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
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.
"I don't think that they are [footprints]," he said. "There are
no trails of footprints that are consistent, the shapes don't really
look like footprints, and, most importantly, there's a huge
diversity of shapes, sizes and arrangements of these things."
Professor Renne measured the age of several different grains in
the ash and got the same age for each. Dr Gonzalez, though, says
that the volcano responsible is complex.
It interrupted explosively underneath a lake and lots of older
material and lake sediment may have been caught up in the ash,
distorting the date.
Furthermore, there do not seem to be the signs of erosion and
weathering that 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 might have been jumbled up and re-deposited
from an earlier material, which, he says, they were not.
But he did find that the magnetic polarity was the opposite of
the Earth's present magnetic field. The Earth's magnetic poles do
"The last time the Earth's magnetic field had consistently
reversed polarity was about 790,000 years ago, so the fact that we
found reversed polarity magnetisation in this rock tells us that
it's older than 790,000 years," he said.
Silvia Gonzalez' view? "We know that there are short-term
'excursions' of the magnetic field, and one of those happened 40,000
years ago, very interestingly."
Professor Renne: "How did I know they were going to say that?
There is a finite possibility that that is correct, but the
probability is extremely low."
It seems this debate really is going to run and run.
To answer the criticisms, Dr Gonzalez and her colleagues hope
now to get permission to excavate for further footprints that would
not be associated with any quarrying marks and to get more secure
and consistent dates for the rocks.
"That would convince even the most intense critics," she said.
"We need to talk to each other to make a continental model of human
migration across the Americas. It won't be done in a few years. It
will take a lifetime, but we are not afraid to do that.
If she succeeds, this little quarry could become one of the most
important archaeological sites in the Americas.
A final comment from Professor David Meltzer, from Southern
Methodist University, Dallas. He has researched and written
extensively on the subject of the "first Americans". He said: "I'm
not averse to the idea of 40 000-year-old people in the New World -
but I'm sceptical because we've been fooled before.
"We want to see it confirmed with all the evidence laid out so
that we're not buying in to something that isn't there."
The weight of evidence clearly seems to be mounting in favour of
that something actually being there. Only time will tell.