Missing Links

man stares at skull

Standing next to the mounted cast of the boy's skeleton, his hand on his hip, Alan Walker looks a bit smug.The skeleton comes up to Walker's chin. Knock-kneed and complete with a toothy smile, the five-foot-four-inch "Nariokotome boy" (as Walker and the skeleton's other discoverers named him) looks fully human. Only after years of study did Walker, a leading paleoanthropologist, realize that Nariokotome boy is not "one of us."

What he lacks, says Walker, is imagination, for which the catalyst is language. "Once people began to speak, they could imagine the future and remember the past in a very different way, and they could pass that knowledge on," Walker says in a crisp British accent. He crosses his arms and rests them on his untidy desk. "Language is the tool by which we make abstractions, and it's imagination that changes the world we live in.

"All animals can think. My cat thinks, even my wife's dumb horse thinks, but they don't think about abstract things—they don't think about next Tuesday. We have lots of abstract thoughts because we can communicate them through language."

Anthropologists have just begun to realize that human language is very late in evolution, maybe as late as 50,000 years ago. This idea overturns the long-standing view that hominids could speak over two million years ago, when they first started making tools. "This new theory explains what has been called the 'creative explosion,'" Walker says. "I mean, humans made relatively little difference on this planet until 50,000 years ago. Then, with abstract thinking, like that expressed in the earliest paintings, humans began to view themselves in the world." After the development of language, humans started to travel, to share ideas, and to change their environment—actions that continue with modern humans. It is these characteristics, Walker believes, and not strictly anatomical ones, that define what we mean by "human."

man holds stares

The "missing link," the fossil that would show definitely when humans separated from apes, has been sought for centuries. Most early paleoanthropologists thought they were looking for something similar to a character from The Planet of the Apes: a large ape, barely sturdy on his new bipedal legs, knuckles still dragging, with a large brain. Basically a human brain, a mind capable of contemplating abstract ideas, stuck in an ape's clumsy body. This "brain-first" theory of evolution was so popular that it sustained the famous Piltdown skull scandal for decades. This fossil of an ape-like jaw connected to a human-sized cranium was a puzzlement. It supported scientists' theses of big-brained ape ancestors, even though it was the lone specimen with these features. Years later, radiocarbon dating proved a prankster had tricked the scientific community by fabricating the Piltdown skull out of an ape's jaw, properly stained and fitted to a fairly modern human cranium.

The discovery of the hoax left a hole in human evolution. Anthropologists had to re-examine their theories of how hominids had changed through time. Though some scientists retained their old biases toward "brain-first" evolution many more doubted the theory—and they went looking for proof. But the possibilities for the missing link had broadened: Was it a human-like mind in an ape body? Or a smaller cranium with human anatomical features? For Walker, the search for answers began with the discovery of Nariokotome boy in the Lake Turkana region of Kenya in 1984.

skeleton

It was a Sunday, a day when the excavation team led by Walker and Richard Leakey (the son of Louis and Mary Leakey) rests, writes letters home, and does its laundry. Everyone rested, that is, except Kamoya Kimeu, a real hominid-fossil hound dog, who was walking near camp when he found a piece of skull. Blending perfectly with the black lava pebbles, the fragment of frontal bone was only the size of a matchbook. It would take the team five years to excavate the rest of the skeleton—the most complete Homo erectus ever found and a "find of the century," according to paleoanthropologists worldwide.

As they were found, the fossil bones were passed from the huge excavation pit to a wooden table that was shaded from the desert sun by a thorn tree. There Walker and Meave Leakey (Richard's wife) hunched over the fragments, a collection of tiny tools, and some glue. Forty percent of the skeleton of this 12-year-old boy was found. And as the remains of this 1.5-million-year-old hominid made their way to his table, Walker's own imagination proved to be his finest tool.

"I like to think of myself as a paleoanthropologist with binoculars," Walker says, "as though I am sitting there in the past looking at the animals, rather than at some old bones. I picture myself there.

"If you look across the desert you see a mass of waving grass, big animals like elephants, giraffes, antelopes—just like you'd see in Africa today. Then, alongside the river, you see a huge forest and these Homo erectus walking in the distance. You think they are humans; from a distance you can't tell. Then when you get close, you see they are very different. They are a completely different species, and you seem to have nothing in common, even though you both walk upright. They have tiny heads; tall, muscular, very dark bodies; and are probably very sweaty."

What would happen next? Walker wonders. Would the Homo erectus attack or run? Would they be able to interact with the anthropologists? Would they make some signaling sounds to each other, what Walker calls "vocal communication with specific meaning," as some monkeys can? Or, of course, would they speak?

"We know language must have happened in the human lineage," Walker says, as he cracks open the fresh, ivory-colored pages of a book that takes up most of his desk top. He points to a branching evolutionary diagram. "People have suggested language came with the development of stone tools, for teaching purposes. But I think if you wanted to teach someone how to make a stone tool the only words you would need would be, 'Watch me.'"

As the pieces of Nariokotome boy came together, Walker started to question the early acquisition of language. He wondered if language came later than anthropologists had thought. Homo erectus is associated with the stone tool technology known as Acheulian. The primitive tools date back possibly as far as 2.4 million years. If language developed to pass on the Acheulian culture, Homo erectus must have been capable of speech. This theory leaves an uncomfortable gap of at least 200,000 years between the extinction of the species Homo erectus and the "creative explosion" marked by the beginning of cave paintings 50,000 years ago. Walker began to suspect the theory was wrong, but he needed proof.

