brunet of the University of Poitiers. Struck by the skull's unusual mix of apelike and evolved hominid features, the discoverers assigned it to an entirely new genus and species — Sahelanthropus tchadensis. It is more commonly called Toumai, meaning "hope of life" in the local language. About a million years older than any previously recognized hominid, Toumai lived close to the time that molecular biologists think was the earliest period in which the human lineage diverged from the chimpanzee branch. The next oldest hominid appears to be the 6-million-year-old Orrorin tugenensis, found two years ago in Kenya but not yet fully accepted by many scientists. After it is Ardipithecus ramidus, which probably lived 4.4 million to 5.8 million years ago in Ethiopia.

Scientists suggest several possible explanations. Toumai could somehow be an ancestor of modern humans, or of gorillas or chimps. It could be a common ancestor of humans and chimps, before the divergence. With new fossil discovery, fossil hunters are more likely to find something unrelated directly to living creatures — more side branches to tangle the evolutionary bush. So the picture of human genealogy gets more complex, not simpler.

Although overshadowed by the news of Toumai, the well-preserved 1.75-million-year-old skull from Georgia, Europe, was also full of surprises, in this case concerning a later chapter in the hominid story. It raised questions about the identity of the first hominids to be intercontinental travelers, who set in motion the migrations that would eventually lead to human occupation of the entire planet.

The discovery, reported in the July 5 issue of the journal Science, was made at the medieval town Dmanisi, 50 miles southwest of Tbilisi, the Georgian capital. Two years ago, scientists announced finding two other skulls at the same site, but the new one appears to be intriguingly different and a challenge to prevailing views. Scientists have long been thought that the first hominid out-of-Africa migrants were Homo erectus, a species with large brains and a stature approaching human dimensions. The species was widely assumed to have stepped out in the world once they evolved their greater intelligence and longer legs and invented more advanced stone tools.