Archaeologists agree that by 11,000 years ago, people were spread across North and South America, but evidence is building for an earlier entry into the New World, a date that would put human population of North and South America firmly in the Pleistocene.

"We want to know what it was like back then," says Graham. "What did they have to deal with?"

The Pleistocene Holocene transition took place about 11,000 years ago and caused the extinction of a large number of animal species including mammoths, mastodons and ground sloths. The Holocene looked very different from the Pleistocene.

"We now realize that climate changes extremely rapidly," Graham told attendees at the annual meeting of the American Association for the Advancement of Science today (Feb.19) in St. Louis, Mo. "The Pleistocene to Holocene transition occurred in about 40 years."

As a result, animals and plants shifted around and the people living in the New World had to adapt so that they could find the necessary resources to survive. Graham likened the change to the difference between shopping at a WalMart where there is great abundance and large variety � the Pleistocene � to suddenly having to shop at a corner convenience store � the Holocene. In human terms this means that what grandparents knew to be true about finding resources, could be untrue and not helpful to grandchildren.

During the Pleistocene large eastern coastal resources existed, including walruses, south, as far as Virginia, seals and a variety of fish. Mammoth, caribou and mastodons were plentiful across the continent as well as smaller animals. The situation was not identical in all places across North America because, during segments of the Pleistocene, large portions of the Eastern North American continent were covered in ice, while western locations were ice free much further north.

"The Holocene climate is much more stable than the Pleistocene � warmer but more stable," says Graham. "The environment, however, became more homogeneous, there was less variety."

Graham argues that the Pleistocene experienced a series of rapid climate changes that created patchiness in the environment, but that once the climate change that signaled the beginning of the Holocene occurred, the climate settled down. Humans coming into the New World during the late Pleistocene would have encountered an environment shaped by rapid changes creating variety in available food sources both animal and vegetable. The groups of people would have to adapt continually and find new resources, but the variety of resources was out there. After the Holocene took hold, there was less need to adapt constantly, but also fewer options in resources.

Archaeologists and geologists debate whether the climate change at the Pleistocene Holocene transition caused the extinction of the mega fauna or if the influx of humans did in the large animals. Graham believes that it was the unstable changing rapidly changing climate, not human predation that killed the large Pleistocene animals.

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