Did viruses help make us human? As weird as it sounds, the question is actually a reasonable one to ask. And now scientists have offered some evidence that the answer may be yes.

If you’re sick right now with the flu or a cold, the viruses infecting you are just passing through. They invade your cells and make new copies of themselves, which burst forth and infect other cells. Eventually your immune system will wipe them out, but there’s a fair chance some of them may escape and infect someone else.

But sometimes viruses can merge into our genomes. Some viruses, for example, hijack our cells by inserting its genes into our own DNA. If they happen to slip into the genome