Freshwater locked in the ice sheets caused global sea level to stabilize approximately 120 m below its present level (e.g., Clark and Mix, 2002).

For thousands of years, the Bering Land Bridge existed as a broad treeless lowland connecting eastern Siberia and Alaska (Hopkins et al., 1982).

For example, how long it took for innovations like pottery and milking to become widespread practices, and whether these were adopted in specific revolutions or by a slow constant stream of change.

"Key questions such as how the earliest settled communities were organised, and how they responded to climate change, can be addressed.

By isolating carbon-14 in the pulp cavity of the ivory, experts can compare the amount of the isotope found to the carbon’s abundance at a certain time, and determine how long ago an elephant died.

When the US and Russia engaged in a barrage of nuclear testing during the 1950s and 1960s, they almost doubled the amount of carbon-14 globally.

But for humans whose life span rarely reaches more than 100 years, how can we be so sure of that ancient date? Even the Greeks and Romans realized that layers of sediment in rock signified old age.

Paleoindian hunter gatherers – the first humans to enter the New World – moved across the land bridge, or along its coasts, into North America about 14,000-12,000 years ago (Dixon, 2001; Yesner, 2001).

As the world's glaciers and ice sheets melted at the end of the last glaciation, rising sea level inundated the land bridge.

But it wasn't until the late 1700s -- when Scottish geologist James Hutton, who observed sediments building up on the landscape, set out to show that rocks were time clocks -- that serious scientific interest in geological age began.

Before then, the Bible had provided the only estimate for the age of the world: about 6,000 years, with Genesis as the history book.