Determining the age of the deposits from the Valsequillo Basin has been a challenge for scientists during the last 40 years. The original excavations of Cynthia Irvin Williams and Juan Armenta in the 1960’s and 1970’s, suggested very early human occupation (at about 22,000 years ago) at some archaeological sites in the Basin. Later work lead by Virginia Steen McIntyre in the 1970’s suggested dates of 240,000 and 360,000 for some layers with large animal bones (mastodont, horse and camel) associated with stone tools.
These ages were regarded as impossible and widely rejected by the scientific community as they were regarded “too old” for the existing models for the settlement of the Americas.
The deposits have been difficult to date because they are very heterogeneous and in particular the Xalnene ash, in which the suggested footprints are found, has particles derived from different compositions suggesting perhaps different origins (Figure 1, Figure 2).
The eruption that produced the ash was highly explosive, in association with magma-water interactions and potentially could have incorporated some older rocks into the ash. We have obtained Optically Stimulated Luminescence (OSL) dates of around 40,000 years old from some backed particles incorporated into the ash and now they need to be explained in view of the new dates obtained by Renne’s group.
Figure 1
Figure 2
It is clear that the dates reported by Renne’s group need to be replicated and independently confirmed. This is important because we applied the Ar-Ar method as well and had no good results, and concluded that they were not reliable. Also, it is not clear from where exactly they took their samples and which fraction was dated. We took our samples directly from the footprint horizons.
Even if we are wrong and the Xalnene ash is indeed 1.3 million years old, as suggested by Renne’s et al, that is not automatically a reason to disregard interpretation of the features reported as “footprints”, simple because they are not in agreement with the established models for the settlement of the Americas.
This highlights the desperate need of further dating with different methods on the deposits in the Valsequillo Basin to make sure they make sense between themselves and to be able to establish a reliable chronology, considering the palaeontological and archaeological evidence included in them. This is exactly what we are doing at the moment with a grant from NERC (Natural Environment Research Council), during the next three years.
What we have ahead is a new era in the scientific research of the archaeological evidence found in the Valsequillo Basin and that is what Science is all about.
Our own dating efforts up to now and arguments explaining why we interpret the trace fossils preserved on the ash as human footprints will appear in the January issue of the Quaternary Science Reviews.
The Oldest American?
Footprints from the Past
Right human footprint, showing toe impressions and typical figure of eight shape
Human left footprint showing toe impressions and slight heel impression. Modern size 43 for scale
The discovery of 40,000 years old human footprints in Central Mexico challenges accepted theories on when and how humans first colonised the Americas.
The timing, route and origin of the first colonisation of the Americas remains one of the most contentious topics in human evolution. Experts from many disciplines are searching for the answers to three seemingly straightforward questions:
• From where did the first people come?
• How did they enter the Americas?
• When did they arrive?
Until recently archaeologists thought they had the answers to these questions. Evidence suggested that the Americas had been colonised towards the end of the Pleistocene period by hunter-gatherers migrating from Siberia into Alaska across the Bering Land Bridge, an exposed continental shelf, when sea levels were lower. This is known as the Clovis-First Model.
According to this model the earliest occupation of the Americas began 11,500 years ago.
The discovery of fossilised human footprints in the Valsequillo Basin, Central Mexico challenges this accepted viewpoint and provides new evidence that humans settled in the Americas as early as 40,000 years ago.
Location of Mexico and Mexico City
Location of the modern Valsequillo reservoir and Cerro Toluquilla south-east of Puebla.
Toluquilla quarry and the upper surface of the Xalnene Ash where the footprints are preserved.
The footprints were discovered in the summer of 2003, on the floor of an abandoned quarry by Dr Silvia Gonzalez, Professor David Huddart (Liverpool John Moores University) and Professor Matthew Bennett (Bournemouth University). At the time of the discovery the team were working on dating and mapping the geology of the Valsequillo Basin, Puebla, Mexico.
Dr Gonzalez is one of a growing number of scientists who believes that the first Americans may have arrived by water rather than on foot, island hopping along the Pacific coast. Click here to find out more about her research on the Pacific Coastal Migration Route.
The footprints research was carried out as part of a wider project funded by NERC through the EFCHED programme, entitled ‘Human dispersals and environmental controls during the Late Pleistocene / Early Holocene in Mexico: Implications for the Peopling of the Americas’. The research is also supported by INAH (Instituto Nacional de Antropologia e Historia) in Mexico.