Who were the first human beings to populate the New World?

The answer to this long-debated question may lie within the genes of Americans living today. Genes with an ancient history, copied and recopied with each new generation, are being decoded in the hope of solving the mystery.

These telltale genes are found not in the cell's nucleus, where the 46 chromosomes reside, but in mitochondria, sausage-shaped structures that serve as the cell's power plants. Unlike chromosomal genes, which are shuffled and reshuffled with each generation during the union of sperm and egg, the handful of genes carried within mitochondria are passed on unchanged from mother to daughter, providing researchers with a much clearer record of a population's genetic history.

Emory University physician and geneticist Douglas C. Wallace heads a team that has analyzed mitochondrial genes from the blood cells of modern Indians living throughout North and South America and has compared them with mitochondrial genes from people living in Asia. They found four rare and distinctive genetic markers in Asians which also occur in modern American Indian tribes, including the Mayas who settled the Yucatan peninsula, the Pimas in the southwest United States and the Ticunas who populate the Brazilian rain forest.

The fact that these unusual genetic markers show up in widely separated modern tribes "supports the idea of common original ancestors" who probably moved from Asia to America during a single, major migration, said Wallace. "If there had been hundreds of migrations" by separate, genetically unrelated groups, "you would not expect these rare markers to appear at high frequency in the Americas." One Migration or Many?

The new findings, published earlier this year in the American Journal of Genetics, may help to settle a long-standing dispute among anthropologists over whether the paleo-Indians, the earliest New World migrants, arrived in one major migration or in hundreds of waves between 12,000 and 14,000 years ago.

Anthropologists whose data come from more traditional sources, such as bones, artifacts and languages, have long believed that the New World was settled by many separate migrations. It is thought that the settlers crossed a 1,500-mile-wide land bridge connecting Siberia with Alaska, the gateway to the New World.

Today, there are hundreds of native American languages, a fact that many linguists say supports the notion that there were hundreds of separate migrations to the New World. In this view, each different language could have been brought by a separate wave of immigrants. Clues in Language, Teeth

But other experts disagree, including Joseph H. Greenberg, professor emeritus of anthropology at Stanford University. Greenberg believes similarities among Native American languages support the view that there were three key migrations to the New World.

According to this theory, the first and largest migration was composed of a group of paleo-Indians from Siberia who moved into Alaska about 12,000 to 14,000 years ago and then spread throughout North and South America. They were the ancestors of most of the Indians populating the New World.

Two smaller migrations followed about 2,000 years later. One of these later groups was the Na-dene Indians, the forebears of some Alaskan and Canadian tribes and of a few tribes of the southwestern United States, including the Apaches and Navajos. The second group gave rise to the Aleut Eskimos, who moved northward through Alaska toward the Arctic Circle and eastward to populate Canada and Greenland.

Clues from more than 32,000 fossil teeth now in museums lend support to the theory of a single major paleo-Indian migration. The size and shape of teeth are genetically determined. Arizona State University anthropologist Christy Turner II said evidence from ancient teeth suggests there was one paleo-Indian migration rather than many. And now, Wallace's study of mitochondrial genes points to the same conclusion.

What the genetic findings do not reveal is exactly how the paleo-Indians moved into the Americas. They could have arrived in several ways, Wallace said. Perhaps a small number of Siberian tribes walked across the land bridge and then quickly fanned out to populate the vast spaces of the New World.

"Or they may have filled Siberia and then migrated across the land bridge in a larger group," Wallace said. But he added that, in any case, the genetic evidence suggests these early settlers from Asia were all closely related.

Using mitochondrial genes to solve anthropological mysteries is a relatively recent idea. It wasn't until the 1960s that scientists learned that mitochondria contain their own genes. A decade later, they discovered that mitochondria break the fundamental rules of Mendelian genetics. Mendel taught that every individual inherits equal numbers of genes from each parent. But because sperm cells contribute no mitochondria to a new being, the genes carried by the mitochondrial DNA are passed on only by mothers through their eggs.

As a result, mitochondrial DNA preserves an unbroken record of genetic transmission spanning thousands of generations of mothers and daughters. By comparing information from mitochondrial genes found in widely scattered populations living today, scientists can deduce information about whether such groups originated from a common ancestor, and about how long ago that individual lived.

Although other researchers are intrigued by Wallace's evidence supporting a common Asian origin for many American tribes, they are still debating the timing of the migrations. "It's much more complex than what any of these researchers are trying to make it," said archaeologist Dennis Stanford, who heads the paleo-Indian project at the Smithsonian Institution. Stanford agrees there may be evidence that native Americans arose from one general gene pool in northeast Asia. "But when that gene pool developed and when people moved out of that area is unanswered by any of these studies," he said. Not an Easy Journey

Researchers do agree that immigrants who made the crossing from Asia were hardy people who faced harsh conditions and great adversity.

"The land bridge was probably like the steppe country of Mongolia today," Turner said. "It was just terribly, terribly cold in the winter and fairly dry and hot in the summer."

The ancestors of the Aleut Eskimos, who settled the Arctic region, "were very, very sophisticated," Turner said. They had a "whole series of technological innovations for which we have no records." Most perishable material, such as crafts, hides, bone and wood were too fragile to survive thousands of years.

Scientists also believe it was man's best friend -- the dog -- that provided the means for humans to settle the New World. New research presented last month at an international conference in Kyoto, Japan, suggests that dogs were domesticated in Siberia by the paleo-Indians. Dogs provided transportation, protection, and in an emergency, could be eaten.

Anthropologists like Turner are elated by Wallace's genetic findings. "I have long argued that there were three migrations to the New World," Turner said. "This theory matches up very well with archaeological and linguistic information, and now we can add Doug Wallace's fascinating information."