Newsweek
                       October 3, 1994
                              
                Out of Africa, a Missing Link

Science: The fossil of our unknown ancestor is what
researchers expected. Its lessons aren't.

BY SHARON BEGLEY

SCIENTISTS DREAM OF DISCOVERing something that no one
predicted, but anyone can make a splash with that sort of
find. Anthropologists who study the origin of humans face a
tougher challenge: to draw dramatic implications from the
expected. Ever since the 1974 discovery in Ethiopia of the
fossil of a little protohuman christened Lucy, it was clear
what sort of human ancestor must have walked the Horn of
Africa just before her: one that was more apelike. Last
week, in the British journal Nature, three anthropologists
announced what Tim White of the University of California,
Berkeley called "the oldest known link in the evolutionary
chain that connects us to our common ancestor with the
[apes]." The fossils look almost exactly as everyone
expected. But their implications for human evolution are
completely unexpected. For the new species suggests that, at
the dawn of humanity, what made us become different from the
apes was a better way of raising a family.
     It started with a tooth. On Dec. 17,1992, as Gen Suwa
of the University of Tokyo walked across the badlands of
Ethiopia in Aramis, 45 miles south of Lucy's resting place,
a glint from the ground caught his eye. "I knew immediately
that it was a hominid"--a humanlike primate, he says. "And
because we had found other ancient animals that morning, I
knew it was one of the oldest hominid teeth ever found. "
Over that winter and the next, the 20-person team uncovered
additional specimens locked m 4.4 million-year-old sediment,
coming up with teeth, arm bones and parts of a skull and jaw
that turned out to belong to 17 individuals. To the
scientists, they clearly represented a new species. White
(codiscoverer of Lucy), Suwa and Berhane Asfaw of the
Paleoanthropology Laboratory in Addis Ababa named it
Australopithecus ramidus. Australopithecus means "southern
ape"; ramidus means "root."
    The diminutive creatures--adults would have weighed 65
pounds and stood four feet tall--are so perfectly positioned
between humans and apes that Darwin must be smiling in his
crypt. Members of the new species, write its discoverers,
"share a wide array of traits with A. afarensis"--Lucy. But
ramidus lacked some of 3.2 million-year-old Lucy's
definingly human characteristics. It had canine teeth
comparable to ancestral apes', for instance, and an
unmistakably chimpanzeelike skull. As paleoanthropologist
Bernard Wood of the University of Liverpool puts it m an
accompanying commentary in Nature, ramidus "lies [extremely]
close to the divergence between the lineages leading to the
African apes and modern humans . . . The metaphor of a
'missing link' has often been misused, but it is a suitable
epithet for [this] hominid." Not that there was much doubt,
but ramidus shows unequivocally that, as White says, "Darwin
was right: humans evolved from an African ape."
     More surprising is how recently that happened. Ever
since Darwin, anthropologists have believed that the lineage
leading to Homo sapiens and that leading to today's African
apes were rooted in a common ancestor. For decades they
believed that our last common ancestor lived 15 million to
20 million years ago. But then molecular biologists horned
in. They compared blood proteins in today's apes to blood
proteins in humans, found them strikingly similar and in
1973 pronounced humans a lot closer to their simian cousins:
the two lineages could not have split more than 4 million to
6 million years ago. (If they had, the proteins would have
diverged more.) This became known as the "Eve" hypothesis,
for the woman who would have been the first humanlike child
of that common ancestor. Anthropologists told these
interlopers to crawl back under the test tubes where they
belonged. But virtually every recent fossil find has put the
naked ape and the hirsute one on ever-closer branches of the
genealogy. Ramidus confirms once and for all that the common
ancestor lived just a little more than 4.4 million years ago
(chart). Humans are barely down from the trees.
     Now the question is, what brought them down? There is
wide agreement that walking upright, on two feet, was the
fateful change that spurred the evolution of humans. It made
possible quintessentially human traits such as tool use
(easier if you're not using knuckles to get around), a big
brain (spurred by use of tools) and families (bipedalism
allowed early humans to carry infants and food). Did ramidus
walk upright? Although the fossil hunters have not yet found
any hip or leg bones that would clinch the case, two other
bones are suggestive. A forward opening for the spine at the
base of the skull and canine teeth smaller than apes' are
both typical of later creatures that walked on two feet.
Moreover, ramidus's arm wasn't built for swinging through
trees.
     If it turns out that ramidus walked upright, then the
implications for human evolution will be dramatic. Aramis
was a thickly wooded flood plain 4.4 million years ago,
filled with the chatter of colobine monkeys, prowling
saber-toothed cats, browsing kudu, pigs and bears. (The
habitat may explain why humanlike fossils older than 4
million years are so rare: in a woodland acid from
vegetation destroys bones before they fossilize.) But the
conventional wisdom holds that our ancestors did not walk
upright--and thus take the first steps toward becoming fully
human--until climate change turned forests into savannas. On
these vast grasslands, say textbooks, natural selection
favored creatures that could get around other than by
swinging from vines. But there's a rogue view, too. "I have
argued that bipedalism could only have evolved in a
protected environment, like a woodland, not an exposed one,"
says anatomist Owen Lovejoy of Kent State University. "On a
savanna, a creature just learning to walk, and therefore
very slow, would have been exposed to predators."
     Sharp Teeth:  Since learning to walk in a carnivore-
filled forest is not exactly a prescription for longevity,
ramidus must have derived a different advantage from the new
posture. A clue to what that might be lies in its teeth. "In
apes, the sharp projecting canine teeth are used as weapons
of threat and display," explains White. But ramidus had low
canines that wouldn't have frightened a kudu. So females
must not have chosen mates based on how convincingly they
bared their teeth at rivals, argues Lovejoy. Instead,
females sought out mates who could help care for offspring;
the best caregivers may well have been those who walked
upright, using their hands to carry food and infants.
According to natural selection, protohumans that became
bipedal would have been more apt to mate, launching their
genes for that trait into the next generation. "Lately,
people have claimed some pretty bizarre reasons for walking
upright, such as reducing exposure to solar radiation," says
Lovejoy. "'This discovery promises to wipe such claims of
the slate."
     The fossil hunters are returning to the field in
November. If further finds confirm that our oldest direct
ancestor learned to walk in the primeval forest, it will
push to center stage a very '90s notion: the crucial spur
toward becoming human was changing from a tooth-baring
aggressive ape to one that carried home dinner and helped
raise the kids.