Where Did Viruses Come From?

Summary

Viruses are too small and fragile to leave conventional fossils, but pieces of viral genetic material can become integrated into a host’s DNA. If this material enters a cell that produces eggs or sperm, it may be inherited by later generations. These inherited sequences act as molecular fossils, allowing scientists to investigate viral evolution through paleovirology.

A basic method for dating these traces is to compare similar DNA sequences across organisms:

1. Find sequences of viral origin in host genomes.
2. Compare the sequences across species.
3. If related species share a sequence, infer that they inherited it from a common ancestor.
4. The virus must therefore be at least as old as that ancestor.

Examples include:

• Circovirus traces found in dogs, cats, and pandas, suggesting that these viruses may date back to a shared mammalian ancestor as much as 68 million years ago.
• Bracoviruses, which infect wasps. A 2011 study suggested their lineage may extend back about 310 million years, to the Carboniferous.
• The mammalian gene CGIN1, which a 2009 study dated to roughly 125–180 million years ago and proposed originated from a retrovirus.
• An estimated 8% of the human genome, which consists of sequences originally derived from viruses.

Hypotheses for the origin of viruses

The video presents three debated hypotheses:

• Virus-first model: Viruses arose before cells from simple self-replicating units, perhaps based on RNA.
• Escape hypothesis: Viruses evolved from pieces of cellular genetic material that gained the ability to leave cells and spread.
• Regressive model: Viruses descended from more complex, once-free-living organisms that became dependent on hosts and lost complexity. Giant viruses such as Mimivirus, discovered in 2003, have been discussed in connection with this idea. However, some scientists argue that Mimivirus’s extra genes may have been acquired from hosts rather than inherited from a complex ancestor.

Are viruses alive?

Whether viruses are alive depends partly on how life is defined. Viruses can evolve and reproduce, but they cannot reproduce independently, generate their own energy, or maintain an internal environment as cells do. The video describes them as occupying a gray area between living and nonliving things.

Researchers and sources mentioned

• No individual researchers are named in the subtitles.
• A 2011 study on bracovirus history.
• A 2009 study on the mammalian gene CGIN1.
• The 2003 discovery of Mimivirus.
• CuriosityStream, identified as a supporter of PBS Digital Studios.