At Real ClearScience, Alex B. Berezow advises us to “Simmer Down” because viruses are “Not ‘Fourth Domain’ of Life”:
The biggest difference between cells and viruses is their method of replication. All three domains of life replicate by cell division, which implies that this trait was derived from the Last Universal Common Ancestor (LUCA). (In other words, LUCA is the theoretical ancestor of Bacteria, Arcahea, and Eukarya.)
Viruses, which do not replicate by cell division, probably evolved independently multiple times, “here, there, and everywhere,” as the authors conclude. Some probably evolved before LUCA, and others well after LUCA. Many have likely exchanged genetic material via horizontal gene transfer. Lumping them all into a fourth domain, therefore, makes little sense.
Little sense? Not necessarily. A fourth domain can consist simply of “Others.” What they have in common is that they are thought to have evolved multiple times and exchanged genetic material by horizontal gene transfer. (Where they got all that material is a separate question.)
The viruses that set this fourth domain idea in motion are the recently discovered giant ones. See, for example,
The Scientist asks, Should giant viruses be the fourth domain of life?
Biggest virus ever: “We don’t understand anything anymore!”
Largest virus genomes hint at fourth domain of life
Berezow tells us that the whole affair is not academic but rather raises anew the question “What exactly is life, and how did it evolve?”
Indeed. Here is a small slice of the problem:
The definition of life has reached the point where science historian George Dyson tells us, “Life is whatever you define it to be.” Richard Dawkins has suggested it is “anything highly statistically improbable, but in a particular direction.” And at a year 2000 international “What is life?” conference, no two definitions were the same. Biochemist Edward Trifonov noted that there are 123 definitions available and, undeterred, promptly proposed his own: Life is self-reproduction with variations. Which was just as promptly contested. In a 2012 issue of philosophy journal Synthèse, Edouard Machery concluded that “scientists, philosophers, and ethicists should discard the project of defining life.”
Still in the game, astrobiologist Charley Lineweaver proposes a new, non-Darwinian approach to defining life:
If we don’t know what life is, the question of whether viruses are a fourth domain is academic, actually. In fact, contra Berezow, it is a classic illustration of an academic question. It cannot be settled with available information.
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