Perhaps the boy drowned.
The way his bones and teeth scattered led scientists to believe his body floated, face down along the edge of a reedy swamp. After the bones sank, hippos broke some, leaving their footprints.
Eventually silt blanketed the remains, and they rested for 1.6 million years.
The bones fossilized, the swamp became a scorching desert, and the rock that held the fossils eroded.
In 1984, Kenyan fossil hunter Kamoya Kimeu spotted a matchbook-size, dark brown fragment of the boy's skull near the Narioko tome River on the western shore of Kenya's Lake Turkana. He radioed his employers -- Dr. Alan C. Walker, a paleontologist and human anatomy professor at the Johns Hopkins Medical School, and famed paleontologist Richard E. Leakey.
The four-year excavation that followed would unearth a nearly complete skeleton of man's immediate ancestor -- Homo erectus.
The skeleton provides the first clear glimpse of early man's brain and body halfway across what had been a vast gap between the 3-million-year-old Australopithecus skeleton called "Lucy," and the first known Neanderthal burials about 100,000 years ago. Previously, Homo erectus was known mostly by skulls and a few bone fragments from Africa and Asia.
"It remains one of the single most exciting and important discoveries ever made in documenting our evolutionary history," said Dr. Alan Mann, a professor of anthropology at the University of Pennsylvania.
Doctors Leakey and Walker recently published a 480-page book detailing a decade of scientific scrutiny of the Nariokotome skeleton. The book sheds new light on Homo erectus, a species that appeared in Africa about 1.7 million years ago and spread to Europe and Asia. The species evolved to become, or was replaced by, Homo sapiens.
Neanderthal people, who appeared about 200,000 years ago, were Homo sapiens. Truly modern people appeared about 40,000 years ago.
"The Nariokotome Homo erectus Skeleton," (Harvard University Press, $125) brings together the work of 14 ranking specialists in physical anthropology, geology, paleontology, biology and natural history.
Scientists will continue to debate the book's conclusions, Dr. Mann said, "but I find them really an extraordinary advance in our knowledge of what was going on at this time in human evolution. Unless we get infernally lucky soon, this will stand for a long time."
Dr. Richard Potts, director of the human origins program at the Smithsonian Institution, said the research depicts Homo erectus "an amalgam of human and ape-like traits. . . . We are not dealing with the kind of human we thought we were dealing with."
Instead of a nearly modern big-game hunter, "what we see is an individual that is much more a part of the ecosystems he had to live in. For him, it was more a matter of survival [than] domination."
The book reveals surprisingly tall, slender creatures supremely adapted to their rugged, tropical environment. They used simple stone tools and cared for their helpless young, but still were not quite "us."
They probably had no symbolic language, the scientists concluded. And, contrary to a popular notion, these were not "people" you could dress up and pass for modern humans on the street.
"He's obviously got a much bigger face than you have," Dr. Walker said last week, nodding toward a replica of the boy's skull in his office in Baltimore. "And he's only a kid, so he would have grown up with very protruding brow ridges."
The skull slopes sharply back from the brows, like a chimp's, with nothing resembling our steep forehead. Homo erectus was bigger-brained than his ancestors, but two-thirds as brainy as modern man.
Addressing a point that is hotly debated, Dr. Walker noted that the boy's skull has little room for the frontal lobes of the brain -- the seat of abstract thinking and much of what makes us human.
From a distance "you would see there are people there, but when you got up close you wouldn't be able to communicate," Dr. Walker said. "There would be a deadly unknowing between you." The book's contributors conclude from the Nariokotome child's bones that Homo erectus probably had not yet evolved symbolic language.
"When we found the spinal column and were digging it out of the ground, we noticed that the spinal canal was very narrow in the neck," Dr. Walker said.
British biologist Ann MacLarnon found that the channel of the boy's spinal cord through the spine matched a human's at the lower back but was narrower higher up.
