During an ancient viral infection, a protein was embedded into the human genome, allowing the placenta to develop as it has.

This animation was produced by Diana Gradinaru, supported by the Genetics Society in celebration of their 100th anniversary in 2019.

Further reading:
https://www.pnas.org/content/109/7/21…
https://www.nature.com/articles/35001608
https://www.nature.com/articles/ng.3449