There is no agreed-upon definition of biological life within the scientific community, though this is not for lack of trying. In the fourth century B.C., Aristotle wrote that anything capable of movement and “self-sustenance” is alive. In the nineteenth century, vitalists argued that all living things are animated by a vital spark—an élan vital—while mechanists believed that the whole is no more than the sum of its parts, and that life functions could be characterized by their component chemical and physical systems. In 1859, Charles Darwin published On the Origin of Species, and subsequent definitions of life have, for the most part, included the stipulation that a living thing must be capable of evolution.
New definitions of life must account for a slew of edge cases, including synthetic molecules and extraterrestrial life. In a Science paper from 2002, Daniel Koshland, a former professor of molecular biology at UC Berkeley, describes a meeting of the scientific elite that attempted to formulate such a definition:
“The ability to reproduce—that is the essential characteristic of life,” said one statesman of science. Everyone nodded in agreement that the essential of a life was the ability to reproduce, until one small voice was heard. “Then one rabbit is dead. Two rabbits—a male and female—are alive but either one alone is dead.”
The discussion ground to a halt. “At that point, we all became convinced that although everyone knows what life is there is no simple definition of life,” Koshland writes. In lieu of a simple definition, he offers a list of traits, deemed essential to all living things, that he calls the Seven Pillars of Life.
The classification of viruses has remained as elusive as the definition of life. By Koshland’s criteria, viruses aren’t alive—they require a host cell in order to reproduce—but the existence of giant viruses has called our previous understanding of viral “life” cycles and life, in general, into question.
Chantal Abergel and Jean-Michel Claverie, both virologists at Aix-Marseille University, are specialists in outsize viruses. The husband-wife team discovered Pithovirus, a genus of the world’s largest known viruses, in 2014. This finding unseated Pandoravirus and Megavirus (discovered in 2013 and 2011, respectively, by Abergel and Claverie) as the previous holders of this title. Viruses belonging to all three genuses are so large that they resemble bacterial cells under a light microscope (an electron microscope is needed to see most viruses).
For their part, Abergel and Claverie maintain that all viruses are living, regardless of size. More specifically, the viral particle, called the virion, is non-living until it has invaded a host cell and commandeered its machinery, at which time both virus and host are very much alive. “You wouldn’t assume that an organism is its gamete,” Abergel explains. Claverie offers a similar analogy: “Viruses are sometimes alive and sometimes not, like a plant’s seed.” Furthermore, a virus’s parasitism shouldn’t preclude it from classification as a living thing, Abergel says. Parasitic cells—including certain bacteria and protozoa—are considered alive.
Ultimately, a binary classification fails to do proper justice to the complexity of the viral life cycle. Instead, it is useful to think of life in gradations, according to Claverie—certain viruses are more alive than others. He compares the viral genome to an instruction manual for functions that a virus can independently execute without a host cell.
One giant virus, the mimivirus, has shown evidence of genes necessary for metabolism, despite the prevailing assumption that all viruses must rely upon their hosts for energy production (“This dogma will fall down, as well,” says Abergel). The enormous mamavirus, whose genome encodes over 900 genes, can become infected by a smaller virus, called the Sputnik virus. Bacteriophages are viruses that infect bacteria; the Sputnik virus is a virophage. That the mamavirus can become sick, coupled with its size, makes it seem all the more bacteria-like and, therefore, alive.
Claverie and Abergel dispute the validity of comparisons between viruses and bacteria, however, and prefer to wash their hands of the entire living-or-not debate when possible. According to Claverie, a virus isn’t an object, but a concept. Viruses aren’t distinguished by a single, common physiological trait (bacteria, for instance, all have ribosomes), but by their unique approach to replication. For this reason, comparisons between viruses and bacteria aren’t useful, says Claverie, and existing definitions of life are hard-pressed to illustrate the true diversity of viruses.
It seems that a sliding scale would be more useful where there have only been black-or-white definitions. “We—humans—like to put things into boxes,” says Abergel. “For me, a continuum is much more relevant.”