Lorena Becerra-Valdivia, UNSW
Chiquihuite Cave is an archaeological site more than 2,740 metres above sea level in Zacatecas, Mexico. Ciprian Ardelean of the University of Zacatecas has been leading excavations of the site for more than seven years. Nearly 2,000 stone tools and pieces created through their manufacture have been found.
The tools belongs to a type of material culture never before seen in the Americas, with no evident similarities to any other cultural complexes. Importantly, more than 200 specimens were found below an archaeological layer that corresponds to the peak of the last Ice Age. (Archaeologists call this peak the Last Glacial Maximum.)
During this time, between 26,000 and 19,000 years ago, ice sheets were at their greatest extent. Evidence from Chiquihuite Cave, therefore, strongly suggests that humans were present in North America well before Clovis.
|A stone tool found below the Last Glacial Maximum layer|
Given the significance of the discovery, myself and a team of international researchers joined in the interdisciplinary study of Chiquihuite Cave. Some of us had the opportunity to visit the site following a four-hour long journey by foot, and see the evidence at first hand. Our aims were to reconstruct the environment humans lived in and define exactly when they occupied the site.Clovis first has been dead for quite a while, but this is a pretty amazing finding, more or less continuous occupation of the same site by a succession of pre-Clovis people and cultures starting at least 33,000 ago.
My own research at Chiquihuite Cave focused on the latter. I helped to build a chronology of more than 50 radiocarbon and optical dates.
Combined with the archaeological evidence, the results showed humans inhabited Chiquihuite as early as 33,000 years ago, until the cave was sealed off at the end of the Pleistocene period (around 12,000 years ago).
It's pretty clear that while humans may have reached the Western Hemisphere either early in the last glaciation, or in the Eemian Warm period, they didn't really make much population growth or environmental impact until the planet warmed again, and another wave of population came from Eurasia.
In a second paper, I explore the wider pattern of human occupation across North America and Beringia (the ancient land bridge connecting America to Asia). This involved analysing hundreds of dates obtained from 42 archaeological sites in North America and Beringia, including Chiquihuite Cave, using a statistical tool called Bayesian age modelling.
The analysis showed there were humans in North America before, during and immediately after the peak of the last Ice Age. However, it was not until much later that populations expanded significantly across the continent.
This occurred during a period of climate warming at the end of the Ice Age called Greenland Interstadial 1. The warming began suddenly with a pulse of increased global temperature around 14,700 years ago.
We also observed that the three major stone tool traditions in the wider region started around the same time. This coincides with an increase in archaeological sites and radiocarbon dates from those sites, as well as genetic data pointing to marked population growth.