There has been a flood of articles on a report “The Limits of Organic Life in Planetary Systems,” published by the National Research Council of the National Academies of Sciences and posted on the NAS web site. It suggests that life elsewhere in the universe may be quite different than life on earth and that our search parameters should be expanded to accommodate this possibility.
The Times of India carried a column on July 12 2007 by Mukul Sharma. In his column Mr Sharma stresses how diverse life on earth is, a point which is undisputed and uncontroversial but then suggests that
"life on this planet itself has turned out to be so staggeringly diverse and bizarre that to lump it all under one universally descriptive umbrella is beginning to make very little sense now".
I feel this is a case where a columnist having read the press release about alien life got real excited and decided to conjure up some aliens on planet earth. Statements like these serve no purpose except to impress the uninitiated public into believing that scientists have been in the dark all along about all this diversity on earth and don't know what to do with it. What on earth does he mean by one universally descriptive umbrella? I suspect he means a way to organize life in one inclusive classification framework. Mr Sharma is saying that some forms of life are so different that they cannot be included in the classification framework biologists use today. To make his case he describes extremophiles, varieties of bacteria which live in extreme conditions and other organisms that don't use oxygen but chemicals like hydrogen sulphide to power their lives. In stressing how different extremophiles are, he ignores the fact that all life including the extremophiles is united by fundamental molecular similarities. All life has a DNA based genetic system and a nearly identical genetic code, the instructions to translate information stored in DNA into proteins via RNA intermediates. This is taken as compelling evidence that all life forms arose from a common ancestor. The reason for so much diversity? Evolution. Lumping all life in one universally descriptive umbrella is not that senseless. Biologists classify life forms based on their evolutionary relatedness, a method that make a lot of sense, since it is giving us an increasingly clear picture of just how life forms are related to each other and how they diversified. The universal umbrella is not an arbitrary and increasingly unwieldy creation of scientists to pigeon-hole life forms into. The umbrella emerges due to the fact that all life forms on earth are related through common ancestry. As molecular genetic data pours in, science is slowly revealing the one true tree of life, the universal umbrella. If at some point we have clear evidence that some life form on earth is using a radically different molecular system, perhaps one that does not use DNA or one which utilizes a different genetic code, we will need another umbrella to lump those life forms under. Until then, one universal umbrella to describe all life, even bizarre extremophiles, is sufficient.
Which brings me to the second point regarding alien life. I am not an expert on biochemistry and so I cannot judge how realistic are assessments that life using a silicon based chemistry or one that uses ammonia or methane as a solvent instead of water may be possible. The sentence in the press release of the original report and quoted without question by Mr. Sharma that caught my attention was the one which suggested that fundamental requirements for life as we know it may not require " a molecular system capable of evolution and the ability to exchange energy with the environment". Life without evolution? Scientist may disagree on the chemical makeup of alien life but surely they agree that life anywhere will have the properties of replication and hereditary variation. Evolutionary change is a natural consequence of these properties. Without evolution life will come to a dead-end, literally. Required also is exchange of energy. How can chemical reactions be sustained indefinitely without an input of energy? Maybe the full report which I have no access to says something about this. But most science lovers will read only the press report. I hope the people who wrote this report will clarify these astonishing statements.