Scientists Make Grand Strides In Linking Species

IT'S been a great year for old bones.

In February, a research team reported that Homo erectus - the immediate ancestor of our own Homo sapiens species - had been in Asia a million years earlier than experts had thought. Last Thursday, another scientific team announced discovery of a long-sought ``missing link.'' It's a hominid species close to the point where the evolutionary lines that lead to modern apes and modern humans diverge. In between these two epochal announcements has been a report of reconstructing the first nearly complete skull of the ancestral human species that lived in East Africa over 3 million years ago. Also, scientists discovered a species marking the transition between H. erectus and Homo sapiens in that same region.

Bernard Wood at Britain's Liverpool University notes, ``The metaphor of a missing link has often been misused, but it is a suitable epithet for the [new] hominid.'' Dr. Wood was commenting in the journal Nature, which also carried the report of last week's discovery.

One marvels at the chutzpah of scientists who try to reconstruct human evolution from a few old bones. Yet, since form often suggests function, they can read much into the fossils. Also they now have a powerful aid in the radioactive dating technique that places fossils in the correct time frame. They date the bones by dating the volcanic tuff in which the fossils generally are found. Volcanic crystals contain potassium, which has been decaying radioactively into argon at a steady known rate since the crystals solidified. That dates the tuff precisely. The time scale of interest runs back roughly 6 million years. Paleontologists have sketched a speculative scenario for that time period that starts with the earliest pre-humans - the genera of hominid species called Australopithecus. This line evolves into H. erectus, which spreads from Africa to Asia and Europe and eventually evolves into Homo sapiens.

That scenario is neat, but very uncertain. The oldest H. erectus fossils in Africa date to about 1.8 million years ago. Last February, geologists Garniss Curtis and Carl Swisher - then with the Institute of Human Origins in Berkeley, Calif. - reported in the journal Science that H. erectus fossils in Java are just as old. This raised questions of what came out of Africa, when it came out, and whether or not H. erectus did evolve into Homo sapiens.

In June, J.D. Clark with the University of California at Berkeley and 10 colleagues reported in Science that fossils from Bodo in Ethiopia are intermediate between H. erectus and Homo sapiens. An important transition between those two species was thus found in Africa dating to about 550,000 years ago.

Meanwhile last March, William Kimbel, Donald Johnson, and Susan Shea at the Institute of Human Origins described in Nature how they had put together more than 200 3-million-year-old fragments to reconstruct most of the skull of a member of the Australopithecine species called afarensis. This is the species that may have given rise to H. erectus or its immediate ancestor. The fossil called Lucy - found in 1974 and now dated to 3.18 million years ago - is the most famous afarensis. The skull is much younger than Lucy. It shows that the species was stable for about a million years before rapid evolution produced H. erectus.

Now, last week, Tim D. White of the University of California at Berkeley and colleagues have described the 4.4-million-year-old ``missing link'' fossil that appears to be transitional between the ancestor common to both apes and humans and Lucy's line. They call it Australopithecus ramidus - meaning root of the human family tree. It appears to have been a creature about the size of a chimpanzee that could walk upright and lived in a forested environment at Aramis, Ethiopia.

Key high points of human evolution have become a little clearer this year. But the gaps in that story are enormous. The story line itself remains puzzling. Stay tuned.