Can There be Only One?

The toughest problem in defining a general theory of life is the paucity of examples. That statement sounds like an oxymoron - isn't life famously diverse? Man, elephant, mouse, carrot, redwood, mushroom, coral, sponge, and bacterium seem pretty darn different. At the molecular level, however, they aren't. The most essential machinery at the molecular level is almost exactly the same.

They all use the same genetic code, and the same basic mechanisms for the most basic operations of life. These commonalities, and the patterns of both commonalities and differences constitute the most dramatic proof of the fact of common descent. However all that common machinery came into existence, it all seems to have existed in the most recent common ancestor of all known Earthly life.

Despite a large number of apparently contingent and arbitrary elements in life, exceptions are not known. Proteins and complex sugars have definite chiralities but we don't know of any fundamental reason why life based on the opposite chiralities couldn't exist. The particular amino acids from which are proteins are built are just a small subset of many similar amino acids. So why couldn't a different set of twenty, or thirty have been chosen. Many other possibilities for the genetic code would also seem to be possible.

We wind up knowing about only one kind of life, and the mystery of how all that elaborate and apparently necessary machinery of life developed seems at least as mysterious as ever.

This hasn't stopped speculation about other possibilities. The cover story of the December Scientific American was on the possibility that there might be Alien life among us but undetected - probably small bacteria like cells with a different hereditary and metabolic history. (An online version here) Well maybe, but of course there is no evidence.

Suppose we find life elsewhere in the solar system. That should be completely different, right? Again, the answer is maybe. Most scientists are doubtful that the ballyhooed meteorite from Mars, AH84001, really demonstrates Martian life, but it would seem to show that if life developed in one part of the solar system it might have made the interplanetary trip to other planets.

Robert Shapiro and physicist Gerald Feinberg have suggested that life might be much more exotic still - perhaps based on ammonia on cold planets or substituting silicon for carbon on a very hot planet, or even being encoded as patterns in the plasmas in stars or interstellar clouds.

In any case, discovering any other form of life would seem likely to clarify the thorny questions of the hows of the emergence of life on Earth.