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10/29/2009
New Planets and an Old Relative
HARPS was in the news last week. No, not harps, the musical instruments. It was the High Accuracy Radial Velocity Planet Searcher, HARPS. HARPS is the spectrograph associated with the 3.6-meter telescope of the European Southern Observatory (ESO) in Chile. ESO is an organization supported by 14 European countries. In one fell swoop, at meetings in Spain and Portugal, the HARPS team reported the discovery of an amazing 32 new exoplanets, planets orbiting stars other than our own Sun. Normally, such a discovery would be a no-brainer and I would devote the whole column to this topic. However, earlier this month, in the October 2 issue of Science, another equally astounding scientific triumph was reported involving a long dead female, Ardipithecus ramidus, Ardi for short. How appropriate for this year of Darwin that Ardi has caused a major evolution in our understanding of the evolution of our own human species.
But first let's deal with those exoplanets. We've talked before about how, as a planet orbits its star, the star "wobbles", changes its position due to the gravitational attraction between the planet and star. Say that we're looking at a star with a planet at the right of the star in our field of view. The star will be pulled slightly to the right. When the planet circles around to the star's left, the star is pulled slightly to the left; hence the "wobble". Most of the planets found to be orbiting other stars have been detected by this wobble. As a star moves back and forth with the position of its orbiting planet, HARPS detects changes in the star's radial velocity. The amazing thing about HARPS is that it can detect changes as small as about 2 miles an hour! I can walk faster than that, even at age 81!
When ESO put out a call for a team to build HARPS for the telescope, ESO promised the HARPS consortium a hundred nights a year observing time for five years. The five years is now up and what a wise decision ESO made. HARPS is responsible for more than 75 of the more than 400 exoplanets found to date, including some we've talked about in the past. For example, HARPS has found "super-Earths", one in a habitable zone of a small star, a planetary system of three Neptune-size planets orbiting a star, the lightest planet detected around a normal star and a planet with a density similar to that of Earth.
Longtime readers will know that two of my favorite themes are anything to do with such astronomical findings and anything related to our origins. When it comes to the latter, I've got to give the nod to Ardi. Until now, it was "Lucy" that held the crown as the most honored and influential hominin fossil. In fact Lucy has been making a tour of the U.S. and may have even been in nearby New York recently. Lucy, a member of the Australopithecus afarensis family, was found in Africa back in 1974. Enough of Lucy's remains were uncovered to reconstruct her skeleton sufficiently to make it clear that, unlike our relatives the apes, chimps or gorillas that knuckle-walk and swing through trees, Lucy, at about three and a half feet tall, was an accomplished upright walker.
The finding of 3.2 million-year-old Lucy helped to promote the view that our own upright-walking Homo species evolved from ancestors that came down out of the trees and began living mainly in the plains and grasslands of Africa. No longer having to swing through the trees, they gradually evolved to walk upright. The prevailing opinion was that our last common ancestor with our very close relative, the chimpanzee, was more like a chimp, adapted to tree life in the jungle, than like Lucy, striding on the savannah walking tall. Bits and pieces of hominid fossils much older than Lucy were found but these bits and pieces were not enough to permit any significant reconstruction of a skeleton that could shed light on how the creatures walked.
Ardi, who lived some 4.4 million years ago, is the exception. A skull, with teeth, arm bones, hand bones, pelvic bones, leg bones, foot bones and a few other bones were painstakingly extracted from the terrain and rock in which they were found. When I say a skull was found, that's not to say it was intact. Indeed, the skull was crushed into a hundred pieces and scattered and the fossil fragments were extremely fragile and tended to crumble when handled. And it wasn't only Ardi that was found and studied. Thousands of fossil fragments of other Ardipithecus ramidus specimens were collected along with many thousands of remains of birds, animals, seeds etc. were collected and studied. All this provides a close look at Ardi's lifestyle, diet, environment, etc. It took three field seasons just to gather up the various pieces of Ardi.
All this is due to the Middle Awash project, a project that has involved an international team of hundreds of scientists, technicians, support staff and laborers who have had access to the fossil-rich areas of the Afar valley in Ethiopia. The support of the Ethiopian government is/was critical in allowing prolonged access to the areas combed by the project. The 11 papers in Science involve over 40 authors. Two News Focus articles by Ann Gibbons provide good overviews of the project and its findings along with an account of her personal visit to the Afar Depression, where she encountered Tim White, of UC Berkeley, one of the co-leaders of the project.
