Donald Johanson is always looking at the ground.
“I find more quarters by parking meters than anybody I know,” he says.
As he was looking at the ground four decades ago, in a region called Hadar, named for a dry riverbed in Ethiopia, he saw something a lot more exciting than a quarter. It was a fossil bone.
“I found a little piece of elbow,” he said last week in Columbus, Ohio, while addressing a conference of science writers. “And I knew from studies of osteology and comparative anatomy that this had to be from a human ancestor.”
Next month paleoanthropologists will celebrate the 40th anniversary of Johanson’s discovery of the elbow bone on November 24, 1974. In the intervening four decades, many more fossils along with other clues have been discovered, rewriting the story of the human race. The evolution of earlier humanlike species and eventually modern humans has grown from the outline of a play with a small cast to an elaborate production with more characters than an Agatha Christie mystery, many remaining enigmatic with relationships still unclear.
“These fossils tell us a great deal about who we are, where we come from and how we fit into the natural world,” said Johanson, of the Institute of Human Origins at Arizona State University. He summarized the fossil story in delivering the annual Patrusky Lecture, honoring Ben Patrusky, emeritus director of the Council for the Advancement of Science Writing.*
The prelude to the modern story goes back to 1856 — three years before Charles Darwin published his treatise on evolution by natural selection — with the discovery of a fossil in Germany’s Neander Valley in Germany. That fossil was the first specimen from Neandertal man. A dozen years later, fossils of Cro-Magnon man turned up in France.
But Neandertal and Cro-Magnon turned out to be relative youngsters in human history. Darwin and his champion, Thomas Henry Huxley, knew that the whole story would get much more complicated.
“Darwin knew that the tree for evolution of any species had to be a branching tree, with lots of false starts, mostly dominated by that grim reaper of evolution, extinction, and survival of very few branches,” Johanson said. “He and Huxley made the specific prediction … that on the basis of similarities in teeth and jaws and anatomy, of us and particularly chimpanzees, the earliest ancestors for humans would be discovered in Africa.”
Sure enough, Raymond Dart found “Taung Baby” (official name Australopithecus africanus) in South Africa in 1924. It was probably about 2.5 million years old, vindicating the predictions by Darwin and Huxley of an African origin.
From then until the late 1950s, “discoveries in southern Africa pretty much dictated our view of the earliest humans in Africa,” Johanson said. “This shows you how evidence from one particular region or place can have such a long-lasting influence on our understanding of a question such as where we come from.”
But then in 1959, Mary Leakey found the massive cranium of the 1.8 million-year-old Nutcracker man (originally called Zinjanthropus, now known as Paranthropus boisei) at Olduvai Gorge in Tanzania, near the edge of the Serengeti Plain. That discovery turned the spotlight from south Africa to east Africa, Johanson said, leading to decades of advances in understanding human origins.
Lucy, whose species was designated Australopithecus afarensis, was the first human ancestor found to exceed 3 million years in age. She walked upright, a clear step along the lineage leading from apes to humans. But as subsequent work showed, just as Darwin had anticipated, the line from apes to humans was not straight, but branched. Today the mapping of those branches is still being worked out.
By 1970, when Johanson entered the hunt, paleoanthropologists had identified seven prehuman species: three of the genus Australopithecus, and four of the genus Homo: erectus, neandertalensis, habilis and heidelbergensis. Today there are more like 15 to 20 or so species in the mix, and Johanson alluded to three species that experts are now evaluating that may be announced soon. “I suspect there will be even more,” he said. “This is a reflection of the ingenuity of natural selection and evolutionary change — that it is not a single line from ape to angel. There are many extinct branches.”
Today the human family tree extends back more than 6 million years, if you count Sahelanthropus tchadensis, although Johanson noted that some experts consider S. tchadensis an ancestor of modern apes rather than as an early branch of the human lineage. Another early prehuman, denoted by the genus name Ardipithecus, is possibly at the root of the human family tree, but various considerations have made this conclusion controversial, Johanson said. It’s not certain, for example, that Ardi really walked upright. “There are many people who think that Ardi is an interesting example of some morphological change in the direction of humanness,” Johanson said. But whether or not it was the human ancestor, we don’t know.”
Lucy’s species, on the other hand, indisputably walked upright, which was what made her discovery so exciting 40 years ago, even though she had an apelike brain and jaw.
“This was an incredibly important and dramatic discovery in the mid-1970s that caused many of us to step back and take another look at the family tree,” Johanson recalled.
As more fossils from Lucy’s genus turned up in the region, other new species were identified, including Australopithecus anamensis and Australopithecus garhi, allowing studies of the relationships among them and other early prehumans.
“It is quite clear that there is a single lineage in East Africa that goes from anamensis at 4.2 million (years ago) to afarensis (Lucy) to things like garhi,” Johanson said. “We’re holding open the possibility that Ardipithecus was the ancestor to Lucy.”
In any event, Lucy clearly occupies “a pivotal place on the family tree, giving rise to various forms of Homo that have through the whims and caprices of evolutionary time evolved ultimately into us, Homo sapiens, supposedly ‘wise man,’” Johanson said. “Although I wonder if that’s the right moniker for us.”
Now Johanson and others are busy trying to fill in the details in the rest of the story, such as how factors like climate and diet led to the cognitive complexity, linguistic flexibility, capacity for culture and cooperation that characterize the modern human race. To Johanson, this story is not just about the need to know where we came from, but to understand where we are going.
“I’m convinced that understanding our evolutionary journey is going to play a role on our future — our future survival as well as the survival of all creatures on this planet,” he said.
“We know we have a united past. We know that we are the same species, we know that we have inherited the same capabilities. And I think hopefully this is going to lead to a world in which we’re more responsible to the natural world — the natural world that ultimately was our creator on this planet….
“We view the world with minds of hunter-gatherers who were intimately tied to Mother Nature. And this has, I think, brought about an imbalance between who we are and where we come from. And I think it’s time for the species, the most powerful, the most destructive and at the same time the most creative species on planet Earth, Homo sapiens, to stop acting as if there was some other place for us to move to.”
*Disclosure: I am an officer of CASW.
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