Thanks to our friends at Minute Earth for supporting this episode of SciShow.

Click the link in the description to check out their new book MinuteEarth Explains: How Did Whales Get So Big? And Other Curious Questions about Animals, Nature, Geology, and Planet Earth. [♪ INTRO] Most of Earth’s history happened before humans arrived on the scene.

And if you condense all it into one calendar year, all of human history would fit in the last few minutes before midnight on December 31st. In those few minutes, modern humans spread worldwide, developing complex societies and civilizations. And we still don’t know exactly how and when early humans migrated all over the world, which is a fundamental part of our history as a species.

But some ancient footprints might hold the answer. How we arrived in the Americas from places like Africa or Asia is a critical part of human history. Nowadays, we’re pretty sure that modern humans originated in Africa and from there moved into Europe and Asia, which are connected to Africa by land.

But North and South America are an ocean away, and it wouldn’t have been easy to reach. Yet, according to recent research, this migration might have happened earlier than we thought. There are a lot of theories on how and when this journey happened.

One of them is that humans arrived in what’s now Alaska sometime during the last Ice Age, between 26,500 to 19,000 years ago. Back then, sea levels were much lower than they are now, because of the massive ice sheets that dominated most of the Northern Hemisphere. That meant that shallower portions of the coastline were exposed, forming land bridges in certain parts of the world.

One of those land bridges spanned what’s now the Bering Strait, the 88-kilometer gap between Russia and Alaska. Humans most likely crossed from Asia into North America from there, and settled in Beringia, a landmass that’s now underwater. But they wouldn’t have been able to move south because of the massive ice sheets around them.

At the time, almost all of Canada and parts of the northern United States were covered by the Laurentide Ice Sheet, a block of ice that was around 2.4 km thick, or even more in some places. The ice should have made it impossible to cross on foot to the rest of the Americas, at least until the glaciers started to melt around 13,500 years ago, which formed a corridor that let people move south safely. So for a while, that was when researchers thought humans first reached the rest of the Americas.

But that idea got tossed out when scientists found evidence from mitochondrial DNA that shows that humans could have been in South America as far back as 16,000 years ago. That made some researchers think that the first humans reached the rest of the Americas by boat, sailing around the ice from Beringia. Other research groups hypothesize that they took advantage of an earlier corridor that would have opened in the ice along the Pacific coast around that time.

Researchers also found some items that appeared to be tools in Mexico’s state, Zacatecas. That led this set of researchers to think that early humans could have traveled south even earlier than that. But researchers found no evidence of human DNA on the tools.

They also found no evidence of a central hearth, or that the tools had been cut, or any other sign of human activity. So, it’s also possible that the shape of the stones could have been a coincidence. Footprints, however, are an unmistakable sign of a human presence, and the recent discovery of some ancient footprints in New Mexico suggests we might continue to revise the timeline.

These footprints were discovered on outcrops of Lake Otero, located in White Sands National Park. Since the footprints are embedded into distinct sediment layers, researchers can tell how old they are by measuring the sediments' age or other objects embedded in the layers. This is called stratigraphy.

In this case, scientists used seeds of an ancient plant, trapped in the sediments next to the footprints. These seeds were found to be between 21,000 to 23,000 years old, well before our estimates of when humans first reached the Americas. And since multiple footprints are found at different layers of sediment that date different periods, the evidence suggests people lived in the area for at least two thousand years.

So, humans have been in the area far earlier than archaeologists and historians realized. They would have lived alongside megafauna species that are now extinct, like giant sloths, mastodons, dire wolves, and saber-tooth tigers, and would have lived alongside these and other incredible creatures for more than two millennia. By looking at the footprints, researchers can even gain some insight into the society of the people that lived there.

The footprints mostly appear to belong to children and teenagers, with adult footprints being rare. So, researchers hypothesized that there was some kind of division of labor. Adults stayed home and did skilled tasks, while teenagers worked on menial things like fetching and carrying, with the kids following them.

Currently, other artifacts haven’t been found at the site to tell us more about the people who left the footprints or the specific way they got to the Americas in the first place. But it’s clear that we still have a lot to learn about Earth’s past, and our own human history. If you’d like to help a kid in your life learn more about our planet and inspire their curiosity, you can check out MinuteEarth’s new book!

This book answers children’s most curious questions like “Where Earth’s water came from” and “Why leaves change color in the fall.” And they’re accompanied by gorgeous illustrations. And honestly, I don’t think this book is just for kids. I’m learning some cool things from it, too, like how much food there is on earth.

Spoiler alert: It’s not as much as you’d hope If you like what you see and would like to snatch a copy of this book head out to minuteearth.com/books or click the link in the description. [♪ OUTRO]