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MICHELE NORRIS, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Michele Norris.

MELISSA BLOCK, host:

And I'm Melissa Block.

The reason humans rule the Earth could be due to a huge collision that took place 160 million years ago. At least that's what a team of planetary scientists now claims. They say somewhere out between Jupiter and Mars, two asteroids smashed into each other. The debris hurdled into space and eventually a big piece hit the Earth. And these researchers say that's what wiped out the dinosaurs and gave us mammals our chance.

NPR's Christopher Joyce has the story on a piece of interplanetary sleuthing.

CHRISTOPHER JOYCE: The asteroid belt is a sort of rock pile in space for debris left over after the planets coalesced. The two asteroids that smashed together were big. One about 30 miles across, the other, more than three times that size. Rocky pieces cartwheeled off into space.

According to planetary scientist William Bottke, the gravitational pull of Mars and Jupiter then acted as a sort of superhighway, guiding some of those fragments toward us.

Dr. WILLIAM BOTTKE (Planetary Scientist, Southwest Research Institute): You'll think about a boulder on top of a big mountain, and you break up the boulder, and all of a sudden, you create all these pieces and they all start rolling downhill. And somewhere at the bottom of the hill is some little village called Earth, and most of the fragments aren't going to hit the village, but in some cases, big boulders might roll through it.

JOYCE: Bottke, who works at the Southwest Research Institute in Colorado, says the fragments from this collision doubled the rate of asteroid traffic in our neck of the solar system for tens of millions of years. And when he calculated their journey over that time, he realized that some pieces would have intercepted our planet at a very important time in history.

Dr. BOTTKE: We think we've found a very positive link between an impact event that happened 65 million years ago, which is believed to have wiped out the dinosaurs. This happened in the Yucatan Peninsula in a place called Chicxulub. That impact appears to be linked to a very large fragment from our asteroid breakup, and which is one of the reasons we're very excited by this.

JOYCE: And why the scientific journal Nature published Bottke's work this week. Bottke is 90 percent sure the killer asteroid came from this one family of fragments. He notes this family has its own elemental fingerprint that identifies its members as special — they are carbonaceous chondrites. And that's the same kind that knocked off the dinosaurs.

Geologist Philippe Claeys of the Free University in Belgium says the finding illustrates something people often overlook about life on Earth.

Dr. PHILIPPE CLAEYS (Geologist, Free University of Brussels): The solar system is a very violent environment, and this is a very good example of a collision taking place in the asteroid belt very far from us, but has a major influence on the evolution of the biosphere on Earth.

JOYCE: In fact, we mammals may owe this asteroid a debt of thanks.

Dr. CLAEYS: Because without the collision, the dinosaurs might still be the dominant organism in this planet, and we might still be crawling at night from one hole to the other to feed ourselves.

JOYCE: It's a coincidence that took 100 million years to play out, says Bottke, and one that should not be lost on the survivors.

Dr. BOTTKE: We're still in the tail end of this asteroid shower, and this asteroid breakup will continue to provide fragments to the inner solar system for an extended period of time.

JOYCE: So we're not out of the woods yet.

Dr. BOTTKE: We are not.

Christopher Joyce, NPR News.

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