720 migration and population movements: The Americas present levels, so landmasses we would not recognize today remained above water One of these was the sprawling landscape of Beringia, an area encompassing parts of modern-day Alaska and easternmost Siberia This landmass provided an inhospitable yet solid land bridge between Russia and North America Siberian hunter-gatherers on the trail of large prey, such as mammoth, musk ox, and saiga antelope, could have crossed some 60 miles of open tundra into Alaska There, they gradually would have become stranded by rising sea levels They might have settled near Beringia’s rivers for an easy supply of fresh water but would eventually have moved on in search of territories more fertile than this dry scrub land As the ice age ended, much of Beringia became submerged beneath the icy waves of today’s Bering Sea Ocean levels continued to rise The submergence of the Bering Strait (the narrow passage between modern-day Alaska and Siberia) took many generations, so the new settlers would have had an opportunity to return to the Siberia of their forbears It is possible that some did rejoin their countrymen on the Russian side of Beringia, but today’s Native Americans are descendants of the ones who attempted to push farther and farther into the new territory Once they were in North America, their path to the rest of the continent was not yet clear Glaciers still covered much of Alaska and Canada so that these early Americans may have lived for centuries trapped between their submerged land bridge to the north and Canada’s glacial ice sheets to the south In the unstable climate the ice sheets sometimes convulsed violently, occasionally advancing toward the settlers As they did so, the glaciers slowly crushed and enveloped entire forests, diminishing vital habitat and destroying food and shelter Often the ice would suddenly retreat again, leaving a flattened landscape in its wake Even in times of retreat, glacial meltwaters could inundate grazing land and woodlands and cause massive flash flooding as millions of cubic tons of water came bursting out from behind dams of half-melted glacial ice Enormous boulders and jagged chunks of ice tumbled within these floods, smashing everything in their path It was a millennium or more before the two major Canadian ice sheets, the Cordilleran in the west and the Laurentide in the east, parted and permitted the new settlers to enter the rest of the Americas From there the migrants diversified into more than 2,000 societies with more than 1,000 languages and produced some of the greatest civilizations in human history It is apparent from studies of Native American languages and DNA that American settlement began with migrants from Siberia The linguist Joseph Greenberg and other scientists have proved that ancient America was settled not by just one but by several separate waves of eastward migration from Asia Greenberg had been suggesting since the 1950s that Native American languages fell into three distinct families By the 1980s, after many years of collaboration and comparison among several disciplines, scientists were able to demonstrate that North America had been settled in three separate waves: Paleo-Indians had arrived near the end of the last ice age and spawned the majority of native languages, a second group consisting of Na-Dene speakers (the Athabascan family from whom the Apache and Navajo descend) arrived some 9,000 years ago, and just 4,000 years ago a fi nal group arrived speaking a language from which modern Inuit (Eskimo) and Aleut have developed Discovering the exact means by which these later migrants arrived, whether by land or sea, has become a new quest of archaeologists FROM BERINGIA TO CLOVIS By the time they were cut off from their Siberian heartland the ancient Alaskan settlers had already begun to adapt to their new home Different bands of settlers quickly diversified into distinct hunting, fishing, and foraging cultures in reaction to their maritime, tundra, valley, or highland environments As they began to differ from each other, they also differed considerably from their Siberian relatives in culture, technology, and language Anthropologists call these distinct people Paleo-Indians (“ancient Indians”) to distinguish them clearly from their Asian antecedents For some 2,000 years they may have lived confined to northwestern Canada, but as the ice age ended and the Cordilleran and Laurentide ice sheets finally melted, the new settlers were free to issue forth into the vast American landscape The oldest sites in North America south of the ice sheets are located in the western United States The sites of Folsom and Clovis in New Mexico date back as far as 11,000 and 11,500 years ago, respectively At both of these sites stone projectile points from spears and arrows were found among the bones of extinct ice age mammals, indicating that people had been in America at least 10,000 years ago In fact, the finely wrought projectile points had been found between the ribs of a bison skeleton at Folsom and between the ribs of several mammoths at Clovis Thus these ancient “kill sites” contained different prey and slightly different kinds of projectiles but illustrated similar hunting techniques The Clovis and Folsom people appeared to be hunter-gatherers who stalked large prey that would supply them with large amounts of food at a time, which they could eat on the spot or preserve until the next hunt When radiocarbon dating was introduced in the 1950s, almost two decades after the first excavations at Folsom, it was determined that Clovis was the older site All biological organisms absorb carbon isotopes while they are alive Some of these isotopes decay at a steady rate after an organism’s death Radiocarbon (or carbon 14) testing measures the degree of isotopic decay to approximate the year or period of death of an organism Radiocarbon dating is commonly used to fi x the age of artifacts in conjunction with other contextual information, such as location, materials, and methods of manufacture Although varying designs of projectile points