This is CBS, I Am the World. I'm John Bachelors with Professor Dan Flores. The book is Wild New World, the epic story of animals and people in America.

The large predators, the large mammals, mastodons, dire wolves, saber-toothed tigers, removed, opens up what Dan introduces me to call niches.

And those niches have replacements. This is a profound turn for the hunter-gathering groups, societies that spread across North America.

Then the filling of these niches, that's part of a story that's ongoing through all of these epochs that we're talking about.

That there's a balance, and when that balance is disturbed, nature seeks stability again. Is that the way to read this, Professor?

Well, that is certainly a way to read it. I would say that whenever a niche opens where there is, for example, the possibility of grazing on a mits grasslands that once had been home to mammoths and other grazing creatures,

what tends to happen in response, and what happened in North America 10,000 years ago, is that an animal that is left like the bison is going to expand into that niche that's been abandoned by other creatures.

And this is when we see the huge population explosion of the American bison into the millions that we think of in our last four or five hundred years of history.

And it's a smaller animal than the bison that had preceded it in the Pleistocene and hernac stink.

In fact, it's an animal that probably is shaped by the presence of human hunting and predation. So it has anthropogenic origins even in its natural history.

And in that 10,000 year period after the Pleistocene is over, from 10,000 years ago down to the time when old worlders began arriving in America, is this kind of marvelous period of a vastly long time when native people, most of them in the beginning of the 10,000 years hunter gatherers, although agriculture does emerge in the last 2000 years or so.

That 10,000 years or so about 10,000 who spread across North America and develop a set of ecological adaptations and economies that allow them to actually preserve the diversity, though wonderful richness of the remaining animals that it survived the Pleistocene and North America.

And then we'll go down to the time when old worlders arrive 500 years ago, remarkably with only in that 10,000 year period, only one extinction.

There is a flightless sea dot on the Pacific coast that becomes extinct during this 10,000 year period, but that is the sole extinction over 10,000 years.

So it's a remarkable kind of story to know about.

You make the point that the hunter gatherers and there's an illustration and Dan spoke to show you where the hunter gatherers were.

The largest number was either in the Southwest or in the Northeast 715,000 in Northeast 908,000 in the Southwest was still a predator's economy.

And the predation, however, seemed to find a balance because what is the rule, Professor, that populations grow to the limits and beyond of their resources to set the rule?

Well, that's the common story, at least in human history, but what seems to have prevailed in North America over this 10,000 year span is that people understood that, especially with a hunter,

especially with a hunting and gathering economy, what you need is a vast open landscape where wild creatures, birds, reptiles, mammals can survive well.

And so they tend to very consciously try to keep their population levels low enough that they don't stress the populations of the animals they depend on.

So one of the remarkable things about this period of time is that even after the introduction of agriculture, as I mentioned, which is in the last couple of thousand years of this 10,000 years span, the human population in North America never goes over.

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