She said those spinal nerves on the upper back that have enlarged in humans but not in apes are those that control our breathing as we speak. It is a capacity far more complex and demanding than apes require for their simple vocalizations.
"I had been brought up by my colleagues to believe that human language started 2 million years ago with the advent of stone tools," Dr. Walker said. Now, "I don't believe humans had a spoken symbolic language until very late, perhaps 100,000 years ago."
In their excavations Doctors Leakey and Walker and Mr. Nimeu picked and sifted through 1,500 tons of rock and soil. Slowly, bone by bone and tooth by tooth, they came to know the Nariokotome boy.
Dr. Walker refers to him simply as "the kid." The child's permanent canine teeth had just erupted, and his wisdom teeth were poised to come in. He also had recently lost one of his baby molars, and he badly needed a dentist.
"We think two pieces of the root of the milk teeth were left behind and didn't resorb properly," Dr. Walker said. That appears to have led to gum disease, erosion of the jaw bone and infection.
"That would have hurt. Whether it also led to sepsis [blood poisoning] that killed him, we'll never know."
But the boy was not malnourished. In life, Dr. Walker said, "he was healthy and big for his age."
The Nariokotome boy was 5 feet 3 inches tall. If he were a modern child, his teeth would suggest an age of 11 or 12. But Dr. Mann said scientists have debated how fast early man grew to adulthood.
If Homo erectus matured quickly, as apes do, the Nariokotome boy might have reached early adolescence by age 7 or 8. Others argue that the rate of maturation was like that of modern humans, allowing more time for the acquisition of knowledge and skills for survival.
University of Michigan anthropologist B. Holly Smith compared the dental development of modern humans and apes and found that of the Nariokotome boy fell between them. She concludes in the book that he had lived about 10 years, but would have been fully mature by 15.
That suggests the human trait of slow maturation was well-evolved 1.6 million years ago. "That's a very exciting result," Dr. Mann said.
Had the Nariokotome boy lived to adulthood, researchers believe, he would have topped 6 feet 1.
"That stature is quite remarkable," Dr. Walker said. And it defies common perceptions of our ancestors as having been short.
Thinking the boy simply may have been a big individual, Dr. Walker re-examined earlier Homo erectus fragments and found "we had not appreciated how big they were."
They averaged about 5 feet 7, including females. Only 17 percent of modern males are taller.
"Clearly this population of early people were tall, and fit. Their long bones were very strong. We believe their activity level was much higher than we can imagine today. We can hardly find Olympic athletes with the stature of these people," he said.
Modern man may have evolved a shorter stature after the invention of agriculture, which favors smaller people able to survive periods of crop failure and famine.
Homo erectus' size and vigor, coupled with associated fossil evidence of a grassland environment rich in game, suggest to Dr. Walker that they had learned to hunt or scavenge and thrived on the animal protein.
A far less complete Homo erectus skeleton found earlier showed evidence of bone disease that Dr. Walker said he believes was an overdose of vitamin A -- probably the result of eating the liver of a carnivore. Dr. Walker believes that Homo erectus had probably already evolved the human traits of hairlessness and ,, sweating to cool the body. He probably also was dark-skinned to protect against the equatorial sun.
The Nariokotome boy was extraordinarily slender, at about 106 pounds, with long arms and legs. That reflects an adaptation to hot tropical climates favoring a large skin area and relatively small body volume for efficient cooling.
The same evolutionary reasoning, in reverse, led to stocky builds among the Neanderthals living near Ice Age glaciers.
DISCOVERY BRIDGES EVOLUTIONARY GAP
A fossil skeleton discovered inKenya has given scientists their first head-to-toe glimpse of Homo erectus, showing man's ancestors as they existed almost 1.6 million years ago. The so called "Nariokotome Boy" bridges a gap between the 3-million-year old Australopithecus afarensis skeleton, "Lucky," and early Neanderthal skeletons of 100,000 years ago.