Humans are one of the few  animals found on every continent.

This isn’t just because we’re adaptable. We’re also really good at moving around.

But while we know that humans  are excellent travelers, we still don’t know how or when our  ancestors ended up in certain places. Like the Americas. The when, how, and even who of  human migration to the Americas has been a running debate  in anthropology for decades.

Some scientists even think that humans weren’t the first hominin species to make it here. But that idea is not without its critics,  and scientists love a good debate. So let’s take a look at a  few different ideas about who might’ve been the winner of  that great American race. [♪ INTRO] Before we get into the hot takes, let’s  start with what we know with some certainty.

Most researchers agree that the earliest  Americans likely got here by crossing the Bering Strait, the 85 kilometer  gap between Russia and North America. During the Last Glacial Maximum,  about 19 to 26 thousand years ago, sea levels were much lower than they are now, because so much ice was frozen into glaciers. That lower sea level meant that previously  submerged land between the continents was surface-level for a while, so people  and animals were able to just walk across.

The other thing that researchers agree on is the most recent possible time of arrival. We’re fairly confident that humans were  living across the entirety of the Americas by about 13,000 years ago. Which is a good start, but it  still leaves a lot unexplained.

Like, there might have been one group  of people who crossed the land bridge and then spread out over the  entirety of the continents, or there could have been  multiple waves of migration. There’s also a precursor question  before we get to when people reached the Americas, which is when  they reached the Bering Strait. We don’t know if they entered the region  around the Bering Strait, AKA Beringia, and hung out there for a while  before expanding their range, or if they just marched right on through.

These early Beringians most  likely got there from Siberia, but again, when they got there is a bit unclear. See, up until about 13,500 years ago, Beringia was surrounded by ice sheets  that would have been impossible to cross. If they had left Siberia  before the ice sheets melted, they probably would have had  to stay there for a while before they were able to reach  the rest of the continent.

But if they got to Beringia  closer to when the ice melted, they could have just come across  and continued on their way in one big migration wave, which is  called the Clovis-First hypothesis. The idea is that all of the people  living across the Americas descended from one migratory group called the Clovis people, who got their name from an archaeological  site in Clovis, New Mexico, where researchers first found  their signature stone tools. Tools that were constructed in the  same way have also been found at sites across North and South America, which  would make sense if the people inhabiting all of those different sites  had learned to make tools the same way from the same ancestors.

However, there’s another option for how  humans may have gotten to the Americas if not by land, or in this case, land bridge. They could have taken the seafaring route. Some have hypothesized that when  modern humans entered Beringia, they didn’t have to wait  for the ice sheets to melt.

They could’ve hopped in some boats  and navigated through passageways where the ice had already melted and  then down along the Pacific Coast, stopping to camp on the shoreline along the way. They would have had a good source of food in the kelp forests close to the shore, too. This would also open up the  possibility that humans could have gotten to the Americas much earlier  than the ice sheets opened up.

This is a super hard hypothesis to test though, because the sea level rise that  sank the Bering Land Bridge would also have left any of the evidence  of these occupations underwater. Without any evidence in the form of, like, a boat, most anthropologists agreed that the  most likely answer was that humans came in one big wave from Siberia, and  then spread out pretty rapidly across the continents about 13,000 years ago. The genetic evidence suggests that there  were at least two other populations of ancient people who came over from Asia  and settled in the Americas after that.

But in the past few decades, we’ve  found evidence of human populations in the Americas dating back  several thousand years earlier, meaning that if they’re legit,  the land bridge would’ve been unavailable for those people to get here. There are sites from Canada to Chile  that are at least 16,000 years old. Some of these are more  widely accepted than others, but anthropologists do now tend to agree that humans have been here at least that long.

But there are some sites that  researchers claim contain evidence of human occupation earlier  than that 16,000 year mark, and these are the ones that get archaeologists to throw down the gauntlet. Or trowel, if you will. So there’s one site in Texas that’s  between 16 and 20,000 years old, where stone tools were discovered that  don’t look like the ones from Clovis sites, which the researchers argue means that the people who made them  predate the Clovis people.

They use this to contest both  the Clovis-first hypothesis and also the earliest possible arrival of humans, which they say is at least 2,500 years  before the Clovis people got there. It’s a bit surprising, but not  outside the realm of possibility, especially if the site is at  the younger end of the range. But in 2021, researchers announced  that they had found human footprints in White Sands National Park in New Mexico that pushed human occupation back even further.

The researchers used carbon-dating on  seeds embedded in the footprint layer to determine that these footprints  were between 21 and 23,000 years old. But other researchers pointed out a problem. Sometimes when you carbon-date  the type of seeds that they found, they can look thousands of years  older than they actually are, due to the type of carbon the  plants use in photosynthesis.

With some real “per my last email” energy,  the original researchers fired back, acknowledging that while that was true, they’d already considered it and  that their original paper, quote, “presented several lines of evidence  that argued against such a problem”. Trust me, to academics, these are fighting words. And while they can’t agree on  how old the footprints are, there’s no question that humans made them.

Unlike some of other finds. Like the stone tools that researchers  found in a cave in Zacatecas, Mexico called Chiquihuite that  date back as far as 33,000 years. Another research group reanalyzed  the images the authors provide and determined that what the  first paper argues are stone tools are far more likely to be just  broken-up rocks than human-made tools.

