Archaeologists have long accepted the theory that the ancestors of modern Indians came to the New World from Asia in two waves of migration. But an Arizona State University anthropologist suggests that there was a third wave of migration, and her theory is based on evidence of the hardest sort - teeth.
Christy Turner has spent 20 years inspecting thousands of human teeth from archaeological sites in Asia and the Americas to determine the origins of the earliest settlers of the New World.
Teeth are excellent subjects of study since they are the hardest part of the body, and are less affected by the ravages of time. In addition, teeth have very distinctive features that change little from generation to generation.
Dr. Turner has identified 30 distinct features of teeth, including ''shoveling,'' the inward curl of the edges of Asians' incisor teeth, and the number of roots on molars - the first bottom molar usually has two roots in Europeans, and three roots in Asians.
After examining thousands of teeth, Turner has gathered enough data to determine the original homeland of the ancestral Indians, and to propose his controversial third wave of migration.
Based on ancient stone artifacts and blood types of modern Native Americans, anthropologists have theorized that the original settlers of the New World came over the then-dry Bering Strait in two waves.
The first group, the Paleo-Indians (''Old Indians''), arrived in North America between 15,000 and 12,000 years ago. By comparing teeth found in the New World to those found in the Old World, Turner has traced the origins of the Paleo-Indians to the Lena River in northeast Asia.
The early Paleo-Indians were big-game hunters, who followed the migrating herds of mammoths up the Lena River basin and across the northern part of the Bering land mass into a world untouched by man, according to Turner.
''If ever there was a Garden of Eden on Earth,'' Turner says, ''it was North America about 12,000 years ago. Food was abundant and there were no people here, so animals had not learned any fear of man.''
Descendants of these people spread quickly in the Americas and became most of the Indian peoples associated with the New World, from the Mohawks of New England to the Inca empire builders of South America.
Some 9,000 to 10,000 years ago another group crossed the land bridge and settled in the Aleutian Islands and northern Canada to become the Aleuts and Eskimos of today, Turner says.
Dental evidence links the ancestors of the Aleut/Eskimos to prehistoric people living by the Amur River in northern China, Turner says. Those people moved across the southern expanse of the Bering land mass, where some remained in what became the Aleutian Islands, and some continued west into modern Canada, he adds.
Between those two waves, Turner theorizes another distinct group moved into the New World about 10,000 years ago, forming what he calls the Na-Dene (nah-din-NAY) people. Na-Dene descendants currently live in the interior of Alaska and along the Northwest coast from around Seattle to Prince William Sound. Isolated pockets of Na-Dene are also found in northern California, and constitute the Navajo and Apache tribes of Arizona.
The teeth and tools of the Na-Dene seem to be a mixture of Paleo-Indian and Aleut/Eskimo features, Turner says. Anthropologists who dispute Turner's theory of a third wave say the Na-Dene people were formed by intermarriage between the Paleo-Indians and the Aleut/Eskimos after they migrated into the Americas.
But Turner points out that teeth change very slowly - much too slowly for a distinct group to have formed in the New World. It is simpler to place the mixing of Paleo-Indians and Aleut/Eskimo ancestors in Asia and postulate a third wave of migration, he says.
Turner has traveled to the Soviet Union twice to study ancient teeth, and has received ''outstanding'' support from Russian anthropologists who have worked closely with him on his research, he says. He is planning another trip this summer to study skulls in collections throughout Europe, the Soviet Union, and China. The two-year project is being sponsored by the National Science Foundation. Turner has also received support from the National Geographic Society.