The 1 to 3 percent Neanderthal DNA in your cells — if your ancestry includes any non-African heritage — is the diluted legacy of first-generation hybrids like Denny who lived tens of thousands of years ago, whose mixed ancestry was passed to their children, and gradually diluted through generation after generation of reproduction, until only a small fraction remained.

The specific Neanderthal variants you carry in your genome may influence your immune system, your risk of certain diseases, your skin and hair characteristics.

They are not neutral historical artifacts.

They are functional pieces of genetic code that are still operating in your body, the product of evolutionary pressures that acted on the Neanderthal populations that first developed them, hundreds of thousands of years ago.

If you have Oceanian ancestry, the Denisovan DNA in your genome is a similarly active legacy — and the high-altitude adaptation in Tibetan populations is one of the clearest examples anywhere of a gene that was transferred from one ancient human lineage to another through interbreeding and that has been positively selected in the population that received it.