https://www.pbs.org/wgbh/nova/article/endogenous-retroviruse...
From the NOVA episode which gets to the human related part, (but that whyy article is on the protomammal rather than primate and thus may be of more interest to you).
> All was going as planned until McCoy’s bioinformatics specialist Steve Howes rushed into his lab in 1997 to show him the sequence of a gene they called syncytin, which their work showed was secreted by placenta tissue.
> Before McCoy could go public with his discovery, he needed to figure out exactly what syncytin did, a job he passed to bench scientist Sha Mi, who everyone called Misha. Misha’s experiments seemed to be going as planned until, a few months later, she, too, rushed into McCoy’s lab with findings of her own.
> Syncytin is produced only by certain cells in the placenta, and it directs the formation of the cellular boundary between the placenta and maternal tissue. Approximately one week after fertilization, the egg, now a hollow ball of cells called a blastocyst, implants itself into the uterus, stimulating the formation of the placenta, which provides the fetus with oxygen and nutrients while removing carbon dioxide and other wastes. It also serves as a barrier to prevent infection and keep maternal and fetal blood separate. (Mixing the two could cause a fatal autoimmune response.) The cells in the outer layer of the blastocyst form the outer layer of the placenta, and those in direct contact with the uterus are the only ones that made syncytin.
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This then links to
Syncytin is a captive retroviral envelope protein involved in human placental morphogenesis https://www.nature.com/articles/35001608
Retroviruses push the envelope for mammalian placentation https://www.pnas.org/doi/full/10.1073/pnas.1121365109
Also the wikipedia page on it: https://en.wikipedia.org/wiki/Syncytin-1 (and all the references that one finds in a science related wikipedia article)
For any virologists out there, is this question similar to estimating the date of a Git commit? Or maybe it's similar to finding the nearest tag to a given commit, where a "tag" is some external reference point for a known age (like a fossil or ice core)?