Study Raises Doubts About 40,000-Year-Old ‘Footprints’

Impressions buried in volcanic ash in Mexico described by British scientists as 40,000-year-old human “footprints” are actually 1.3 million years old, indicating that they are not human in origin, according to a report by UC Berkeley geologist Paul Renne and colleagues published in this week’s edition of the journal Nature.

The volcanic impressions were discovered in 2003 in a rock quarry southeast of Mexico City by a team led by geologist Silvia Gonzalez of Liverpool’s John Moores University, who said this summer that the finding proved that humans had colonized the Americas much earlier than 11,500 years ago, as previously believed.

Renne and his team used radioactive dating and magnetic analysis of the rock to come up with an age estimate, which dated the “footprints” more than one million years before the first known appearance of Homo sapiens in Africa.

Renne said there are only two possible explanations: “One is that they are really old hominids -- shockingly old -- or they’re not footprints.”

Renne used a technique, known as argon/argon dating, which measures two argon isotopes. It can date rock from 2,000 to 4 billion years old.

The British-led team primarily relied on carbon-14 dating of underlying sediments to come up with their age of 40,000 years. The carbon method can’t reliably date material older than about 50,000 years.

Gonzalez said the Berkeley findings didn’t settle the issue because they had yet to be confirmed by other scientists.

But Renne said that a person didn’t need advanced equipment to question the validity of the “footprints.” You have “to wonder why any people would want to walk on top of these rocks,” he said, adding that the volcanic material would have been about 500 degrees Fahrenheit.