Recent African Find

                          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.