[SmithNathan]

Ancient Footprints May Rewrite American History

July 6, 2005 — Humans colonized the Americas 30,000 years earlier than previously thought, according to a set of human footprints unearthed beside an ancient Mexican lake.

Dated to about 40,000 years ago, the impressions challenge the traditionally held view that settlers first came to North America after the last Ice Age ended about 13,500 years ago.

oday, the volcanic Xalnene ash, in which the footprints were embedded, is hard as concrete and used locally as a building material.

It took Gonzalez's team nearly two years to date the material. Various methods, from radiocarbon dating to argon-argon, uranium series and electron spin resonance techniques had been used.

The key date came from shells in sediments just above the layer of ash, which the team carbon-dated to 38,000 years ago.

Sand grains baked into the ash, dated using optically stimulated luminescence, confirmed the results.
If it survives the controversy, the discovery would point to Central Mexico as one of the most important areas for the study of early human occupation and provide fresh evidence that humans settled in the Americas as early as 40,000 years ago.