Gibbons tells of White being sick and also having to worry about threats of being killed by tribesmen living nearby. This explains the six Afar policemen carrying AK-47s that accompany the group! Gibbons also tells of fossil-finder Kampiro Kayrento, a 30-year-old local farmer who White calls "the best person in the world for finding little pieces of fossilized bone." Over the last 19 years the team has collected some 19 thousand vertebrate fossils and about 300 specimens from 7 different hominins. (Note: if you're wondering whether it's hominin or hominid, apparently both are OK. I've seen both spellings in the same article, even the same paragraph!)
To understand the outstanding, careful and daunting task that the Middle Awash team has accomplished one only has to look at the timeframe involved. The first hint of Ardipithecus ramidus was in 1992, when an alert worker spotted what turned out to be a tooth. The area was a treasure trove of remnants of many of this new species. It was in 1994 that Ardi herself was found; at least the first signs of a skeleton for the ages that was to be uncovered. Although the finding of the new species was reported relatively early on, it's taken these 15 years or so to publish the findings on Ardi.
Why? Let's just take her skull and pelvis. Putting Ardi together from the fragile fragments of her skeleton not only required the utmost in care to preserve the fragments during excavation and transport but exceedingly delicate work in the labs using microscopes, dental drills, porcupine quills and the utmost in patience! Sophisticated computer modeling in distant parts of the globe also came into play. C. Owen Lovejoy in Ohio and Gen Suwa in Tokyo (Suwa found the tooth in 1992) collaborated with Breanne Asfaw in Addis Ababa in Ethiopia. Some fossil fragments were taken to Tokyo to be scanned in a special CT scanner and the results scanned into computers to make virtual skulls and pelvises. The virtual skulls and pelvises were refined and finally, the 10th virtual reconstruction of the skull and 14th virtual reconstruction of the pelvis satisfied the researchers that they had the real Ardi in hand. Lovejoy has made physical models of the skull and pelvis based on the virtual images.
Ardi was not an imposing figure by modern standards, standing only about 3 feet, 11 inches, a bit taller than Lucy. Both Ardi and Lucy had smallish brains, significantly smaller than ours. But there was a major difference between Ardi and the later Lucy. Look at your hands and what do you see? You have an opposable thumb. Now look at your feet, You do not have an opposable big toe. If looked at Lucy you would come to the same conclusion.
Ardi, who lived roughly 700 thousand years earlier than Lucy, did have an opposable toe, much like our opposable thumbs. Ardi's hands, however, were not of the type that the researchers considered suitable for knuckle-walking or swinging in trees. Ardi walked upright and the opposable toe permitted her to walk upright on branches of trees. She was not a swinger, but rather a careful climber and, though not as dexterous an upright walker as Lucy, she was comfortable walking upright in grassy flat terrain as well as in the wooded areas that were part of her habitat. The major change in the generally accepted view of our human evolution that Ardi has led to is that we are not evolved from a chimp. Well, it's not quite that much of a change. The prevailing view was that our last common ancestor with the chimp was more like a chimp than like us modern humans. With Ardi, it seems likely that our last common ancestor was more like us, perhaps a more satisfying view to those who don't like to think of being descended from a chimp.
The Middle Awash project has now found fossils of hominids spanning a time period of some 6 million years starting with Ardipithecus kadabba, dating back to 5.2 to 5.8 Mya (million years ago). Ardipithecus ramidus, comes in at 4.4 Mya, followed at 4.1 Mya by Australopithecus anamensis, thought to be the ancestor of 3.4 Mya Lucy, Australopithecus afarensis. Next comes Australopithecus garhi, at 2.5 Mya and hints of what's to come with the first evidence of the use of stone tools to butcher animals. At 1.0 Mya the Middle Awash project finds Homo erectus and the use of hand axes. Finally, very recently, only 160 thousand years ago, we Homo sapiens appear upon the scene there in Ethiopia. For better or for worse, the world would never be the same.
Now, as with any major claim that upsets conventional wisdom, there's controversy. Some say the evidence for bipedalism, walking upright, is not conclusive. Others refuse to believe that our last common ancestor with the chimp did not have chimp-like features. In the last paper in the Science issue, Lovejoy, Suwa, Simpson, Matternes and White conclude that Ardi "implies that African apes are adaptive cul-de-sacs rather than stages in human emergence." Aside from using the word "implies", the general tone of the various papers and other articles is certainly a much more positive view that we are not descended from the chimpanzee (or a chimp-like creature).
Perhaps someday, somewhere in Africa, another striking fossil, 7 to10 million years old, will appear and it will actually be that long sought last common ancestor.
Next column to be posted on November 12.
Allen F. Bortrum
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