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From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.

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And I'm Michele Norris.

Anthropologists feel pretty lucky when they find a fossilized bone from a human ancestor. Even more unusual is a fossil footprint. Scientists have now discovered not one, but two sets of such footprints. They were left in the mud 1.5 million years ago.

NPR's Christopher Joyce has the story.

CHRISTOPHER JOYCE: It was a long way to go for fossils, a four-day drive across a desert in Kenya, then months in blazing sun digging through dirt. But the day in 2006 when anthropologist Brian Richmond and his crew found the fossil footprints, they knew they'd made one of the rarest of discoveries.

Dr. BRIAN RICHMOND (Anthropologist, George Washington University): We looked at this, and at first, we had to look at each other, like is - do you think - this is a human footprint, isn't it? You know, and we almost wanted to have our - we wanted to have our colleagues look and tell us that what we were seeing is, in fact, true. And we couldn't believe it. It was really exciting.

JOYCE: Exciting because of the extreme age of that layer of sediment, 1.5 million years. Richmond, from George Washington University, knew there was only one set of human-like footprints older than that. They'd been found 30 years ago, but they were from a much more primitive ancestor. These new prints looked, well, like Richmond's own footprint.

Dr. RICHMOND: These footprints are the earliest footprints of early humans.

JOYCE: They probably belonged to a species called Homo erectus or a close relative, species that eventually evolved into us, and apparently, they looked a lot like us.

Dr. RICHMOND: The prints match a men's shoe size of about nine, which gives you a height of about five feet, nine inches.

JOYCE: The prints had been laid by someone walking through mud that had dried and then been washed over with sand and naturally preserved. The shape of the prints and their spacing leads Richmond to conclude that Homo erectus could walk the walk that humans do.

Dr. RICHMOND: Here we have really compelling evidence that sure enough, they were walking with a long stride, they had an arch in the foot the way we have, and the arch gives us a spring in our step, in a way, and actually makes our walking more efficient.

JOYCE: When the team went back to the dig in 2007, they got another surprise. Digging in a level about 15 feet higher, they found another set of Homo erectus footprints. They calculated that they'd been made about 1,000 years after the first set. In all, they now had about 20 prints.

John Harris, an anthropologist from Rutgers University who led the dig, says the odds of finding two sets 1,000 years apart are off the charts.

Dr. JOHN HARRIS (Anthropologist, Rutgers University): It's just incredible. I'd never excavated anything like this before.

JOYCE: There were lots of other prints there, too: antelope, zebra, even a form of lion. The place had been a river valley near a lake, a hopping place, says Harris, but the climate was drying out with greater distances between trees and water. Evolving a more efficient walk would have been a big advantage.

Dr. HARRIS: It shows a creature that was capable of increasing its home range. So they would've had to have moved over greater areas of the ancient landscape to sustain themselves in terms of their food requirements.

JOYCE: The footprint's shape and the pronounced arch suggests that this creature, like us, had a spring ligament in the foot. That ligament absorbs energy and then returns it as the walker moves forward. That, says anthropologist Dan Lieberman at Harvard University, suggests that a hominid like Homo erectus was not just a good walker, but a runner, because he had to be.

Dr. DAN LIEBERMAN (Anthropologist, Harvard University): Imagine you are a Homo erectus, and you're hungry, and you want to kill something for dinner. The weapons available to you are incredibly primitive. So, one thing that early hominids might have included as part of their repertoire of hunting strategies was to run animals in the heat.

JOYCE: Until, Lieberman says, the prey collapsed. The footprints are described in tomorrow's issue of the journal, Science. The team will return this summer, and Richmond says he can't wait.

Dr. RICHMOND: It's almost an emotional kind of connection when you realize that the last time someone stood in that spot where you were standing at the moment was 1.5 million years ago, and it may have even been a direct ancestor.

JOYCE: Christopher Joyce, NPR News.

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