A Population Too Thin

There were never many Neanderthals. Even at their peak, the whole species may have numbered only in the tens of thousands, spread from Spain to Siberia. Modern humans later poured into the same land in far greater numbers.

Tiny numbers carry a hidden cost: inbreeding. With so few mates nearby, Neanderthals often paired with close relatives. One Siberian individual's genome shows her parents were as related as half-siblings. That thins out genetic variety, lets harmful mutations build up, and leaves a small population less able to adapt when conditions turn harsh.