MICHELE NORRIS, host:

This is ALL THINGS CONSIDERED from NPR News. I'm Michele Norris.

MELISSA BLOCK, host:

And I'm Melissa Block.

Today for another story in our Climate Connections series with National Geographic, we head to Peru. A mysterious civilization sprang up there about 5,000 years ago many centuries before the Inca empire. Yet these people were sophisticated. They cultivated crops and orchards, and they built huge monuments of earth and rock. Archaeologists are trying to prove that an abrupt change of climate created this ancient culture.

NPR's Christopher Joyce went to Peru's arid, costal valleys to see the evidence.

CHRISTOPHER JOYCE: It is so desolate here. Just desert, spongy on the surface but dry as dust underneath, a few cactus is poking up from the sand. Mist, that's the only source of humidity. It's really a moonscape.

(Soundbite of radio transmission)

JOYCE: Several Peruvian and American archaeologists tried to cross this moonscape. In the clammy fog, they communicate with walkie-talkies. They're looking for artifacts left behind by an ancient and mysterious civilization - a civilization that built giant mounds of earth and rock.

Archaeologist Jonathan Haas, with the Field Museum in Chicago, first saw these mounds eight years ago in a region called Norte Chico. As fascinating as they were, it was stuff down in the dirt that really got him thinking.

Dr. JONATHAN HAAS (Archaeologist, The Field Museum, Chicago): It's sand and a little bit of gravel. You get down on your hands and knees and what you find is little pieces of seashell. And then you go, how do I get little pieces of seashell out here? I don't know. I don't know, and I don't really care.

JOYCE: But eventually, he did care.

Dr. HAAS: And I puzzled and I puzzled and I puzzled over it, and I finally realized it was the people who were building these mounds who were coming out here, and I bet they were fishermen.

JOYCE: Fishermen who had come up from the coast bringing shellfish. But why? The story starts thousands of years ago, when people from eastern Asia flowed into North America and then South America.

Dr. HAAS: People are going where the good resources are right down to this very beach.

JOYCE: This beach is called Barranca. Early Americans — hunters and gatherers — Same here to fish and collect mussels and clams. And that worked fine until about 3000 B.C.

Dr. HAAS: At around 3000, the environment begins to change.

JOYCE: Haas suspects that what changed was El Nino, the cycle of warm ocean water and torrential rains that regularly descends on western South America. Some shift in the coupling of the atmosphere and the Pacific Ocean made El Ninos more frequent. Haas doesn't know why it happened, but he believes more frequent El Ninos had a drastic effect on coastal life.

Dr. HAAS: They were pushing out the cold-water fish and bringing in warm-water fish. They kill off local clams and mussels.

JOYCE: The fishing got bad, the weather unpredictable. So people moved inland, to the desert valleys. It was only about 10 miles or so, but it might as well have been the moon.

One of the places they went is now called Huaricanga. They built a mound here about 5,000 years ago. Haas' team is now excavating it.

Dr. WINIFRED CREAMER (Professor of Archaeology, Northern Illinois University): I'm Winifred Creamer. I'm a professor at Northern Illinois University. And today, what we're going to do is clear a profile.

JOYCE: The profile is a 10-foot high wall of dirt and rock created when farmers dug an irrigation ditch through the mound. After people built the mounds, they sometimes lived on it. This ditch is a lucky find; it exposed the mound from top to bottom like a layer cake. But working conditions aren't good.

Dr. CREAMER: We're standing here working outside somebody's house. We're on the edge of the highway, and we're standing in a ditch which may or may not fill with water. And the area right behind where we're working is where people throw trash. So it's not really the romance of archaeology, is it?

(Soundbite of laughter)

Mr. ALVARO RUIZ (Project Co-Director): My name is Alvaro Ruiz; I am the co-director of this project. The people here is really worried about we don't want to cause any damage to their irrigation, because people live here, they have many problems.

JOYCE: Life was even harder 5,000 years ago. People had to learn how to grow crops and irrigate them from the few rivers and streams. The weather controlled life, especially El Nino.

Standing on a hill, Creamer says El Nino storms would have brought life-giving water, but also destruction.

Dr. CREAMER: You look up this dry wash that we're situated in and, well, imagine a 40-foot-high wall of water rolling out of that. That would have a pretty life-changing impact on everybody in this valley.

JOYCE: But people learned a new way of life. Culture grew more complex. Trade flourished. Coastal people brought shellfish — the shells Haas found in the desert — and took back squash and cotton. And they brought their labor to build the mounds. It was massive architecture on a scale never seen before in the New World.

Dr. HAAS: We're going to the site first of Porvenir, which is one of the biggest and earliest of these great big sites.

JOYCE: Porvenir is one of dozens of mounds that lie almost hidden in the creases of these valleys. It's at least 1,000 years older than other mounds in the New World. It looks like a pile of rubble, but 30 feet high and maybe 200 feet across. Originally, it was terraced, with a flat top, and the product of enormous labor.

Dr. HAAS: Well, you have to think of a large stone birthday cake, and then it would have been covered with plaster, and you can have it pink, you can have it light orange.

JOYCE: Whoever these people were, archaeologists don't yet have a name for them. They built these monuments even before the Egyptians built the pyramids.

(Soundbite of footsteps)

JOYCE: As we climbed the mound, we walk by pits dug by present-day looters. Human bones and trash litter the ground.

Dr. HAAS: This isn't the coolest archaeology in the world in terms of the stuff you find. There are no beautiful ceramics, no gold masks, our treasure is trash, residential architecture, and all of a sudden those start bringing together this incredible picture of the origins of civilization in South America.

JOYCE: Haas believes it was a change in climate that seems to have started all this.

Dr. HAAS: If you think about going from a hunter-gatherer society to this highly centralized society with an organized religion, it's a pretty dramatic change to take place over a very short period of time.

JOYCE: Haas says he still has a lot of work to do to prove that. But whatever this society's genesis, Haas believes its way of life — the mounds, sunken plazas, irrigation agriculture, religion — eventually spread across South America, like a map unfolding.

Dr. HAAS: There's a very distinctive Andean pattern that starts here and then spreads and forms the foundation for Andean civilization for the next 5,000 years. It's pretty cool.

JOYCE: Christopher Joyce, NPR News.

BLOCK: And there's a photographic tour of Jonathan Haas' archaeological dig at npr.org/climateconnections.

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