Prehistoric Americas

It was a sensational discovery - human footprints said to be 40,000 years old, preserved by volcanic ash in an abandoned quarry in Mexico.

The announcement was made in July 2005, and it created a flurry of excitement. Then it was promptly dismissed by a second team of researchers who re-dated the rocks at 1.3 million years old, impossibly ancient to bear human traces. The original claim did not go away, however.

The first widespread evidence for human occupation in North America came from the town of Clovis in New Mexico.

The beautiful fluted stone spear points made by the Clovis people are found on many sites and they date back to around 11,500 BC. They are believed to have been left by people who descended from those who crossed a land bridge which once existed between Siberia and Alaska.

But there is an increasing body of evidence for earlier occupation of the Americas, dating back to a time at which the overland route through the ice would have been impossible.

'Car park'

The best evidence probably comes from Monte Verde in Chile and dates back to at least 12,500 BC.

But to have reached so far south by that date, people must have entered the continent earlier still. There have been many claims of earlier dates, but few have been substantiated. So the announcement of 40,000 year-old footprints from Mexico was greeted with scepticism and caution.

It came from a team led by Silvia Gonzalez, a Mexican working at Liverpool John Moores University (JMU), UK. In 2003, she was visiting a site to the south of Puebla, about a hundred kilometres south-east of Mexico City.

It is a dry environment with many small volcanoes and, in the distance, the smoking peak of Popocatepetl. She was hoping to find the geological context of deposits which had yielded animal bones showing possible butchery marks which dated back between 20,000-30,000 years.

The researchers were looking for a vertical section through the rocks in the side of a small quarry, but it was overgrown and strewn with debris. As they were about to give up, they noticed that the floor of the quarry was made of a single layer of hardened volcanic ash called the xalnene tuff. This stuff looks a bit like a badly asphalted car park.

'Mechanical' marks

Silvia Gonzalez had studied much younger human tracks in Lancashire and thought she could see similar markings in the volcanic ash. Then she spotted some marks on the surface of the ash, and recognised them as human footprints. At first, her colleagues laughed at her, but soon they were brushing away the dust with excitement.

A large area of the quarry was quickly cleared and the researchers carried out a detailed digital laser survey of the marks.

Some were clearly animal tracks left by perhaps deer and buffalo and running for several metres. Others did seem to resemble human footprints, although there were few in a continuous track. They were of several sizes, suggesting both adults and children.

The ash was too coarse to have left clear toe prints but some certainly appeared to be from left or right feet with a raised arch and material bunched up behind the toes as the person pushed forwards up the gentle slope.

Professor Mike Waters of Texas A&M University was sceptical. He thought the marks were products of the quarrying process and subsequent erosion. There were certainly some marks at the site which were very obviously due to that, and these seemed much fresher - pick marks and tracks from mechanical diggers.

'Complex' scenario

More controversial still are the dates. Colleagues of Silvia Gonzalez at Oxford University used a technique called optically stimulated luminescence (OSL) which records the last time rocks were exposed to sunlight or heat. That gave a variety of dates from the overlying sediments, but when applied to small fragments of what looked like brick or burnt clay within the volcanic ash, it produced a date of about 40,000 years.

That initially shocked Dr Gonzalez as it implied by far the earliest evidence of humans in the Americas. But it fitted in with dates of up to 38,000 years based on carbon 14 in shells in the sediments above.

In December, however, Paul Renne of the Berkley Geochronology Center in California published dates for the volcanic ash itself based on the powerful argon-argon technique. That gave an age of 1.3 million years, far too old to be compatible with human footprints. Professor Renne measured the age of several different grains in the ash and got the same age for each. Dr Gonzalez, though, pointed out that the volcano responsible for the ash is complex.

It erupted explosively underneath a lake and lots of older material and lake sediment may have been caught up in the ash, distorting the date.

Further work

Furthermore, there didn't seem to be the signs of erosion and weathering which would be expected if there had been a gap of more than a million years between the ash and the overlying sediments.

Professor Renne also looked at the magnetisation of the rocks, partly to see if they may have been jumbled and re-deposited from earlier material which, he said, they were not. But he did find that the magnetic polarity was the opposite of the Earth's present magnetic field.

It's well known that the planet's magnetic poles do occasionally flip. The last time the Earth's magnetic field had consistently reversed polarity was about 790,000 years ago, so the fact that reversed polarity magnetisation was found in this rock means that it's older than 790,000 years.

However, it's also well known that the magnetic field can experience short-term 'excursions' (even Professor Renne, above, was careful to emphasise a 'consistently reversed polarity'). One of those 'excursions' happened 40,000 years ago, very interestingly.

To answer the criticisms, Dr Gonzalez and her colleagues next hoped to get permission to excavate for further footprints which would not be associated with any quarrying marks and to get more secure and consistent dates for the rocks. That sort of evidence should convince even the most intense critics. If she succeeded then this little quarry could become one of the most important archaeological sites in the Americas.

As for the initially controversial theory that modern humans entered the Americas 40,000 years ago, the weight of evidence clearly seemed to be mounting in favour of something actually being there. Only time would tell.

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