White Sand National Park in the New Mexico desert has one of the world's largest collections of fossil footprints.

Here, mammoths, camels, giant bedbugs and humans have left traces in the soft ground.

Footprints of children and young people

Now researchers have managed to identify footprints from humans and with the help of seeds from seaweed have been able to date them to be between 21,000 to 23,000 years old.

And there are traces of small feet from children and young people that have been preserved as fossils.

"Our hypothesis is that the younger children followed the teenagers and that their task was to be collectors and to carry things while the adults had more specialized tasks," writes Matthew Bennett, professor of geology at Bournemouth University and lead author of the study published in this week's Science.

Lakes and steppes

Then there were lakes here and the landscape was more like a grassy steppe than the dry desert that this part of New Mexico consists of today.

It was temporarily relatively warm, even though it was in the middle of the last ice age that ended about 12,000 years ago and began 115,000 years ago.

Blocked hiking trails

In addition, it was just before the period known as the ice age maximum that occurred 20,000 years ago when it was the coldest and the ice sheet was the largest.

At that time, people's hiking trails from the drained Bering Strait between Russia and Alaska and to the south were completely blocked by inland ice.

Therefore, immigration must have taken place either before or after the maximum 20,000 years ago.

To this day, the oldest completely reliable evidence of human presence in America has been the preserved remains of people who lived 13,000 years ago.

But tools have been found that are dated to be up to 40,000 years old.

But the dating has been uncertain and questionable.

Istidsmaximum

- The new dates certainly show that people have been in North America before the ice age maximum, and then they have also come quite far south, says Fredrik Charpentier Ljungqvist who is a historian and climate researcher at Stockholm University.

The dating of the footprints now puts an end to a several-year-long debate among researchers about when the first modern humans migrated across the drained Bering Strait between Asia and the Alaska Peninsula and continued south.  

- Probably the first humans migrated in several thousand years before the ice age maximum, says Fredrik Charpentier Ljungqvist.