"When people try to define life, they choose a few of the features of living things and make them the very essence of life. And that, Cleland and Chyba argue, is a mistake. "We don't want to know what the word life means to us. We want to know what life is," says Cleland"
This is the same way Maturana and Varela introduced the problem 34 years ago. They rejected definitions of life that were merely empirical-descriptive lists such as homeostasis, reproduction, and DNA. They emphasized that the search must be to identify the process itself from which these properties emerge; More than a description, a "theory of life" is needed. Cleland provides a good example to illustrate this point:
"In this sense, scientists who try to define life today make the same mistake that alchemists did in the Middle Ages. Alchemists tried to define substances by their properties, without any understanding of the underlying chemistry. Water, for example, was defined according to its ability to dissolve different solids. This definition led alchemists into confusion. Since ice couldn't dissolve anything, it couldn't be water. Alchemists gave the name "water" to things that we know now are nothing of the sort.
They called nitric acid aqua fortis, or strong water, because it could dissolve most metals. Aqua regia, or noble water, was actually a mixture of hydrochloric acid and nitric acid that was powerful enough to dissolve even gold and platinum, the so-called noble metals. Searching for a better definition of water would have not gotten alchemists out of this mire. A solution only came in the 18th century, as scientists formulated a theory of chemistry. The behavior of water and other substances suddenly makes a lot of sense when you realize that they are all composed of atoms, which are in turn composed of smaller particles. Chemists can now say water is H2O. However, "'Water is H2O' isn't a definition," says Cleland. "It's a discovery."However, Cleland then takes a wrong turn; she proposes that a defintion of life is impossible. I agree that in absence of a theory of life, defining life is nothing but a "barbaric" description. But once we DO have a theory of life, definitions should not only become possible, but they may allow an "acid test" for the clarity of the theory. Cleland's posture seems a bit mysterianist to me, as if no true theory of life will ever be possible. For instance, without a theory of life, how can we know if "having DNA" is or not directly related to life? Cleland certainly does not think that genes are crucial:
"Cleland, for example, doubts that Darwinian evolution, the core of the NASA definition of life, is essential. "I think those arguments are weak," she says. She envisions alien microbes filled with enzymes but lacking genes. The enzymes build more enzymes and the microbes split in two. They couldn't evolve through Darwinian evolution, because they wouldn't have genes. But they might still change, as their environment changed. Cleland doesn't claim any evidence that such things exist, but she argues that scientists can't rule them out"
Cleland's problem is that she does not have any truly sound THEORETICAL response if she were required to explain why she thinks DNA is unnecessary. We know already that many scientists would refuse to acknowledge such extraterrestrial things as truly alive; we cannot naïvely hope that the discovery of such extraterrestrial life would provide an empirical shortcut to "eliminating DNA" on what is truly a conceptual, and not an empirical, problem. After all, they can reply right back to Cleland that she cannot rule out the future discovery in other planets of autocatalytic self-replicating molecular "genes". Would she say those cannot be life? Why, exactly?
Because Maturana and Varela's autopoiesis is a systemic theory of life, it has no requirement for the presence of any particular type of molecule such as DNA; therefore, this theory explains why having DNA is not a requisite. According to Maturana and Varela's theory of organisms as "Autopoietic machines":
"An autopoietic machine is a machine organized (defined as a unity) as a system of processes of production (transformation and destruction) of components networked in a way such that: (i) through their continuous interactions and transformations they generate the processes (relations) that produced them; and (ii) constitute the machine as a concrete unit in physical space"
They emphasize that the production of components and the borders of this well-defined topological unit help produce each other: a distributed network. It is a little system; not a "selfish" self-replicator. Importantly, reproduction is not a requisite of the definition (yes, you can be a mule and still be autopoietic). Reproduction can occur as a RESULT of the growth and fracture of a previously existing autopoietic system; Maturana and Varela point out that many things can reproduce that are not autopoietic systems nor universally acknowledged as living (for instance, crystals, viruses, and prions).
The exclusion of reproduction from the core of the theroy of life is a pretty conspicuous difference from the "darwinian" perspective on the origin of life; according to those theories, the first life forms were self-replicating, reproducing molecules (RNA is a clear favorite), "genes" in themselves. This would put in motion a darwinian process of natural selection of favorable mutations, that therefater would have added DNA, membrane, proteins, etc. To ensure better self-replication, of course.
Now, as final words, allow me to indulge in some biochemical speculation (despite this not being my area). I dare say that it is becoming increasingly clear that no molecule of RNA complex enough to self-replicate can be produced without some vesicle or "border" separating it from the outer environment; nowadays most researchers seem to be trying to fit one biochemical cocktail or another into some kind of membrane or miscela of some kind. It seems probable to me that the new more impressive claims of "synthetic life" in the future will involve such little systems. Probably nothing too impresive will be coming from any isolated self-replicators....
-A. Vargas
Ref:
Maturana, Humberto & Varela, Francisco ([1st edition 1973] 1980). Autopoiesis and Cognition: the Realization of the Living. Robert S. Cohen and Marx W. Wartofsky (Eds.), Boston Studies in the Philosophy of Science 42. Dordecht: D. Reidel Publishing Co.
