May 21, 2001
Bernard Korzeniewski of the Institute of Molecular Biology at Jagiellonian University in Kraków, Poland, says that definitions of life usually list the attributes an organism must have, such as genes, a certain level of complexity, the ability to reproduce and evolve, and so on. But this, he says, merely describes life rather than providing a useful way of deciding what's alive and what's not.
So Korzeniewski set out to formulate a fundamental definition of life, "which would apply not only to life presently existing on our planet but also to the first living organisms on Earth, as well as to life-like phenomena existing presumably on other planets in the Universe", as he puts it.
His definition is this: "A network of inferior negative feedbacks subordinated to a superior positive feedback." In other words, it's a system that tries to regulate itself to preserve its identity.
It might not sound like much, but Korzeniewski is convinced that this broad formulation - which he calls a cybernetic definition of life - pinpoints the defining function of life, rather than listing its attributes. "In spite of the enormous diversity of life forms, the most fundamental principles of biochemical construction and function are astonishingly alike," he says.
Under his definition, sterile worker ants are not alive because they rely on others to reproduce and preserve their own identity. And although it would be anathema to most people, he says the same applies to infertile humans.
But an ant colony is a living system, according to Korzeniewski, because individuals work together to preserve the colony as a whole. "An ant is alive in the manner, say, a liver or a heart is - only as a part of some bigger system," he says. Individual ants may be complex systems, but complexity is not unique to life - just look at computers.
Similarly, viruses, cancers and parasitic DNA that has no function but gets passed down the generations, all count as alive. All are capable of producing regulatory enzymes that ultimately help them reproduce.
But prions and viroids - disease-causing particles of RNA smaller than a virus - do not have this ability, and should not be considered alive.
One reason why life is such a great mystery is that no one has yet been able to define it, says Mark Bedau, a philosopher of biology at Reed College in Portland, Oregon. Bedau welcomes the generality of the new definition. "But the test of a good definition of life is whether it can settle ambiguous cases," he says.
Korzeniewski says his definition also pinpoints when life emerged. He believes life began with RNA - the first molecules capable of perpetuating their identity through replication.
More at: Journal of Theoretical Biology (vol 209, p 275)
Abstract: New Scientist magazine