Nariokotome boy's 1.5 million-year-old skeleton was so well-preserved, Walker was sure it held the key. He knew he couldn't find "imagination" in a skeleton, but a good anatomist, he thought, should be able to find evidence of language.

A piece of vertebra with a nerve canal much smaller than a human's was the clue Walker needed. When modern humans speak, they control how much breath is expelled to make each sound of each word in a sentence. This breath control requires the muscles on the chest wall to develop in a very specific way—one that varies depending on the language spoken. Fine control of these muscles is necessary to push the air from the lungs through the larynx. The nerves that control this muscle action come out of the spinal chord. Without enough of these nerves, humans would not be able to breathe and speak at the same time.

"I noticed when we found the vertebra," says Walker, "that the canal was too small to contain these nerves. The size of the canal is a lot closer to that in apes than in humans. Not enough nerves were connected to the muscles to control the breath and sustain a sentence." Nariokotome boy was speechless. Thus Homo erectus was very different than modern humans, despite the deceiving human-like appearance of skeletal remains like Nariokotome boy. Just as when the Piltdown hoax was uncovered, Walker's examination of Nariokotome boy would change much of what scientists thought they knew about the missing link.

I knew from the age of eleven that I was going to be a paleontologist," says Walker, who grew up in Leicester, England. "I was always riding around on my bike out in the countryside near my home finding fossils. I took them to my teacher and asked him what I'd found. It got to the point that he didn't know anymore and sent me to the library." He chuckles and runs his hand across the top of his thick, curly hair. "I then became a museum brat, hanging around the Leicester Museum and pestering people. Eventually I was given a position as a very lowly paid field collector. I had two bags and a carrier on the back of my bike. I put my tent on the carrier and the two bags could be filled with fossils. I'd go out and camp overnight finding fossils for the museum."

Walker was awarded a grant for graduate study at London University in 1962. His independent learning style fit in perfectly with the British system—graduate students didn't take classes, but instead worked solely on research with a professor. If a deficiency was discovered in a student's education, he or she was sent off to sit in on a few classes. Walker admired his mentor, John Napier, who had hurriedly been trained to be a hand surgeon just before World War II and had later focused his research on the evolution of the human hand. "No one had a better experience as a graduate student than I did," recalls Walker. "John had this gift of making everything he was doing—and therefore everything you were doing—the most important thing in the world that day. Often I think I don't have that magic touch with my students. With him I couldn't wait to get to work to see what we were doing that day."

Walker began making detailed studies of the anatomy of lemurs, small monkey-like animals from Madagascar. Then National Geographic published the exciting tales of hominid discoveries by Louis and Mary Leakey at Olduvai Gorge in Tanzania, and human evolution became a hot field. Walker found himself in the perfect place for discovery. In 1965 he made his pilgrimage to Africa. "When I first went to Africa, I went on a boat," he remembers. "We chugged through the Mediterranean, through the Red Sea and on to Mombasa." Although he intended to continue studying lemurs, the call of the missing link proved too enticing and Walker found himself wandering into Louis Leakey's territory. At first Leakey sought to get the young pest out of his way, but with his training in primatology and his sharp eye for anatomy, Walker soon became an asset to Louis's son, Richard Leakey, whose team was working on a Homo habilis discovery in northern Kenya. Thus began Walker's life-long relationship with the Leakey family.

sands with green bushes

After years of work and countless discoveries in Africa, relocation to America, a series of professorships at medical schools including Harvard's and Johns Hopkins', Walker finally settled at Penn State in 1995. Since then he has been working with Meave Leakey on some recent examples of Australopithecus, the oldest known human ancestor, found near Lake Turkana. Walker and Leakey are looking for another missing link—the point where humans and chimps began evolving independently. Walker estimates it happened about 4.5 to 5 million years ago. "Meave Leakey and I have been looking for the oldest Australo-pithecus we can find, which up to now is about 4.2 million years old." The search seems endless. In a field like paleoanthropology, Walker says, most of the links are missing.

Walking over the African desert looking for fossils, a naturalist like Walker might notice snakes or insects, but really he's there to think. How do the fragile bones fit together to create something bigger? How do they reflect human lineage and explain our species? And why does it matter? For Walker the expeditions are motivated by problem-solving: finding an anatomical feature and figuring out why it evolved that way and when the change occurred in time.

"We're made of a mosaic, some old bits and some new bits," Walker says. "What I'm interested in is what order they came in. Did bipedalism come first, or big brains, or larger spinal columns? And when did certain behaviors, like language, evolve?" For Walker the discovery of Nariokotome boy was a milestone in his quest to understand the mysteries of human evolution, because it forced him to test and ultimately contradict our long-standing views about language and the origin of the genus Homo.

Walker now stares at the stony face of Nariokotome boy, not with a smug grin, but with a look of reverence for what he has been taught. He squints his eyes to imagine the dark, muscular body and the blank unreadable eyes that would stare back. Nariokotome boy is not an ape and he's not a human. When he came walking across the grassy plain and saw the anthropologist there with his binoculars, he had nothing to say. And it was his silence that told Walker Nariokotome boy was truly his own magnificent being and nothing at all like us.

Alan Walker, Ph.D., is distinguished professor of biology and anthropology in the College of the Liberal Arts and the Eberly College of Science, 315 Carpenter Building, University Park, PA 16802; 814-865-0796; axw8@psu.edu . His research is funded by The National Geographic Society and the National Science Foundation.

Last Updated September 01, 1998