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/109/7/2184.long https://www.nature.com/articles/35001608 https://www.nature.com/articles/ng.3449 The Ri is on Patreon: https://www.patreon.com/TheRoyalInstitution and Twitter: http://twitter.com/ri_science and Facebook: http://www.facebook.com/royalinstitution and Tumblr: http://ri-science.tumblr.com/ Our editorial policy: http://www.rigb.org/home/editorial-policy Subscribe for regular science videos: http://bit.ly/RiSubscRibe