- In 1994, the bones of a family of Neanderthals were discovered in El Sidron Cave in northwest Spain.

They date to around 49,000 years ago, not long before the final days of the Neanderthals.

And times must have been tough when this family lived because their remains show signs of starvation and nutritional stress throughout their lives.

But their bones also give us other clues that food might have been hard to find because it looks like these 13 Neanderthals were probably cannibalized.

And only a few thousand years after their deaths, Neanderthal remains no longer appear in the archeological record.

These human cousins became extinct.

So, what happened to them?

These relatives of ours lived in Eurasia for more than 300,000 years.

They were expert toolmakers using materials like stone, wood, and animal bone.

They were also skilled hunters and foragers and may have even created cave art.

So, what caused the decline and the disappearance of their population?

Well, in a way it could have been us, but maybe not in the way you might have heard.

(somber music) Our species, Homo sapiens, first arrived in Europe around 45,000 years ago and were very similar to Neanderthals.

Similar enough, even, to interbreed with them, which we know we did from the small amounts of Neanderthal DNA that still exist in the human genome today.

But only a few thousand years after humans entered Europe, the Neanderthal population became smaller and smaller before disappearing altogether.

So, what happened between us?

You might picture epic scenes of our ancient relatives battling it out for survival and for good reason.

This has been a popular hypothesis for a long time.

It comes up a lot in debates about Neanderthal extinction and has been used to fuel arguments about human nature, but there's actually very little physical evidence for direct conflict between these populations.

Instead, according to some anthropologists, the reason for Neanderthal extinction and our survival may come down to a very small difference based on the humble calorie.

Now, a calorie is a unit of energy that your body takes from the food you eat and uses to power all the bits and pieces that make you a living, breathing person.

The baseline number of calories you burn in a day just to keep your body functioning is called basal metabolic rate.

It's difficult to accurately estimate the number of calories a person uses each day, but it depends in part on how much muscle mass that person has and how active they are.

The more muscle you have and the harder you use it, the more energy your body needs and the more calories you burn.

And this matters because there are a few key differences between the Neanderthal body and ours.

Overall, Neanderthals were a bit shorter than the average Homo sapiens and they were also probably stockier.

Their bones are shorter, thicker, and chunkier than ours, which indicates that they likely had very dense muscles.

The average Neanderthal was probably more muscular than the average human at the time or the average human now.

And while both groups lived as hunter gatherers there's evidence from Neanderthal skeletons that they had a really active daily life, often doing strenuous activities.

For example, the bones of Neanderthal legs and feet suggest that they were adapted to powerful sprinting rather than long distance running.

Their two lower leg bones, the tibia and fibula, are shorter in proportion to their thigh bone than in Homo sapiens.

This creates leverage against the ground that is great for speed but not as efficient for long jogs.

Even their DNA suggests that Neanderthals were built to be muscular.

The Neanderthal genome contains many genetic variants that in humans today are linked to high level performance in power sports like weightlifting or shot put.

And with those big muscles Neanderthals probably burned through a lot of energy.

Researchers studying the remains of both Neanderthals and early Homo sapiens have used their skeletons to estimate how much muscle each would've had on average and the results suggest Neanderthals, overall, probably had more muscle tissue.

This means they'd start off with a higher basal metabolic rate than Homo sapiens would.

So even if both Neanderthals and Homo sapiens did the same task every day, Neanderthals would still use up more calories.

In fact, according to one study, a Neanderthal would need to eat a couple hundred more calories per day than the average human, around the equivalent of an extra hamburger per person, per day, which might not seem like a lot, but think about it over the course of a year.

If 10 Neanderthals and 10 ancient humans each ate only hamburgers for a year, the Neanderthal population would need 10 extra burgers per day for 365 days.

That's 3,650 extra burgers.

In short, a population of Neanderthals needs more food resources than a human population of the same size to survive, reproduce, and keep their population numbers up.

So, picture a scene when a group of Homo sapiens first make their way into Eurasia.

Over the next next few centuries you've got both Neanderthals and Homo sapiens living in the same areas who need very similar resources to survive.

One group needs more food than the other and there's no grocery store in town.

Each group has to hunt and gather for themselves.

What's gonna happen?

Well, if Neanderthal groups faced competition for prey or other foods from Homo sapiens, they would be at a disadvantage because of their need for more calories.

And if a population can't get enough food to sustain everyone, members of the group will die and reproduction will happen more slowly.

So, based on this model, when Homo sapiens' populations reached the same areas of Europe where Neanderthals lived, a couple of different scenarios might have happened.

Either the modern humans gradually push the Neanderthals farther and farther south or groups of Neanderthals and Homo sapiens mingled together, inter breeding and spreading out farther and farther over time.

And it's likely that a mix of these scenarios is actually what happened.

We know that Neanderthal DNA entered the Homo sapiens' genome at several points throughout our shared history.

Over time, in multiple generations, the two genomes would have blended more and more and as there were fewer and fewer Neanderthals around their genetic contributions to the shared gene pool would have faded into the background.

We also know that sometimes both Neanderthal and Homo sapiens' populations experienced very difficult conditions with scarce resources.

A major climate shift right after Homo sapiens migrated into Europe caused long periods of cold, dry climate with severe winters.

Those conditions would've made it pretty difficult to be a hunter gatherer, no matter which each group you were from.

So, if another population of a different kind of human joined you on the landscape and was competing for the same resources that you need, it would've made it even harder to survive.

Neanderthals lived through several other periods of cold, dry temperatures during the 300,000 or so years before humans arrived, but it's possible their populations just couldn't survive the combination of cold and competition.

We can see evidence of how hard it was for Neanderthals to survive in glacial periods without competition by looking back at that family from El Sidron Cave.

They lived about 5,000 years before the big climate shift, even before Homo sapiens arrived on the scene, and most of the members of that family had signs in their teeth and skeletons that they had lived a life of pretty constant nutritional stress.

For example, there were defects in the enamel of their teeth that come from dental development being interrupted by things like malnutrition.

Their diet was likely made up of local plants and animals that they could hunt or scavenge.

During colder seasons food would and even more scarce and they may have endured long periods of near starvation.

This also seems to have been the case for other groups in the area because the bones of the El Sidron Neanderthals have cut marks and breaks on them that indicate their bodies were butchered with stone tools and the only types of stone tools present are a kind typical made by Neanderthals.

Archeological evidence for Homo sapiens in that region of Spain doesn't show up until later in time, so it was probably a different Neanderthal group desperate for food that at least partially consumed the El Sidron family.

And it's not the only site with evidence for Neanderthal cannibalism either.

This wasn't an isolated incident.

We've also found cut marked Neanderthal bones at sites in France and Belgium.

But the fossils from that cave in Spain capture a particular moment in time.

A snapshot of what life might have been like for one of our human relatives.

And while this does sound horrifying, it tells us that this local population was probably struggling to survive.

The ultimate cause of the disappearance of the Neanderthals is still an open question and several different causes probably all contributed, but there's no doubt that they thrived in their environments for hundreds of thousands of years.

And archeological evidence shows that they were making similar tool technologies and behaving in similar ways to ancient humans for a lot of the time that the two groups both lived in Europe.

So, it's possible that one big difference between the survival of the two groups might have just been a small unit of energy.

(upbeat music)