You don’t need to look that far: Syncytin, a protein involved in placental development, is derived from an ancient retrovirus[1]. In other words, a virus made m

You don’t need to look that far: Syncytin, a protein involved in placental development, is derived from an ancient retrovirus[1]. In other words, a virus made mammals possible. [1]: https://whyy.org/segments/the-placenta-went-viral-and-protomammals-were-born/ https://whyy.org/segments/the-placenta-went-viral-and-protom... EDIT: If I recall correctly, endogenous retroviruses are involved in brain development as well.