<p> Bones from the Ethiopian desert prove that human ancestors walked
the earth 4.4 million years ago
</p>
<p>By Michael D. Lemonick--Reported by Andrea Dorfman/New York
</p>
<p> Paleontologist Gen Suwa was walking across the pebble-covered
desert of north-central Ethiopia under the searing midday sun,
peering carefully around him for ancient bones. Then he saw
it: the telltale gleam of a fossil tooth partially exposed on
the rocky ground. "I knew immediately that it was a hominid
tooth," says the University of Tokyo scientist, "and one of
the oldest ever found."
</p>
<p> It was more than that. Suwa had uncovered nothing less than
a new chapter in the history of human evolution. He and his
colleagues report in the current Nature that the archaic molar,
along with other fossils they found in the area on expeditions
in 1992 and 1993, belong to a previously unknown species. This
diminutive, humanlike creature walked the earth some 4.4 million
years ago--half a million years earlier than the oldest human
ancestors ever identified. That stretches our family tree back
almost to the era when humans and apes branched off from a single
ancestor. In fact, says University of Liverpool paleontologist
Bernard Wood, whose commentary on the find also appears in Nature:
"It looks to me like this is either the common ancestor or damned
close to it. I think we're splitting hairs not to call it the
`missing link.'"
</p>
<p> Paleoanthropologists have not unearthed anything this revolutionary
since 1974, when the famous fossil skeleton known as Lucy was
discovered about 50 miles north of the current find. That 3.2
million-year-old female hominid had some human characteristics--most notably, she walked on two legs rather than four--but skull and tooth fragments indicated she was somewhat apelike
as well. She fit nicely into the shared-ancestor theory first
put forward by Charles Darwin and supported by modern comparisons
between human and ape proteins and DNA. The divergence between
the ape and human lines, argued the biochemists, came somewhere
between 4 million and 6 million years ago. And some paleontologists
predicted that as hominid species were discovered from periods
closer and closer to the time of the actual split, they should
be even more apelike than Lucy.
</p>
<p> That's exactly the case with the new species, which now bears
the scientific name Australopithecus ramidus (ramid means root
in the local Afar language). Like Lucy and her clan, known as
Australopithecus afarensis, ramidus had teeth with some apelike
and some human characteristics. But at least one specimen--a baby molar still attached to a piece of an immature ramidus
jaw--resembles a chimpanzee tooth more than a molar from any
known hominid. "It's obvious that it belongs to an ancestor
of afarensis," says Tim White of the University of California,
Berkeley, a co-author of the Nature report and a leader of the
international team that uncovered the new fossils.
</p>
<p> The researchers found very few bones from below the neck, and
those they found were in fragments; chew marks on the bones
show that the hominids' carcasses were ravaged by carnivores.
That makes it hard for anyone to be sure what these creatures
looked like and how they walked. The fossils suggest that at
least some members of the ramidus clan were about 4 ft. tall,
but that doesn't establish what the range in height was. In
some African apes, males are considerably bigger than females,
as they were in Lucy's species as well. Says White: "We do know
the arm bones come from an individual that was larger than Lucy,
but we don't know if it was male or female."
</p>
<p> Most experts assume ramidus walked on two legs, as Lucy did,
but the evidence is skimpy and indirect. One clue is a tiny
fragment of the foramen magnum, the opening at the base of the
skull where the spinal cord joins the brain: its location suggests
an upright stance. Moreover, the structure of the arm bones
is different from what anatomists see in knuckle-walking apes.
</p>
<p> If ramidus really did travel on two legs, anthropologists may
have to rethink their notions of what started pre-hominids on
the evolutionary road that led to modern Homo sapiens. It is
already clear from Lucy, who stood upright but had an apelike
skull, that bipedalism came first and a large brain later. But
what prompted the shift to two-leggedness? The conventional
theory is that a change in climate transformed the eastern and
southern African forests to dry, open grasslands, favoring apes
that could walk upright; they would have been able to see predators
from farther away and walk long distances holding food or children.
</p>
<p> It appears ramidus may have lived not on the savannah, however,
but in some sort of forest. Mixed in with the hominid fossils,
the scientists found thousands of fossilized tree seeds and
abundant petrified wood. There were also some 600 specimens
from other animals, including such forest dwellers as monkeys,
kudu antelopes, bats and squirrels. Notably rare were fossils
from grassland beasts like prehistoric horses or giraffes. The
theory that ramidus was a forest dweller is still not proved,
but if it is supported by more fieldwork and analysis, then
theorists will have to form a new explanation for the development
of erect posture by some apes.
</p>
<p> While the evolutionary story is still in some doubt, there is
no question about the fossils' antiquity. Ancient bones cannot
be dated directly, but geochronologists proceed by determining
the age of nearby rocks. It also helps if the fossils have lain
undisturbed since they were buried. In this case, the ramidus
bones could not have been better placed: they were enclosed
in sedimentary rock that was neatly sandwiched between layers
of volcanic ash, which contains radioactive isotopes that make
material easy to date. The volcanic layer just beneath the fossils
turned out to be about 4.4 million years old. That jibes perfectly
with the ages of other fossil animals found, which were already
known from analysis of other sites.
</p>
<p> Scientists are debating whether the new hominid is really the
common ancestor of both humans and apes, whether it's the first
species to appear on the human side after the split or whether
there are still several pre-ramidus hominids left to be found.
Liverpool's Wood thinks White, Suwa and company may have discovered
the seminal species. The man who found Lucy in 1974, paleontologist
Don Johanson of the Institute of Human Origins, based in Berkeley,
California, disagrees. "I still think we're a long way from
the common ancestor," he argues. "We're one link closer, just
as Lucy was a link closer. There could be room for several more
species."
</p>
<p> All these issues--bipedalism, the forest-dwelling theory,
the question of how high ramidus sits in the evolutionary tree--can be settled only with more fieldwork. The team is returning
to Ethiopia next month, to the site, hoping to find parts of
other skeletons and uncover more clues about the Ethiopian environment
of 4.4 million years ago. Says White: "We're going to crawl
on our hands and knees, looking for every giraffe, pig, bird,
rodent, seed and any other fossil we can find." Humanity has
just added half a million years to its heritage; perhaps the
next expedition will give scientists a better idea of how much
