The story of exactly how and when people from other continents populated the Americas is still unfolding. Scientists have uncovered stunning new evidence – score of human footprints in New Mexico dating to 21-23 thousand years ago, 5-7- thousand years older than the previous oldest evidence.

The evidence is pretty clear now that humans evolved in Africa and later spread throughout the world. We were not the first hominid species to leave Africa, that was Homo erectus, who spread to Europe and Asia about 1.8 million years ago. Meanwhile those that remained in Africa continued to evolve into other species, including Homo heidelbergensis, which is the current best candidate for the most recent common ancestor between modern humans and Neanderthals. Heidelbergensis also migrated out of Africa, and in Europe and Asia evolved into Neanderthals (who were well established by 400,000 years ago). Meanwhile their cousins back in the homeland evolved into modern humans.

Modern humans migrated out of Africa about 80,000 years ago, and spread throughout the world. Getting to Europe and Asia is easy, because they are all connected by land. Getting to the pacific islands was probably through a combination of land bridges during times of low ocean levels and traveling across water by some means, probably with short distance island hopping. The same is true of Australia, some combination of land bridges and island hopping.

The Americas present a unique problem, however, because they are mostly separated from the rest of the world through vast oceans, and only a reasonably advanced boat would have a chance of making the crossing. Only extreme northern routes are plausible. One minority hypothesis is called the Solutrean hypothesis – that people from Europe were the first to migrate to the Americas traveling along packed ice during the last glacial maximum essentially from Europe to Iceland, Greenland, and then Canada. They could have walked this route, or traveled by boat along the continuous shoreline of northern ice. There is little evidence for this hypothesis and it is not generally accepted by experts.

The dominant hypothesis for how humans got to the Americas is through the Bering land bridge that connects Asia to Alaska at certain times in history. The Atlantic and Pacific routes, however, do share some common principles worth emphasizing. One is that during the period of time when people may have migrated to the Americas, we were going (and are still going) through an Ice Age with periods of glaciation and inter-glacial periods (right now we are in an inter-glacial period). The Last Glacial Period lasted from 115,000 to 11,700 years ago. Further, during glacial periods glaciers advance and retreat, causing glacial maxima and minima.

According to Clark et al., growth of ice sheets commenced 33,000 years ago and maximum coverage was between 26,500 years and 19–20,000 years ago, when deglaciation commenced in the Northern Hemisphere, causing an abrupt rise in sea level. Decline of the West Antarctica ice sheet occurred between 14,000 and 15,000 years ago, consistent with evidence for another abrupt rise in the sea level about 14,500 years ago.

The timing of the glacial maxima are critical to understanding human migration to North America from Asia. During these times the ocean is at its lowest level, exposing the land bridge to Alaska. But also, the extent of ice could make it easier for boats to travel along the shore, all the way down across the Pacific and down the western coast of North America. Traveling by water in sight of the shore is much easier than navigating open ocean.

Evidence for humans in the Americas has a few forms – human fossils, human artifacts (mostly stone tools), and trace fossils like footprints or cave paintings. For a long time the oldest evidence of humans in North America was the Clovis culture, defined by a characteristic stone arrow and spear heads known as Clovis points. These go back to about 11,500 years ago, right at the end of the Last Glacial Period. The idea of Pre-Clovis humans was controversial, but eventually evidence was discovered of Pre-Clovis humans in the Americas, pushing back the date of the first people on the continents to about 16,000 years ago.

There were also still many questions – how did they get to the Americas, and how quickly did they spread? If we find evidence of humans in South America, how long should we figure it took them to get there? If they traveled down the coast by boat, perhaps not long at all. If they slowly spread by expanding their population and range, then it could have taken thousands of years.

Into this story comes the new evidence – footprints that reliably date to between 21,000 and 23,000 years ago. This time period is significant because it is right during the last glacial maximum, when the Bering land bridge would have been open, so in a way, this makes sense. There are many footprints, mostly children and adolescents, but with a few adults. This was likely in the muddy remains of a dried lake (during a time of relative dryness), later filled with sediment to preserve the footprints. Footprints can be dated reliably because by definition they are embedded in the substrate of the exact moment they were created. Stone tools and bones can sink into older dirt, or become mixed in sediments. But footprints have to be in the layer in which they were made. You can then date the layers just above and below the prints and get your date range, which is what they did here.

We still don’t know who these people were. We have no stone tools or bones (and therefore DNA) to go with the footprints. We therefore also do not know if they are the ancestors to the other Pre-Clovis people from 16,000 years ago, or to the Clovis people. Was there one migration during the last glacial maximum, or waves of multiple migrations during period of relatively (if not always maximal) high glaciation? How and how quickly did they spread throughout the Americas?

There is still so much we don’t know that each new find is a major new puzzle piece, as it is here. Hopefully there are more dramatic pieces of evidence waiting to be found.