See, there are certain signs  that anthropologists look for to determine whether a rock was  shaped by human or natural forces. Things like whether the places where  rock has been flaked off overlap or if they’re randomly distributed,  whether there’s evidence that a sharp rock has been resharpened, and whether the breaks line up with  the mineral structure of the rock. And these other researchers  said that the original authors didn’t do enough to show that  these rocks weren’t just rocks.

In the spirit of constructive criticism, they included a numbered list of ten  different analyses they think the original authors should have done in order  to more strongly support their case. But whether or not these  individual sites are verified, it’s becoming more clear that  the Clovis-first hypothesis can’t be the only explanation, since  there are so many potential sites of human occupation that  predate the Clovis people. Which means there’s still a lot to  uncover, and a lot to argue about.

And then, there’s the Cerutti Mastodon. But before we get into that, all research needs  funding, so here’s a little bit about ours. This SciShow video is supported by Triple

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You can put yourself on that  path using the code “SCISHOW” for 30% off on all their programs. You even get a free career consultation at the link in the description  or the using QR code. So let’s talk about a  mammoth-sized archaeology beef.

Construction workers in California discovered the broken bones of a mastodon  next to some large broken rocks. When anthropologists from the nearby  San Diego Natural History Museum took a look, what they found  was pretty unbelievable. The bones appeared to have been  broken while they were still fresh, which is consistent with human  butchery of hunted carcasses.

The nearby rocks had wear and tear patterns you’d see if they had been used as hammers and anvils. All of which suggests that humans were here. Except there was a problem.

The site was 130,000 years old. Over 100,000 years older than the already  super controversial Chiquihuete site. So it shouldn’t surprise you that  when they published their findings, there was a lot of pushback.

One rebuttal brought up the  possibility that the bones had simply been damaged by  the construction equipment. They also questioned whether the  dating process they used was accurate, and even pointed out that other  mastodons had been found with the exact same fracture patterns, but with  no evidence of human activity nearby. But its overall message really seemed to be “this makes no sense given  everything else we know”.

A second research group jumps  in to back up the rebuttal, saying that they’ve also  seen similar bone fractures on mammoths with no human activity nearby. It’s worth pointing out that  these controversial mastodon bones have plenty of percussion marks, but  really no slashing or cutting marks, which are indications of slicing  meat off a bone and are usually a much more telltale sign that  humans made that thing their dinner. After all, bones can break in all kinds of ways, not just because humans hit them with rocks.

The original authors publish  rebuttals to the rebuttals, then other researchers come off  the top rope to have their say. So now we have the academic  equivalent of a rap beef, with papers flying back and forth, reiterating “here are the reasons you’re wrong”,  “no, here are the reasons YOU’RE wrong”. It is chaos.

Which is kind of to be expected  when you have something that would change everything  we think we know about not just the history of the Americas,  but the history of the world. Because 130,000 years ago, Homo sapiens were still 30,000 years away from leaving Africa. And the earliest evidence  we have of our own species in northeast Asia dates back to 50,000 years ago.

Which would mean that our own  species couldn’t be the ones who broke those mastodon bones,  and some other hominin species would have had to get to  North America before we did. To be clear, we aren’t the first long-distance  trekking species in our family tree. Hominins made it all the way from Africa  to Europe around a million years ago, so an ancient species going on an adventure and spreading out isn’t entirely unprecedented.

But the Bering Land Bridge  didn’t exist 130,000 years ago. It would still be submerged under water  for another hundred thousand-ish years. That’s pretty compelling evidence that  they couldn’t have walked to the Americas.

The other option is boats. Just like with the humans  navigating a frozen Bering Strait, the idea is that this human ancestor species took the water route to get  where they wanted to go. And there is evidence supporting  some degree of Homo sapiens boat migration in the Americas 14,000 years ago.

So getting to the Americas by boat  isn’t out of the realm of possibility, it’s just a question of whether hominins that weren’t us were able to, you know, do that. There is evidence of early hominins  using boats to reach new areas in Asia, with Homo erectus finding their way  to islands a million plus years ago, but those journeys were much shorter, and again, we don’t have any of the boats they used. Homo sapiens only started  taking long-distance sea voyages roughly 50,000 years ago, and even  then, we’re talking trips of 100 km.

So for the pre-human boat hypothesis to be true, it would require that an earlier  hominin species developed long-distance seafaring skills, eighty  thousand years before humans did, left behind no evidence of a single  stop along the way to break up the trip, and then never did it again. Which is a pretty tough sell. So right now, there isn’t really  robust evidence supporting a species before ours making it to the Americas, and the majority of researchers think  these bones are just old, broken bones.

It’s one of those “extraordinary claims  require extraordinary evidence” things, and for a lot of people, these  mastodon bones just don’t cut it. As we’ve seen, it can be really hard to interpret the types of evidence that we do have. But this is what science does so well.

Scientists hypothesize, test, argue, and refine, all with the goal of getting as  close to the truth as we can. So hopefully, once more evidence comes to light, we’ll have a more conclusive  and agreeable answer to exactly how humans made it to the Americas. But that certainly won’t stop researchers  from arguing about it in the meantime. [♪ OUTRO]