Virus
The mammalian placenta depends on a protein that came from a virus. An ancient retroviral sequence was repurposed to make syncytin, which helps cells fuse at the boundary between mother and fetus. Placental mammals may partly owe their birth system to an old viral infection.
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About 8% of the human genome is made of ancient retrovirus sequences. When viral DNA entered sperm or egg cells, the infection did not die with one body; it became inheritable, passed down like a gene. Our DNA carries molecular fossils from viruses that infected our ancestors.
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Researchers found 33 ancient viruses in a Tibetan glacier core, about 15,000 years old. 28 were new to science. Because ice can preserve microbes and viruses for millennia, melting glaciers may expose long-frozen microorganisms to today's environment.
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