Impressions in volcanic ash that were hailed as the earliest traces of humans in the Americas may not be what they seem. A dating study in the journal Nature puts the age of the volcanic ash in which the indentations were found at 1.3 million years — more than a million years before modern humans evolved — which casts damning doubt on the theory that they are 40,000-year-old footprints.

Thomas Higham of the University of Oxford, UK, according to Nature, calculated that they were 40,000 years old, a bombshell of news making the impressions the oldest evidence of human occupation of the New World.

However, experts say that relatives of our species living at this time were incapable of making the journey to the Americas.

Researchers began investigating the site, the bottom of an abandoned quarry close to the Cerro Toluquilla volcano at Valsequillo Lake, near Puebla in southern Mexico, after a British-Mexican team led by Dr. Silvia Gonzalez of Liverpool John Moores University made public the discovery of 269 fossil footprints, both animal and human, in July. According to the researchers, Discovery News said, the footprints were preserved as trace fossils in volcanic ash along what was the shoreline of an ancient volcanic lake.

The 40,000-year date was based on several methods to date minerals and fossils — from above, below and on the footprint layer itself. The researchers used radiocarbon analysis of shells in the layer above the ash at the nearby Toloquilla quarry. They obtained dates for lake sediments incorporated into the ash by a technique called optically stimulated luminescence, BBC News reported. The resulting date was that of the controversial 40,000 years.

The conventional view places the oldest Homo sapiens' fossils as African, dating to 160,000-200,000 years ago. The oldest evidence of humans in the Americas is at Monte Verde in Chile, according to Nature, where occupation is dated to about 14,500 years ago.

Now geochronologist Paul Renne, of the Berkeley Geochronology Center in California, has produced data showing that the volcanic ash layer (Renne calls "eroded gouge marks") is so old it is "highly unlikely" that the markings are footprints.

Renne and his colleagues used two dating techniques on nine samples of the volcanic rock: one examined the ratios of chemical isotopes in the ash; the other looked for magnetic signals from the sediments. They used argon dating (or argon-argon) and palaeomagnetic analysis (the latter technique looking at Earth's magnetic field during past geologic time) to show that the so-called Xalnene basaltic tuff on which the purported footprints were found was actually much older even than Gonzalez and her team suggested.

The tuff, according to these latest results, is 1.3 million years old. Therefore, the "footprints" predate the first known appearance of Homo sapiens in Africa by more than a million years.

As such, either the marks are not human footprints, or humans appeared much earlier than previously thought, or they are footprints of archaic humans such as Homo erectus. The latter two likelihoods, according to the scientists conceding the possibility of each, are remote in the extreme.

"If these really are footprints, and they were made 1.3 million years ago, that would be absolutely revolutionary," LiveScience quoted Renne as having said.

Co-author Michael Waters, of Texas A&M University, in criticizing the use of radiocarbon dating on shells from the sequences, told BBC News "freshwater shell is notorious for producing erroneous ages." Waters also thinks the marks were actually left over from quarrying. Moreover, after visiting the site, Renne believes the markings are actually "impressions left by machines or animals that have passed through the quarry in recent times," LiveScience said.

Gonzalez wants the dates reported by Renne's group replicated and independently confirmed. Her team plans to submit a formal scientific response and evidence for publication in the academic journal Quaternary Science Reviews in January.

References

Footprint claims get stamped on
by Rex Dalton
Nature, Nov. 30, 2005
http://www.nature.com/news/2005/051128/full/051128-7.html

Ancient American Footprints Disputed
by Rossella Lorenzi
Discovery News, Nov. 30, 2005
http://dsc.discovery.com/news/briefs/20051128/footprints_arc.html

Study treads on footprint claim
BBC News, Dec. 1, 2005
http://news.bbc.co.uk/2/hi/science/nature/4488490.stm

Controversial Footprints: Earliest Man or Modern Machine?
by Ker Than
LiveScience, Nov. 30, 2005
http://www.livescience.com/humanbiology/051130_ancient_footprints.html