Carl Zimmer:
In 2000, a team of Boston scientists discovered a peculiar gene in the human genome. It encoded a protein made only by cells in the placenta. They called it syncytin.
The cells that made syncytin were located only where the placenta
made contact with the uterus. They fuse together to create a single
cellular layer, called the syncytiotrophoblast, which is essential to a
fetus for drawing nutrients from its mother. The scientists discovered
that in order to fuse together, the cells must first make syncytin.
What made syncytin peculiar was that it was not a human gene. It bore all the hallmarks of a gene from a virus.
Viruses have insinuated themselves into the genome of our ancestors
for hundreds of millions of years. They typically have gotten there by
infecting eggs or sperm, inserting their own DNA into ours. There are
100,000 known fragments of viruses in the human genome, making up over
8% of our DNA. Most of this virus DNA has been hit by so many mutations
that it’s nothing but baggage our species carries along from one
generation to the next. Yet there are some viral genes that still make
proteins in our bodies. Syncytin appeared to be a hugely important one
to our own biology. Originally, syncytin allowed viruses to fuse host
cells together so they could spread from one cell to another. Now the
protein allowed babies to fuse to their mothers. . . . Syncytin also performs another viral trick to help its human master:
it helps tamp down the mother’s immune system so she doesn’t attack her
baby as a hunk of foreign tissue.
Further research has shown that all placental mammals make syncytin, all using viral genes.
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