But whether humans really made the marks found in the rocks in Mexico is controversial, says Michael R. Waters, director of the Center for the Study of the First Americans and an anthropology and geography professor at Texas A&M.
"We conclude that either hominid migration into the Americas occurred very much earlier than previously believed, or that the features in question were not made by humans on recently erupted ash," the Nature article states.
Waters and a consortium of researchers from California and Mexico tested the age of the rocks using radioactive isotope dating methods and paleomagnetic data and found that the basaltic rock in which the "footprints" are found is about 1.3 million years old. The resulting Nature article does not address whether the marks in the rock were made by early ancestors of modern man, although Waters says that the extreme age of the rocks containing the marks and their similarities to impressions made by quarrying equipment such as that used in the area make him doubt that early men made these "footprints."
Waters collaborated to produce this article with Paul R. Renne, Department of Earth and Planetary Science, University of California, Berkeley and Berkeley Geochronology Center, where the dating work was done; Joaquin Arroyo-Cabrales and Mario Perez- Campa of the Mexican National Institute of Anthropology and History; Patricia Ochoa Castillo of the Mexican National Museum of Anthropology; and UC Berkeley graduate students Joshua M. Feinberg and Kim B. Knight. The Berkeley Geochronology Center, located a block from the UC Berkeley campus, is recognized as one of the world's preeminent anthropological dating laboratories.
Earlier this year, researchers in England (who have yet to publish a peer-reviewed analysis of the footprints) dated the volcanic rock containing the marks at 40,000 years old and touted the "footprints" as definitive proof that humans were in the Americas much earlier than 11,000 years ago, which is the accepted date for the arrival of humans across a northern land- bridge from Asia. They hypothesized that early hunters walked across ash freshly deposited near a lake by volcanoes that are still active in the area around Puebla, Mexico. The so-called footprints, subsequently covered by more ash and inundated by lake waters, eventually turned to rock.
The rock was dated using radioactive argon at the Berkeley Geochronology Center. The argon method reliably dates rock as young as 2,000 years or as old as 4 billion years. Corroboration of the date indicated by the argon method was sought with paleomagnetic data because many rocks retain evidence of their orientation at the moment they cool in the form of iron oxide grains magnetized in a direction parallel to the Earth's magnetic field at the time of cooling. Because the Earth's field has repeatedly flipped throughout the earth's history, it is possible to date rock based on its magnetic polarity.
Waters, Renne, Knight and Mexico City archeologists visited the site at the Toluquilla quarry last year while investigating another potential early anthropological site across the reservoir.
Contact: Michael Waters, Texas A&M, 979-845-5246,
Paul Renne, Berkeley, 510-644-1350 or email@example.com;
Joaquin Arroyo-Cabrales, Instituto Nacional de Antropologia e Historia, firstname.lastname@example.org.