A couple hundred thousand years ago, the planet became a much colder and drier place. In Africa, deserts expanded, species were wiped out and the human race was in deep trouble.

See, humans today may look pretty different from one another but, genetically speaking, there's not much diversity at all within our species. In fact, chimpanzees, which look pretty much the same from one individual to the next, are much more genetically diverse than we are. To scientists, that suggests that humans have come through a genetic bottleneck–a point where our numbers shrunk dramatically, and a relatively small population had to rebuild the species. For about 20 years, genetic anthropologists have been comparing the genes of modern human populations. Over time, they've used bigger and bigger samples, and better and better analysis, to hone in on when our bottleneck likely happened, and how many humans managed to slip through it.

Turns out, somewhere between 130,000 to 190,000 years ago, the human species was reduced to less than 1000 breeding individuals–just a few thousand people in total. Ancient, naturally driven climate change pushed our species to the brink, said Curtis Marean, Ph.D., a professor with the Institute of Human Origins and the School of Human Evolution and Social Change at Arizona State University.

What saved us? According to Marean, the answer may be "shellfish".

"They're a great source of protein," he said. "And shellfish are immune to colder ocean temperatures. In fact, when the water gets colder, those populations go up."

Marean used climate models to pinpoint locations in Africa where human hunter-gatherers could have hunkered down during a long glacial period that dried out the continent and expanded deserts. Of the four-to-six possible locations, he focused in on an area along the coast of South Africa.

"That area has a super high diversity of below-ground tuberous plants, which have high carb loads. People are excellent foragers for them. You need a digging stick and there wouldn't be a lot of animal competitors," he said. "And the tuberous plants are adapted to arid environments."

His team eventually found a site, dating to 164,000 years ago, that shows evidence of humans eating shellfish, working with natural pigments and creating technologically sophisticated tools. He thinks this could be the remnants of the humans of the bottleneck–ancestors of everyone alive today.

Other researchers have theorized that eating shellfish was actually the driver that allowed humans to develop the big brains we enjoy today, because shellfish are high in the Omega 3 fatty acids that the brain needs to function. But Marean thinks the big brain came first. You can't just walk down to the beach and score yourself some sweet shellfish action (at least, not enough to sustain a society) without being pretty bright. Ancient humans would have had to be able to do some pretty complex thinking about concepts like time, Marean said. They would have to be able to make connections between unrelated things, like phases of the moon, tides and when shellfish were most plentiful. And they'd have to be able to communicate all that to other people.

From Marean's perspective, big brains enabled a small group of humans to make the switch to a shellfish diet–an adaptation that allowed them to survive a climactic upheaval that wiped out most of their peers.

Naturally, this all begs the question, "Could humans adapt to and survive modern, anthropogenic climate change as well?" Again, Marean thinks the answer lies in our food supply.

"These people were hunter-gatherers, and beauty of a hunting and gathering economy is that it's very flexible. If you use 80 plants and 14 animals, and you lose 10 plants and two animals, you can just shift your resources. When you commit yourself to agriculture that has very narrow environmental parameters, and your whole population is dependent on that, slight changes in the environment can have catastrophic effects," he said. "I'd say we have the cultural and technological ability to make a change and adapt. But we need to get busy."

This story was inspired by Curtis Marean's lecture at the 2008 Nobel Conference at Gustavus Adolphus College in St. Peter, Minn. You can watch Dr. Marean's lecture online.

Image courtesy Flickr user avlxyz, via CC