英语听力：冰河绿洲—8

But how did one man and a spear produce the huge force behind this blow? The answer lies with small bits of ivory like this. They were once part of an atlatl, or spear thrower. An atlatl acts as a sort of catapult, magnifying the strength of a hunter's throw, allowing him to launch a spear up to 200 metres, with enough power to drive the point through a bison skull. It's this kind of sophisticated technology that helped these early North Americans to spread throughout the entire continent.

But what else did they find once they reached Florida? Florida's ice age wildlife was remarkably rich and diverse. There were many familiar animals, normally found further north, but also creatures unique to the tropics. The result was a mixture of species unlike anything we see today and an abundant food source for the human immigrants.

So what produced ice age Florida's wealth of wildlife? Part of the answer comes from the ice itself.

At the peak of the last ice age, massive glaciers up to 2 miles thick covered over half the North American continent. The ice destroyed the habitat over which it lay, but it also had a profound impact on regions far away. It created a domino effect that rippled down the continent.