The footprints discovered at White Sands National Park, which belong to humans and other animals who lived during the Last Glacial Maximum (c.23,000 years ago), represent the earliest unequivocal evidence of human presence in the Americas, as well as offering a unique snapshot of the relationship between these early people and the megafauna living alongside them.
They were first spotted by David Bustos, Resources Manager at the National Park, during one of his regular patrols. The tracks, which are referred to as โghost tracksโ, are only visible under certain moisture conditions โ when the level of water in the surrounding soil and the level of water in the infill of the prints are slightly different โ at other times they are completely invisible.
In January 2016, David Bustos invited a group of researchers to the park to examine the footprints. The experts agreed that the tracks were definitely created by humans and megafauna, but were uncertain how they were associated, and decided to return to carry out fieldwork later that year. It quickly became apparent that there was a vast area covered with the footprints of both humans and prehistoric megafauna. Among the prints were some that showed humans walking in the footprints of an extinct giant sloth, which provided clear evidence that humans were living alongside these huge animals, and also that they appeared to be stalking and probably hunting them. However, although this discovery showed that the prints must have been created before the giant ground sloth became extinct, it remained impossible to date them precisely, as the site contains very little organic matter.
Getting up to date
What makes the latest research so exciting is the discovery of a set of tracks between layers of sediment containing the seeds of spiral ditchgrass (Ruppia cirrhosa), which provided a datable context around these footprints. Radiocarbon dating of the seeds by a team at the US Geological Survey produced a date of 21,000 years ago for the layer above the prints and 23,000 for the layer below, meaning that the prints can be securely dated to between these two points in time.
This discovery is hugely significant as it indicates that the tracks were created several thousand years before humans were widely believed to have arrived in North America. For a long time, it was accepted that the first humans to enter America did so through an ice-free corridor that opened up across Canada c.13,000 years ago. However, later discoveries indicated that people were present on the continent before this corridor was open, at least 16,500 years ago. This led to the development of an alternative theory: the โKelp Highwayโ hypothesis, which posits that people arrived by sea rather than by land, following the coastline around from Siberia. A variation on this suggestion is that they may have migrated through open seafaring, which, while less popular as an explanation, cannot be ruled out. The new discovery does not provide a definitive answer to this debate, but it does show that humans arrived in North America much earlier than was previously thought. Although dates earlier than 16,000 years ago have been proposed for other sites in America, none of them have been as securely dated as the White Sands footprints. In addition to providing reliable evidence for the presence of humans in New Mexico 23,000-21,000 years ago, this discovery means that some sites where dates were deemed too early could be re-examined with fresh eyes.
Living with giants
The other important aspect of the discovery is what it means for our understanding of the ways in which humans were interacting with prehistoric fauna, and their role in the extinction of megafauna, such as the giant ground sloth and mammoth. It was previously thought that these species were driven to extinction very rapidly after the arrival of humans on the North American continent, a phenomenon attributed to over-hunting. However, the dates of these footprints suggest that humans were coexisting with, and probably hunting, these animals for 10,000 years before they became extinct. This points to much more sustainable subsistence practices that allowed people to live alongside these species for millennia, before the number of humans increased to the extent that megafauna populations were not able to recover from the impact of their hunting.
In addition to contributing to our understanding of the wider impact a human presence can have on the surrounding environment, the footprints offer a unique insight into both a moment in the lives of the people living in the region and the reason for their presence at White Sands.
The area where the footprints were found is currently part of a playa, a large dry desert basin, but at the time when they were created it was the site of a lake: an important source of water in an increasingly arid area and consequently a spot where animals would have congregated. This is demonstrated by the prints of mammoths, ground sloths, canids, felids, bovids, and camelids that have been found in the area. The human tracks themselves appear to suggest fairly relaxed movement, giving the impression of people casually enjoying their time on the lakeshore. In turn, this suggests that they were strategically using the landscape to their advantage, planning ahead, and waiting for prey to come to them.
Although we do not have information about the stature of the people who created the prints and therefore cannot draw definitive conclusions about the correlation between foot size and height or age, a wide range of foot sizes are represented, suggesting that children, teenagers, and adults were all present. The morphology of the toes, which are further apart than those of habitually shod modern humans, reveals that these people were not accustomed to wearing any kind of foot covering, but it is possible that evidence of early footwear may be found in other prints at the site in the future.
Work at White Sands will carry on, and it is hoped that now some prints have been securely dated thanks to the seed layers, it may be possible to date others using alternative methods, which do not require organic material.
Dr Sally Reynolds, who was involved in the research, emphasises that, although the dates of these footprints are exciting and have the potential to change our understanding of the timeline for Americaโs earliest occupation, perhaps the most important thing about the site is the behavioural archive it represents. She observes that even if an older site is identified somewhere else, the prints at White Sands will remain unparalleled for the information that they provide about interactions between humans and megafauna. The unique insight into life in this period that they offer โ which we cannot get from dates alone, or even from stone tools, bones, or other artefacts โ is where the true value of the White Sands footprints lies.
FURTHER READING M Bennet et al. (2021) โEvidence of humans in North America during the Last Glacial Maximumโ, Science (https://doi.org/10.1126/science.abg7586). D Bustos et al. (2018) โFootprints preserve terminal Pleistocene hunt? Humanโsloth interactions in North Americaโ, Science Advances (https://doi.org/10.1126/sciadv.aar7621).
All images: Bournemouth University/National Parks Service.
