PBS:

The rise of the mammals may be feel like a familiar tale, but there’s a twist you likely don’t know about: If it wasn’t for a virus, it might not have happened at all.

One of the few survivors of the asteroid impact 65 million years ago was a small, furry, shrew-like creature that lived in underground burrows and only ventured out at night, when predators weren’t active. The critter—already the product of some 100 million years of evolution—looked like a modern mammal, with body hair and mammary glands, except for one tiny detail: according to a recent genetic study, it didn’t have a placenta. And its kind might never have evolved one if not for a chance encounter with a retrovirus.

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