They dead.

5 theories on why Neanderthals went extinct

There's a reason we're the only humans left on Earth.

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Some 300,000 years ago, nine human species lived on Earth. Today, Homo sapiens are the only members of our genus left. What happened to the rest of the humans is a paleoanthropological cold case.

Of particular interest is the disappearance of our closest relatives — the Homo neanderthalensis, or Neanderthals, who went extinct 40,000 years ago.

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The first Neanderthal fossil was found in Belgium in 1829.

But it wasn't until 1858 that scientists realized they were dealing with a different type of human.

It all had to do with a skull.