Thursday, August 27, 2009

What is Life?

What is life? Biology is the study of life and yet a typical modern biology text with, say, about 1200 pages will devote at most about three pages (and usually less) to answering this question . You will be hard put to find any University courses exclusively devoted to this subject. But after all, Biologists study living things, they are not philosophers.

Everyone has an intuitive feeling for what life is. We can generally tell the difference between something that is alive and something that is either dead or simply material matter. Living things move, they feel warm to the touch, and they react to stimuli. That's enough for most people. But think of a flame or a hurricane. They both move, grow, and die, they react to stimuli and give off heat but they are not alive in the same sense as you or I.

What differentiates a living thing from a flame? Like living things, a flame metabolizes. It takes energy in from the environment and processes it. A flame burns because it's reached a certain temperature and it's fed by fuel. Once the fuel is used up the flame dies. The flame does not go looking for more fuel somewhere else. But a living creature will maintain itself purposefully; it will search out food sources; it will avoid dangers and predators; it will adapt to changing circumstances; it will reproduce and thus maintain some of it's genetic characteristics even after it dies.

This ability to self-maintain is called “autopoiesis”. It's an amazing property because it has created an unbroken link between us and the very first cell. Lynn Margulis and Dorion Sagan, in their book, What is Life? capture it's essence well: “ Once autopoiesis appeared in the tiniest bacterial ancestor it was never completely lost....As sheer persistence of biochemistry “we” have never died during the passage of three billion years. Mountains and seas and even supercontinents have come and gone but we have persisted.”

How does autopoiesis come about? We have to be careful when we try to answer this question because it's all too easy to go in circles. Did the first bacterial cell create itself on purpose? And this is also the place where those of us who are impatient for certainty want to bring in God, alias - “The Intelligent Designer”.

Imagine a sandpile. A constant trickle of grains is being added to the middle of the pile. At some point just adding a single grain can cause an avalanche. Maybe a small avalanche, maybe a large avalanche. There's actually no way predict which it will be. Thus the sandpile exhibits complex behaviour. Per Bak, the Danish Physicist who came up with the sandpile model explains how it works in his book How Nature Works – The Science of Self-Organized Criticality:

“The addition of grains of sand has transformed the system from a state in which individual grains follow their own local dynamics to a critical state where the emergent dynamics are global.... It is clear that to have this average balance between the sand added to the pile say, in the center, and the sand leaving the edges, there must be communication throughout the entire system. There will occasionally be avalanches that span the whole pile. This is the self-organized critical state.”

Of course the sandpile doesn't do anything besides reach a peak and spill over it's edges but the point is that it is a simplified model that gets at the essence of emergence. It shows how complex global behaviour can emerge from the simple addition of individual parts without recourse to purpose or design. This model has been used to explain how complex systems such as living cells, human societies, and economic systems can come about from the bottom up, that is through the action of individuals alone.

Obviously autopoiesis, life's ability to self-maintain, is an emergent phenomenon. It cannot be predicted from the chemical properties of all of the molecules in a cell. But we now know that it is possible for such an emergent property to come about from the bottom up, that is from some critical state brought about by the addition of a sufficient number of certain kinds of molecules. For now how these molecules first came together is a matter of conjecture.

One of the problems with Darwin's theory of Evolution has been that although natural selection can explain the evolution of life from the first cell to all the living creatures that exist today it doesn't explain how the first cell came to be. Many critics have pointed out the fantastically small odds of such a cell ever coming to be by chance. We used to think that there were only two alternatives: chance or design. But now we see a third alternative: self-organized criticality. And this fits better with the continuing scientific project of reading universality into the world. Life is an inevitable property of a certain level of molecular organization. That is, whenever that level of organizational complexity is reached that organization will self-maintain and life will occur. The question is – what are the conditions that make autopoieses possible? This is a question I will address in my next series of articles.

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