We might not know the meaning of life, but a group of scientists working for NASA came up with a definition for it that's just seven words long: "Self-sustaining chemical system capable of Darwinian evolution." But does this truly encompass all life, including the types we have yet to discover?
This definition of life was developed in 1994 by a committee of scientists to offer NASA guidance as it developed missions to detect extraterrestrial life.
There are, of course, competing definitions of what constitutes life—including some scientists who argue that such a definition isn't even possible. And there are researchers who have tried to define life by listing its basic characteristics. But a list needs a theoretical framework. Otherwise, it's difficult to make the case that these characteristics would be universal, found among life forms that we haven't yet encountered.
So, the NASA committee sought not merely to offer a working definition of life, but a basic theory of life. The scientists chose their terms carefully, imbuing their seven-word definition with a conceptual heft that belies its brevity.
Let's look at this definition again and break it down, piece by piece: "Self-sustaining chemical system capable of Darwinian evolution."
At first glance, the term self-sustaining doesn't make much sense. Life needs to eat. To grow and develop, living creatures need foremost to be consumers, since growth includes changing biomass, creating new individuals and the shedding of waste. But, in this context, self-sustaining means that life doesn't need continuous intervention by an intelligent being—be it God, a graduate student or a gardener—to provide its sustenance. Given an environment with sufficient resources, it can survive on its own.
Next, we have chemical system. In part, this recognizes that life is the integration of multiple, interdependent metabolic processes. But, the term "system" also distinguishes between "life" and "living," which are not necessarily one and the same. A blood cell in your body is living tissue, but, by itself, it is not life.
And now, the biggie: capable of Darwinian evolution. There's a lot packed into that phrase. When we speak about Darwinian evolution, we refer to the mechanism behind natural selection that allows life to survive and adapt to changing environments. In the broadest sense, Darwinian evolution means that "life" must be capable of making perfect copies of imperfect information during reproduction, and then passing that information to its progeny, across generations. In terrestrial life forms, that information is encoded within DNA.
This is especially critical in differentiating between an actual living organism and chemical processes that can mimic life. An oft-cited example is crystals.
As molecular biologist Steven Benner and physicist Paul Davies explain in Frontiers of Astrobiology:
A crystal of sodium chlorate can be powdered and used to seed the growth of other sodium chlorate crystals. In this way, these crystals can be said to reproduce. Further, features of the crystal, such as whether its atoms are arranged in left-handed spirals or right-handed spirals….can be passed to its descendants.
However, the replication is imperfect; a real crystal contains many defects. Indeed, to specify all of the defects in any decent-sized crystal would require an enormous amount of information, easily exceeding the 10 billion bits of information contained in a human genome.
But the information in these defects is not itself heritable: the defects in the parent crystal are not reproduced in the descendent crystals. Thus, the information held in the defects in the descendent crystal is entirely independent of the information held in the defects in the parent. Therefore, the crystal of sodium chlorate cannot support Darwinian evolution, meaning that a system of sodium chlorate crystals does not qualify as life.
Lastly, we have the word capable of Darwinian evolution. Gerald Joyce, a renowned professor and scientist at the Scripps Research Institute, who is credited with being the primary author of this definition, explains in an interview:
It's the population, or even the ecology, that makes the living system. Capable doesn't refer to one individual entity; that doesn't constitute a living system. A single individual might seem to be capable of undergoing Darwinian evolution, but may in fact be dead, or a fossil remnant, or about to die, or unable to find a mate. So it is the system that is said to be capable, not the individual.
And, Joyce adds, this also bring us back to another aspect of life being a "self-sustaining chemical system"—which, in this context, refers to the information necessary to undergo Darwinian evolution:
Chemical information is the product of Darwinian evolution. So all of the information necessary for the system to undergo Darwinian evolution must be part of the system.
So then you come back to: Why is Darwinian evolution put front and center in thinking about life? Because it is a way for complex entities to maintain themselves, not as an individual but as a system, in the face of a changing environment that is subject to unanticipated change. A system evolves to adapt to environmental change. It has to be able to invent new functions because those environmental changes may be more than incremental.
There are several conceivable examples of different types of life that do not fit the NASA committee's definition. Benner and Davies turn to one of the most authoritative sources on this subject, Star Trek:
The crew of Star Trek: The Next Generation encountered nanites that infected the computer of the next-generation starship, the Enterprise, in Episode 50 ("Evolution"). These were informational or, perhaps, electromechanical, but in any case not chemical; their evolution was not tied to an informational molecule like DNA (although they did require a chemical matrix to survive). Likewise, the Crystalline Entity of Episode 18 (" Home Soil") appeared to be chemical, but not obviously Darwinian. The Calamarians from Episode 51 (" Déjà Q") were made of "pure energy" (whatever that means), not chemicals. And in Episode 1, Q himself (" Encounter at Farpoint") appeared to be neither matter nor energy, moving instead in and out of the so-called Continuum without the apparent need of either.
If we were to encounter Q, the Calamarians, or any of these other conjectural entities during a real, not conceptual, trek through the stars, we would be forced to concede that they do represent living systems. We would also agree that they do not fall within the NASA definition of life. We would therefore be forced to agree that we need a new definition for life.
We submit, however, that we do not change our definition of life now so as to accommodate Q… or other examples of "weird" life from Hollywood because most of us do not constructively believe that this kind of life is possible.
Benner and Davies, however, don't refer to one Star Trek entity that seems increasingly plausible: Lt. Commander Data. Should an android, or any machine capable of artificial intelligence, be considered life? It's tempting to say so, but "sentience" and "life" are two different concepts. Joyce mentions that, even 20 years ago, the NASA committee touched upon this subject:
Can you have life that is just bits in the aether? At that time, artificial life in the computer was a new thing. There's a statement in the document about how studies of so-called "Alife," life in the computer, should be encouraged to the extent that they cross-inform studies of chemical life.
But NASA's not expecting to find a bunch of supercomputers on Mars or Titan. Or even PCs or iPhones. So at least for NASA's purposes, we wanted to focus on chemical life.
For his part, Benner believes that the first organism we encounter that will not fit this definition of life will be ourselves:
In a few years, we may be able to identify DNA sequences that prospectively help our children survive, and gain the technology that allows these sequences to be placed into our germ lines to generate mutant children that are fitter by design. If this happens, then our species will start to escape Darwinian mechanisms for improving our genes.
The good news is that we will no longer need to see children die of genetic disease; a large number of bad mutations is the Darwinian cost of a few good ones. With gene therapy, we may imaginably be able to scan the germ line for deleterious genes and remove them.
But, Darwinian evolution does not allow prospective mutation. Through this technology, humankind would be able to evolve in a more Lamarckian way….So perhaps we should start thinking now about a better definition-theory of life.