Tracking Ancient Footprints, Human and Otherwise

New evidence contradicts a British team's claim to have found 40,000-year-old human footprints in a quarry last year. But there is less debate about fossil tracks left by an enormous water scorpion.

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SCOTT SIMON, host:

Some very old footprints are creating a buzz this week. One set is in Scotland, the other in Mexico. But in Mexico, there's a new challenge to what were claimed to be the oldest human footprints in North America. NPR's David Malakoff reports.

DAVID MALAKOFF reporting:

Last summer, scientists from Britain made headlines when they announced that they had found hundreds of ancient human footprints in Mexico. They were oblong depressions preserved in a layer of volcanic ash. The team said tests showed that they were 40,000 years old. That claim made a lot of scientists very skeptical. They say most studies show that people arrived in North America just 11 or 12,000 years ago. And now the skeptics say they have proof that the British team is wrong.

Mr. PAUL RENNE (Rock Dating Expert): I don't believe that they found 40,000-year-old footprints. I don't believe that they found footprints at all.

MALAKOFF: That's Paul Renne, a rock dating expert. He works at the Berkeley Geochronology Center in California. A few years ago, he collected ash samples from the site of the tracks, and when he heard the British claims, he decided to do his own dating tests. The journal Nature published the results this week, and they say the ash layer is definitely more than a million years old. That would make the prints far, far older than any human foot.

Mr. RENNE: They would predate the first known Homo sapiens anywhere in the world. The oldest ones that we know of come from Africa, and they're arguably 160,000, 190,000 years old, something like that.

MALAKOFF: So who made the marks and when? Renne thinks it was modern-day quarry workers.

Mr. RENNE: This is a surface that is very heavily traveled. It's in an active quarry. There are tire tracks all over it, people drive on it, and this has just left a lot of gouge marks.

MALAKOFF: Not surprisingly, the British team disagrees, and it plans to do more dating studies.

Now less controversy surrounds another set of fossil footprints. They were recently found by geologist Martin White of the University of Sheffield while he was on a trip in Scotland.

Mr. MARTIN WHITE (Geologist): I had a bit of spare time, it was a beautiful day, and I just wandered along the beach, enjoying the geology. And one point, I climbed over a rib of sandstone and could see the trackway on it.

MALAKOFF: The 20-foot trackway looks like squiggles or scratches but White immediately recognized the signature of a giant six-legged creature.

Mr. WHITE: The name is Hibbertopterus which is a type of water scorpion or eurypterid.

MALAKOFF: Picture a six-foot long horseshoe crab with a thick spiked tail that dragged on the ground. White says it probably didn't sting and it lived about 330 million years ago when what we know as Scotland was much closer to the equator. Now, he says, the tracks will make a great teaching tool.

Mr. WHITE: It's a wonderful feeling to reach out across 330 million years and touch one of these footprints and touch the same sand as this bizarre animal touched.

MALAKOFF: White describes his scorpion tracks in the current issue of Nature. David Malakoff, NPR News.

SIMON: And the time is now 22 minutes before the hour.

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