The most complete skeleton of an early human ancestor ever found has been dug up in Kenya. The bones are those of a surprisingly tall 12-year-old boy of the Homo erectus species who died about 1.6 million years ago.

The specimen is among the earliest of this species, which lived in Africa and Eurasia until giving rise to an archaic form of Homo sapiens about 400,000 years ago. The completeness of the skeleton is unprecedented in a field where discovery of a few teeth or scraps of bone is cause for celebration. The new skeleton is missing only its left arm, lower right arm and most of the foot bones. It is far more complete than "Lucy," a partial skeleton of Australopithecus, which preceded Homo erectus by about 2 million years.

The new find reveals that these ancient people had bodies virtually indistinguishable from our own and that, at least in some cases, they were as tall as Americans today. The skeleton shows that the boy stood 5 feet, 6 inches, taller than many of today's 12-year-olds. In the absence of well-preserved bones, most anthropologists had assumed that Homo erectus was short and stocky, averaging around 5 feet 4 inches as an adult.

"This is a really surprising thing," said Alan Walker, a Johns Hopkins University anatomist who excavated the skeleton with Kenya's Richard Leakey. "This boy would have grown up to be 6 feet tall. It's a myth that these people were always short. In fact, I think we have to conclude that modern people, at least in some populations, are reduced in size."

The discovery was announced here yesterday by Walker and the National Geographic Society, and simultaneously was described in Nairobi by Leakey, director of the National Museums of Kenya.

From the neck down, the bones are remarkably modern in shape. "It looks so human," said Walker, who teaches anatomy to medical students. "I'm not sure whether the average pathologist would notice any differences from a modern human. There are some subtle differences, especially in the thigh bone, but I'm not sure yet what they mean."

The skull and jawbone, by contrast, are more primitive in appearance. This fits the generally accepted view among paleoanthropologists that different parts of the human body evolved at different rates, the body achieving fully modern proportions long before the head. The brain size of the new specimen has not been measured, but Walker estimated it between 700 and 800 cubic centimeters -- a little more than half the modern average.

The skeleton was found on the west side of Lake Turkana in Kenya's northwest corner. Most of Leakey's earlier finds have come from the east side of the lake.

The first piece came to light in August when Kamoya Kimeu, leader of Leakey's team of fossil hunters, spotted it on the ground just yards from base camp. It was a small piece of skull, the sort of scrappy fossil that all too often leads to nothing more.

"Kamoya showed it to Richard and me," Walker recalled. "We were not impressed and left to work at another site." When that dig proved disappointing, the two returned to the base camp. "It wasn't long before we got pretty excited," Walker said. "We had never seen anything like this before."

Digging continued until Sept. 21, when Walker had to return to teach at Johns Hopkins. There is hope that further excavation next year will yield the missing bones.

Geological evidence shows that the boy's body originally lay on the clay edge of a swampy area near a river. In the same deposits there were fossil remains of swamp snails and catfish, and natural casts of the roots of swamp grasses. There is no evidence as to how the boy died; no sign of disease or injury has been found on the bones, although many were broken after death. Preserved footprints suggest that hippopotamuses and other animals trampled the decomposing corpse, breaking the bones and pressing them into the clay that preserved them. After excavation the bones were still encrusted with mineral deposits. Only a few have been cleaned enough that the pieces can be fit together. Most of these were the 70-odd fragments into which the skull had been broken.

"When I put the mandible onto the skull," Walker recalled, "Richard and I both laughed because it looked so much like a Neanderthal."

This resemblance to the much later race of Homo sapiens supports a theory that human beings evolved through a process called neoteny, an evolutionary phenomenon well known among certain lower species such as amphibians. Adults of so-called neotenic species resemble the juvenile forms of their evolutionary ancestors. The adults are, in fact, developmentally arrested forms that have, nonetheless, gone on to mature sexually.

Because baby chimpanzees look much more like modern human beings than do adult chimps, some anthropologists have speculated that the human skull evolved from an apelike form simply by prolonging the growth stage appropriate to a young ape. This is when the brain grows fastest and, theorists suggest, evolution may have favored neoteny because it allowed the brain to grow larger before adulthood stopped the growth.

Neoteny is by no means established as a mechanism of human evolution but, if true, it would account for the fact that a 12-year-old Homo erectus would look more advanced -- more like a Neanderthal -- than an adult Homo erectus.

The age of the skeleton at death was estimated on the basis of its teeth. It had the combination of baby teeth and permanent teeth appropriate to a 12-year-old of today. Its antiquity was determined from its location, sandwiched between layers of volcanic ash that can be dated by measuring the products of radioactive decay.