- [Ella] The Neanderthals were failing to cope with the changing conditions.

A struggle uncovered in a cave known as the Tunnel of Bones.

Oh, wow.

So this is the famous El Sidron cave.

- It is, yes.

- It's got more character than I was expecting actually.

- Here is where we found 2,500 neanderthal bones.

We found 13 neanderthal individuals of different ages and sexes.

- Such a diverse group in terms of individuals, all found in one spot.

- Genetic studies told us that they are related.

So it's a family group.

We know, for example, that one female, we are not completely sure, genetic information said was red hair.

- And you know, when you say that one of those people had red hair, it suddenly brings what are just fossils, really, to life.

(bright music) It's a cave that's filled with ghosts.

- We think these people were killed by other group.

(ominous music) - [Ella] This was not a natural death Cracked skulls and precise cuts on the bones suggest that this was a brutal massacre.

13 people killed by another neanderthal group, but closer analysis of their remains revealed an even darker truth.

So what do the bones actually tell us?

- The first thing the bones tell us is that this neanderthal group cannibalized another group.

- They were really eating these 13 individuals?

- Yeah, yeah, yeah.

- So how do we know that this was cannibalism as opposed to just straight up murder, or for that matter an animal coming?

- Well, because we have seen in the bones what we call cut marks.

And also we have found a lot of bones broken just to get the marrow.

- Yeah.

Ohhh.

- So there are, both things.

We have cut marks and then they broke the bones to get the marrow.

- Yeah, if you're getting bone marrow, - That's- - That is an indication of cannibalism for sure.

- Yes, it is.