Discovery in Kenya Reveals Species of Human Ancestor
By John Noble Wilford
c.1995 N.Y. Times News Service
Fossil discoveries in Kenya have revealed a new species of human ancestor that lived 4 million years ago, were hardly any bigger than chimpanzees, bore striking resemblances to both apes and evolving man, and, perhaps most significantly, were already standing erect and walking on two legs much like modern humans.
Leg and arm bones found near Lake Turkana, paleontologists said, provide the earliest direct and unambiguous evidence for upright walking, or bipedalism, by any members of the human family tree. Until now, the oldest evidence has been the 3.6-million-year-old footprints at Laetoli in Tanzania, together with tiny foot bones from South Africa, first reported last month that are about the same age.
By placing the emergence of bipedalism further back by about half a million years, the discovery supported the growing impression that this novel mode of locomotion might be the defining adaptation that first set human ancestors and their close relatives, known collectively as hominids, apart from the quadrupedal apes. From genetic studies comparing modern apes and modern humans, molecular biologists have established that the split between the two lineages occurred 5 million to 7 million years ago.
The discovery also crowds and probably complicates the base of the hominid family tree. It could provoke a new round of controversy among scientists trying to reconstruct human geneaology.
For two decades, the earliest known hominid was a species represented most famously by the Lucy skeleton. Known as Australopithecus afarensis, this appeared to be the sole hominid species from 3.9 million to 2.9 million years ago. Reading the DNA clock in the genes based on guesses about the rate of mutation, molecuar biologists started asserting that the split from the apes was far more recent than most paleontoligists believed - a mere 5 to 7 million years ago. Thus inspired, paleontologists searched more diligently for signs of earlier hominid life by looking in Africa for fossil-bearing sediments of this critical age.
Two new species have become known in the past year. A 4.4-million-year-old species from Ethiopia was identified and described last year; first lumped in the Australopithecus genus, it was recently reassigned to a new genus and given the name Ardipithecus ramidus. These hominids may have walked upright, but no direct evidence for this has yet been reported.
Now, in a report being published on Thursday [Aug 17] in the journal Nature, a team led by Dr. Meave Leakey of the National Museums of Kenya announced the discovery of a third new hominid, which has been named Australopithecus anamensis. The fossils of 21 specimens were found at two sites near Lake Turkana, Kanapoi and Allia Bay. The name "anam" means lake in the Turkana language.
If the name of the principal discoverer has a familiar ring, it may be because she is the wife of Richard Leakey, the prominent Kenyan fossil hunter who has left the field because of a crippling airplane accident and new interests in wildlife preservation and politics. He is the son of Mary Leakey and the late Louis S.B. Leakey, whose many discoveries this century established East Africa as the ancestral grounds for many human forerunners. The British-born Meave Epps Leakey, following in the family tradition, is an experienced paleontologist with a doctorate in zoology and years of fossil-hunting behind her.
The co-authors of the Nature report are Dr. Alan Walker, an anatomist at Pennsylvania State University who has worked for years with the Leakeys, and two geologists, Dr. Craig S. Feibel of Rutgers University in New Brunswick, N.J., and Dr. Ian McDougall of the Australian National University in Canberra. Their research was supported by the National Science Foundation and the National Geographic Society.
Geologists have dated the new specimens at 3.9 million to 4.2 million years old, making them intermediate in time between the ramidus species discovered by Dr. Tim White of the University of California at Berkeley and the later afarensis species, whose Lucy fossils were found in 1974 by Dr. Donald C. Johanson, a paleontologist now at the Institute for Human Origins, in Berkeley.
The new species seemed to be a blend of primitive and advanced characteristics. The jaw, with its shallow palate, large canines, and the placement of teeth, harks back to the apes. So do the extremely small ear openings in the skull. But the tooth enamel is much thicker than that of the apes or the ramidus hominid, thus resembling a feature of later hominids in the Homo lineage. And the tibia, or shinbone, and humerus, the upper-arm bone, are especially advanced. In one of the most telling clues to bipedality, the upper end of the shinbone is shaped to bear more weight than a four-legged animal would require.
Dr. Ian Tattersall, a paleontologist at the American Museum of Natural History in New York City, said the new fossils seemed to be "more plausibly hominid than ramidus is" and should advance understanding of when and where upright posture first evolved.
Johanson, often a rival of the Leakeys in finding and interpreting early human fossils, endorsed the conclusions drawn from the new finds.
Reached by telephone in Berkeley, Johanson said the anamensis species "does seem to have a series of features different from afarensis" and so is worthy of a distinctive place on the family tree. The species "fits the evolutionary trajectory of these early australopithecines," he said, and it emphasizes how the 4.4-million-year-old ramidus "seems to stick out as not being part of" this trajectory and may, instead, be "an ancestor to later apes rather than later hominids."
After examining the mixture of primitive and advanced characteristics of the anamensis species, Dr. Leakey and her colleagues wrote that the evidence "shows this species to be a possible ancestor to Australopithecus afarensis." But they, too, questioned whether the earlier ramidus hominid was a direct ancestor of either anamensis or afarensis.
Because the ramidus fossils appeared to be so much more primitive than anamensis, Dr. Leakey's team decided it was more likely that this earlier hominid was a sister species to all later hominids and thus represented a different branch of the hominid tree.
"Either evolution occurred very quickly for ramidus to be a direct ancestor of anamensis, or else Tim's fossils are on a side branch," Walker said in a telephone interview, referring to Tim White, the ramidus discoverer.
In a letter to Nature in May, White acknowledged that the ramidus hominid is likely to be a sister branch in explaining why he was assigning it to an entirely new genus. But he has also been quoted as still believing that ramidus was an ancestor to the hominids leading to afarensis and eventually the Homo lineage.
White has found more ramidus fossils but has not yet published his results. "We're on tenterhooks waiting to see what Tim has," Walker said.
In any event, paleontologists generally agreed that the new fossils represented a distinct species and that this added to the growing body of evidence for a great diversity among hominids around 4 million years ago. They could picture an early period when several hominid lines co-existed, and only one led eventually to humans, with the others dying out long ago.
"A. anamensis is seen as a possible ancestor to A. afarensis, although it is recognized that 4 million years ago there may have been a number of newly emergent hominid species with variations based on the novel bipedal adaptation," Dr. Leakey and her colleagues wrote, noting that there "is no theoretical reason why there should be only one early hominid species at any one time." In their view, ramidus is more likely to have been one of the ill-fated lines and anamensis an occupant of the successful line.
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