What if, for instance, viruses weren't always parasites? What if they evolved before life split into eukaryotes, bacteria, and archaea, but subsequently lost some of their independence? In that case they would have every right to be called alive—and they might hold clues, as many clues as the other three groups, about our Last Universal Common Ancestor (LUCA). Since LUCA is practically the holy grail of biology, it doesn't do to ignore the possibility, and the claim is not without foundation. Around half of Mimivirus's genes are unknown to science; no one has a clue what they encode.
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