Where do we come from?: a humbling look at the biology of life's origin - includes related articles

And the exact form seems to be part of the problem. What exactly is life? Physicist Erwin Schrodinger asked that question precisely in such fashion in 1947. While Schrodinger's thinking led him to predict some of the properties of DNA as a necessary component of a living organism, we still have only a vague notion of the boundary between life and inert matter. And so it should be, if we accept the idea that living organisms are made of inert matter that happens to acquire some "emergent properties" when it is assembled in particular ways. To put it another way, living beings are not separated from the rest of the universe by some mysterious force or vital energy.

How then do we know what is life and what is not? We can derive a list of attributes, some of which can be properties also of non-living systems, but with the ensemble defining a living organism:

* Ability to replicate, giving origin to similar kinds (reproduction)

* Ability to react to changes in the environment (behavior, not just limited to the special meaning that the word has in animals)

* Growth (i.e., reduction of internal entropy at the expense of environmental entropy - note that even single cells grow immediately after reproduction, so this is not a property restricted to multicellular life)

* Metabolism (i.e., capacity of maintaining lower internal entropy, including the ability of self-repair)

How did we get from a nucleo-protein to an entity capable of all of the above? And what did this entity (sometimes known as the "progenote") look like? There are very few even tentative answers to these questions, and this - I think - is where the real problem of the origin of life lies. The German scientist Manfred Eigen has proposed a possible scenario that invokes what he called "hypercycles." We can think of a hypercycle as a primitive biochemical pathway, made up of self-replicating nucleic acids and semi-catalytic proteins that happen to be found together in pockets within the primordial soup. It is possible to imagine that some of these hypercycles are made of elements that "cooperate" with each other, i.e., the product of a component of the cycle can serve as the substrate for another. Different hypercycles could have coexisted before the origin of life, and they would have competed for the ever-decreasing resources within the soup (the resources were decreasing because the hypercycles were using up some organic compounds at a higher rate than they were formed by comparatively inefficient inorganic processes). Eventually, this competition would have favored more and more efficient hypercycles, where the "efficiency" would be measured by the ability of these entities to survive and reproduce, that is by the parameters of Darwinian evolution. Life as we know it (sort of) would have begun.

Eigen and modern followers of complexity theory also expect these systems to become more complicated with the addition of new components to the cycle. From time to time, the addition of one component would modify the whole system dramatically, giving it properties that the previous group did not possess (sort of like adding an atom of oxygen to two of hydrogen and suddenly getting something completely distinct and more complex: water). Complexity theorists such as Stuart Kauffman and Christopher Langton have demonstrated, on the basis of mathematical models, that some self-replicating systems can display unexpectedly complex patterns of behavior. The textbook example of this phenomenon is the so-called cellular automata, mathematical entities first imagined by John von Neumann in 1940 and that can now be studied at leisure by anybody who has a personal computer and a copy of a game aptly called "Life."