It has been suggested that the first posts of this book club be devoted to the Universal Rules of Life. So… What is life?
Carl, twice in the book you refer to viruses as “creatures.” Perhaps you used the word metaphorically. In any case I’d love to know whether you think viruses qualify as being alive, and I’d love to hear your reasoning either way.
Historically, viruses have been considered non-living. Some of the first discoverers of viruses, Frederick Twort for example, thought they were enzymes secreted by bacteria. Other biologists, such as Felix d’Herelle, contended that viruses were alive. The distinction lies largely on how “life” is defined.
Viruses certainly do straddle the borderline between the living and the non-living. In some ways, they resemble chemicals. In essence, they are nucleic acids encased in protein. They are inert much of the time, only becoming active on finding and entering a host. They are unable to reproduce on their own. They cannot consume food, breathe, chase prey, respond to the environment in ways typical of the organisms most familiar to us.
But many organisms we classify as living occasionally show the inability to conduct activities we associate with life. For example, is reproducing on its own truly a characteristic of life? Obligate symbioses are common in the biological world. These organisms are unable to survive without the assistance of another organism. For example, some flowers cannot reproduce without assistance from bees. I would argue we are all obligate symbiotes deep down.
The best definition of life I have encountered comes from Salvatore Luria, he of the Slot Machine Experiment ably described by Zimmer in Microcosm.
“An organism is the unit element of a continuous lineage with an individual evolutionary history.”
SE Luria, JE Darnell, D Baltimore and A Campbell (1978). General Virology, 3rd Edn. John Wiley & Sons, New York, p4 of 578.
With this definition, viruses are unequivocally alive. I’ve blogged briefly about this definition previously here.
Photo: A thin section of T4 phages hitting a microcolony of E. coli K-12 by John Wertz.