Now we turn to the modern accounts of life.
In 1828, Friedrich W÷hler produced uric acid without using ?kidney of man or dog?. Prior to that time, there was considered to be something different between organic chemistry and inorganic chemistry. Living things had some ?vital fluid? that other things lacked. Most often this was expressed in Aristotelian terms even if, like Buffon, they were very anti-Aristotelian. But still life was not fully explicable in chemical terms.
Vitalism, as this idea was termed, did not die with W÷hler, though. In fact, we can find instances of it until the middle of the 20th century, particularly on the Continent. The final blow was delivered, not by the discovery of the structure of DNA, as might be expected, but with the final elucidation of the Krebs citric acid cycle and similar metabolic processes.
But what chemists understood had little effect on ordinary biologists, and it was developmental biology that persisted in vitalism longest. So the first general attempt at a ?materialistic? explanation of life was done by a physicist. Erwin Schr÷dinger, living in Ireland during the war, in 1944 produced an influential little book entitled What Is Life? in which he argued that life is a matter of thermodynamics – it feeds on free energy, and thus reduces local entropy. Schr÷dinger called this “negentropy”. Entropy is the tendency of systems to become homogenous in their energy; that is, reach an energetic equilibrium. Life requires a difference in energy levels – this is a metabolic definition of life
This view is not wrong, I think. Life is, as one paper expressed it, a function of the Second Law of Thermodynamics – which states that entropy cannot decrease in a closed system. Life uses the fact that no living thing is closed to ensure that the entropy is increased somewhere else.
But this doesn?t define only life. A good many physical systems are like this, like refrigerators or air conditioners. However, this fact about life has been extended to a larger discussion of ?cosmic evolution? as the local decrease of thermodynamic entropy in parts of the universe, recently by Eric J. Chaisson.
So we need to move on past energetics alone.
The discovery that organic chemistry had a basis in inorganic chemistry was still a major shock, in the early years of the 20th century, and two researchers, Alexander Oparin and J B S Haldane (whose name I have never seen used in full – it is as if he was born with the initials only) both proposed a view that ordinary chemistry could be expected to have given rise to living things somehow.
In the 1950s, a graduate student named Stanley Miller tested the ideas of his advisor, Harold Urey, on the nature of the early atmosphere of earth, and synthesised the basic building blocks of life from a mixture of gases and water. Although the atmosphere Urey hypothesised is not now thought to be the right mix historically, Miller?s experiment showed that organic compounds might be formed through inorganic processes with some ease. A great many other chemistries have been shown to create organic compounds – some of quite complex structure.
William J. Schopf?s book takes the chemical tradition as a definition of life – life is that which is constructed of a half dozen elements in particular forms and compounds. Schopf?s term for it is CHON(SP) – life in composed of Carbon, Hydrogen, Oxygen and Nitrogen (plus Sulphur and Phosphorus).
Schopf?s view is historically true, although we would not be shocked to find a terrestrial organism that, say, replaced some carbon with silicon. Remember Hull?s Rule: there is nothing so absurd that some organism or other doesn’t have it.
The CHON(SP) account is post hoc. It takes life as we see it and makes these the conditions without which something is not alive. This is only partially useful – it points to life as we do know it, but doesn?t help us either delimit life from non-life that happens to be made from the same elements, and it doesn?t tell us what all other life, say on Mars, might be like. It is not, therefore a definition for a ?universal biology? as Kim Sterelny and Paul E. Griffiths put it.
Then there’s Mathematical life – Information and Alife. John von Neumann proposed an idea of “self-replicating robots“, which, being mathematical entities can be given a mathematical definition; life is what replicates its information or program. A number of people have tried to follow up John von Neumann’s suggestion that we might one day build robots that could construct copies of themselves, and which would therefore be subjected to evolution if they sometimes made mistakes (“mutations”). To this end they have proposed formal criteria for life, usually with a mathematical foundation.
Alan Turing, the father of the computer, worked out a mathematics of diffusion gradients that would generate patterns like a zebra’s stripes. When asked if he could therefore explain a zebra mathematically, Turing is reputed to have said, “the stripes, yes, but the whole horse is harder”. Still, a tradition known as process structuralism in biology attempts to describe life as a mathematical pattern.
Gregory Chaitin at IBM published a suggestion in the early 70s that life was a process of information-production. Life generated complexity, which can be understood as something that needs an algorithm of a certain length to describe. Living things were therefore computer programs, or something very like them. This of course ties nicely into the idea of development as a program.
This has flowered over the past two decades into the program known as Alife, or Artificial Life. A number of such programs are available – Thomas Ray’s Tierra is one, in which “animats” evolve through genetic algorithms. Another is Avida. In each case, a living thing in these “toy worlds” is something that can generate copies of itself, and which more or less satisfies a fitness function.
It is very tempting, in these cases, to call these things actual life, rather than simulations of life, and some do just that. I don?t – they are mathematical models that exist in a virtual world. They are abstractions. Life is concrete. But they may identify some general properties of living things?
And this is what Leslie Orgel proposed. For him, the dynamics of things are as important or more than the physical components they are constructed of. Orgel proposes a universal biology. He includes the Chaitin elements: complexity, and information. He makes them concrete objects, and he has them evolve, and evolve by natural selection.
Orgel calls these CITROENS:
- Objects, that
- Evolve by
- Natural Selection
This is a universal biology approach. Orgel specifically says that the elements in our own biota need not apply on a hot planet like Mercury. It is sufficient that objects that reproduce with a high information content through natural selection are alive. The motivation here is clearly the notion of evolution by natural selection, conjoined with the notion of information.
Again, though, some terms are left undefined. What is ?complexity?? He defines information as ?genetic information?, again without telling us how to measure it. When we come to look at this, we will find that it is not so easy to define information in a genetic context, at least, not in any way that addresses the sort of problem Orgel is addressing.
Manfred Eigen, a Nobel laureate physicist, addresses the origins of life as the outcome of what he calls “Hypercycles”. These are chemical reactions that catalyse the reagents needed for the next step, until the last step catalyses the reagents needed for the first step. Hypercycles are chemical merry-go-rounds, and since they are catalytic, that is, since their parts acts as templates for the synthesis of the molecules needed for the next step, they multiply exponentially.
Eigen brings all the traditions together. Life is a dynamic state of matter. It is organised. Information is produced by natural selection. It has a metabolism. This is all the necessary and sufficient conditions for something to be life:
Unlike many of the others, Eigen treats information as a function of the physical structure of the catalysts in his process. DNA is a kind of catalyst – its products, via tRNA or mRNA, themselves catalyse protein assembly. There is no hard and fast ?information-metabolism? divide here.
Again, in contrast to Orgel, Eigen does not make information a prerequisite for life, but instead a product of natural selection. In his account, the thermodynamic, mathematical and chemical aspects of life are combined.