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The oldest confirmed human footprints from the Americas have been found in New Mexico, pushing back the date of our ancestors' arrival on the continent by thousands of years.
The footprints are believed to be between 21,000 and 23,000 years old, and show evidence of children and teenagers alongside a variety of extinct animals. The tracks are thought to be over 10,000 years older than previous evidence of humans in the Americas.
Understanding when humans arrived in the Americas has been fraught with uncertainty and disagreement over the quality of evidence that has been unearthed. While some highly contentious (and now largely rebuked) finds suggested a date as long ago as 130,000 years, most remains have come back as dating to around 15,000 years ago.
Despite this, experts have generally agreed that humans most likely arrived in the Americas earlier than current evidence showed.
Now, ancient seeds found alongside fossilised footprints in New Mexico have allowed for much greater certainty over the age of this new site, pushing back the earliest evidence of humans in the Americas by 10,000 years.
The findings, led by British and American researchers, were published in Science.
Dr Sally Reynolds, from Bournemouth University and one of the paper's authors, says, 'It is an important site because all of the trackways we've found there show an interaction of humans in the landscape alongside extinct animals. We can see the coexistence between humans and animals on the site as a whole, and by being able to accurately date these footprints, we're building a greater picture of the landscape.'
When and how humans first arrived in the Americas has been long debated, with a range of times and methods having been suggested.
Generally, most theories agree that humans came from Asia and down into the Americas. Some have suggested that humans walked across a land bridge between Siberia and Alaska, known as Beringia, as the ice sheets that covered this region began to melt. When the ice age ended, rising sea levels flooded the bridge and separated the continents.
Others argue that humans may have used small boats and gradually followed the coastline from Asia over to the Americas, avoiding the ice sheets that would have blocked their way.
In either case, human arrival was previously estimated to have been around 11,000 years ago, based on evidence from tools found at Clovis, another site in New Mexico, in the twentieth century.
Since then, however, sites dating to 15,000 years ago in the eastern US have been discovered, while one controversial paper suggests a date of up to 130,000 years ago. The validity of these findings have been disputed, with some suggesting natural processes could have led to tool-like remains, charcoal and bone markings.
Firmer evidence has now been found in White Sands National Park, which lies close to the US-Mexican border. Human footprints belonging to multiple individuals have been discovered buried in the sediment of what was once the muddy shore of an ancient lake.
What sets this discovery apart, however, is the presence of seeds trapped in the sediment surrounding the footprints, allowing the researchers to use carbon dating to give an age for the footprints themselves. The results found that the seeds, and therefore the trackways, were between 21,000 and 23,000 years old.
'We have a unique window into life during the Pleistocene in North America,' said Dr Thomas Urban, from Cornell University. 'This new study provides the first unequivocal evidence of a sustained human presence in the Americas thousands of years earlier than most archaeologists thought was likely.'
Based on size and estimated gait, the tracks are thought to be those of teenagers and children. The researchers suggest that they were probably being tasked with less demanding roles, such as fetching and carrying, while their parents were out hunting.
'The footprints left at White Sands give a picture of what was taking place, with teenagers interacting with younger children and adults,' said Dr Matthew Bennett, the lead author. 'We can think of our ancestors as quite functional, hunting and surviving, but what we see here is also activity of play and of different ages coming together. A true insight into these early people.'
However, it's not just humans that were living around this ancient lake at White Sands. The mud also preserved the tracks of a range of other creatures, including extinct animals including mammoths, dire wolves, camels and giant ground sloths.
It suggests that the area, which is thought to have been rich in vegetation thanks to a variety of fresh water sources, was an important site for humans and ancient animals. One set of footprints suggests the area may have been a hunting ground, with human tracks appearing to stalk those of a giant sloth.
But as the climate started to change about 12,000 years ago, the freshwater wetlands turned into the desert it is today. At this point, it appears that both humans and wildlife largely moved on.
The site is, however, still changing and now the recently discovered footprints are under threat as they get weathered away.
David Bustos, resources manager at White Sands National Park and co-author, says, 'It is incredible to have the confirmation on the age of the human prints and exciting, but also sad to know that this is only a small portion of the 80,000 acres where the prints have been revealed bare and are also being rapidly lost to ongoing soil erosion.'