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STEVE INSKEEP, host:
You hear that music, you know we're going to talk about climate. But today, we're not going to talk about how we're changing it. We're going to talk about how climate changed us. The environment largely determines how living things survive. Consider this example: If you're a monkey in a rainforest, full of four-legged predators, you learn to climb trees. If the climate changes and the forest disappears, the lucky monkeys will find a new way to survive, while the others become somebody's dinner.
Adaptations like that make up the story of evolution. And now, a scientist has a new idea about the way the climate affects evolution, in particular our evolution - our exodus from Africa, the human migration to every corner of the world.
As part of our climate connection series with National Geographic, NPR's Christopher Joyce talks with the human whose adaptable mind is responsible for this new idea.
CHRISTOPHER JOYCE: So here's human evolution in a nutshell. In Africa, our ancestors came down from the trees and stood on two legs. Several variations on the early human theme followed. Some left Africa and spread around the world. Their descendants adapted to the climates they found - the Neanderthals in the frigid badlands of Europe, for example. But eventually, only one of these experiments was left: us.
I took a trip to the Museum of Natural History at the Smithsonian Institution in Washington to find out the secret of our success. The museum was closed and pretty dark. I stopped at a diorama. I had the place almost to myself.
I am standing next to a cave, surrounded by the Neanderthals. It's okay, though. They're just made of plastic. It's supposed to be very cold here. The Neanderthals were supremely adapted to cold, but something happened and the Neanderthals disappeared and were replaced by humans like us and like Dr. Rick Potts, who runs the Human Origins Program at the museum. So, Dr. Potts, what happened?
Dr. RICK POTTS (Scientist; Director, Human Origins Program, Museum of Natural History): At the same time that the Neanderthals were evolving and surviving in Europe, in tropical Africa there emerged a different lineage, the modern human lineage, Homo sapiens. They were able to make new kinds of tools. They used their brainpower to expand the area over which they lived. And eventually, this much more adaptable lineage, ourselves, displaced the Neanderthals even in their favored range of the cold Ice Age of Europe.
JOYCE: What was it that made humans more adaptable?
Dr. POTTS: Modern Homo sapiens in Africa emerged during a prolonged period of extreme climate variability. And the idea that we're working on is that this instability, this tendency of the environment to vary to a wide degree, was a real driving force in the emergence of the ability to adapt.
JOYCE: And you have the evidence for that somewhere here?
Dr. POTTS: Yeah. Let's go upstairs, and we can take a look at the artifacts there and some of the fossils.
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JOYCE: The rub of life is this: If you don't adapt to changes in your environment, you die. And what Potts believes is that humans are the world's most adaptable species. Unlike most other living things, we can live in rainforests, deserts, high mountains, even on ice. The remains of some of the also-rans lie on a shinny wooden table in Potts' office.
Dr. POTTS: We have the skull here of a Neanderthal.
JOYCE: When Potts holds the Neanderthal's skull, up next to a modern human one, you can see how well that the Neanderthals adapted to one type of climate. The first thing you'll notice is how much in the Neanderthals face pokes out the front. Experts called it prognathic.
Dr. POTTS: And that actually is an adaptation to cold climate. As you breath in, you want to humidify that air and give as much space inside your nasal cavities to humidify the air, warm it up before it goes down to your lungs.
JOYCE: And that worked just great until the climate change. Neanderthals couldn't adapt to the new environment as well as those ingenious modern humans. So Potts says the secret to human success has been our amazing adaptability.
Dr. POTTS: The question that is in front of us, and I think all of biology, is how to explain the evolution of this capacity to adapt, what I call the evolution of adaptability, that as climate changes, that there is, in fact, a kind of a pump.
JOYCE: Potts thinks rapid climate change - hot, cold, wet to dry, and back again - made life hard, but humans are more versatile, physically and mentally. No pain, no gain.
Dr. POTTS: Uncertainty itself is pain. And climate instability is a very, very difficult thing for organisms to accommodate to, and it probably does drive evolution.
JOYCE: Potts has been thinking about how to prove this for a long time. It's what anthropologists call a big idea. One thing he's done is assemble two lists. One is a climate record over the past five million years. It shows long periods when climate - and Africa's especially - was pretty steady, didn't change much. But there were also 67 periods when climate was all over the place, rapidly changing back and forth. Sometimes these periods of wild climate gyrations lasted as long as 326,000 years. Changes in the earth's orbit and its wobble caused these climatic ups and downs.
It's when Potts compares this climate record with a second list that he gets a thrill. His other list shows the great moments in human history, when we or our ancestors experienced those eureka moments in evolution.
Dr. POTTS: Going from upright walking, the first tools, changes in our body, the invention of fire, the increase in brain size and then the invention of specialized tools and ultimately the ability to take a story of something you saw outside and bring it inside a cave and paint it. All of these things represent a ratcheting up of adaptability in our lineage.
JOYCE: And it will happened during times of high climate change.
Dr. POTTS: And happened during a time of high climate change.
JOYCE: Potts points to three hugely important events that coincided with periods when climate was changing fast and furiously in Africa. First, the first stone tools used by human ancestors about 2.8 to 2.4 million years ago. Then, the first big exodus of human ancestors out of Africa 1.7 million years ago. And third, the appearance of modern Homo sapiens - us - about 200,000 years ago.
All this may seem like an academic exercise until you remember that we are experiencing rapid climate change right now, a change we have created. Potts notes optimistically that we are the most adaptable species. However, other forms of life are not.
Dr. POTTS: Our activity, there's just no doubt about it, is affecting the food web, the food chain all throughout the world. And so our own activities ultimately will be a new experiment in survival.
JOYCE: And how well we survive that experiment may depend on whether we can continue to adapt as well as we have in the past.
Christopher Joyce, NPR News.
INSKEEP: And you can see a very different take on global warming at our Web site. It's the latest episode of our animated series It's All About Carbon from NPR's Robert Krulwich, as you might have guessed, and Public Television's Wild Chronicles. Go to npr.org/climateconnections.
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