April 10, 2005

Looking for alien life on Earth.

Here's a NYT op-ed (about why we should search Earth, not Mars, to find alien life forms) that brings up a question I've been meaning to look for answer to for a long time: why isn't it inconsistent to believe that all the life we know on Earth traces back to a single cell and that other planets that have Earth-like conditions would give rise to life? It seems to me, that if Earth-like conditions get some life-creating cell started, that there should have been many such cells on Earth, starting many different evolutionary lines. Even if in the end, one line became so successful that it rendered the other lines extinct, wouldn't there have been a period in which multiple lines coexisted?

From the op-ed (by astrobiologist Paul Davis):
Genetic sequencing is used to position unknown microbes on the tree of life, but this technique employs known biochemistry. It wouldn't work for organisms on a different tree using different biochemical machinery. If such organisms exist, they would be eliminated from the analysis and ignored. Our planet could be seething with alien bugs without anyone suspecting it.

How could we go about identifying "life as we don't know it"? One idea is to look in exotic environments. The range of conditions in which life can thrive has been enormously extended in recent years, with the discovery of microbes dwelling near scalding volcanic vents, in radioactive pools and in pitch darkness far underground. Yet there will be limits beyond which our form of life cannot survive; for example, temperatures above about 270 degrees Fahrenheit. If anything is found living in even harsher environments, we could scrutinize its innards to see whether what makes it tick is so novel that it cannot have evolved from known life.


Starless said...

This book doesn't directly answer your question, but ,_Rare Earth: Why Complex Life Is Uncommon in the Universe_, by Peter D. Ward and Donald Brownlee (2000, Copernicus) is an excellent popular press book on this subject by two real live scientists. The book doesn't necessarily express a "belief" they hold, but is rather an intellectual exercise countering the more popular position that life must be ubiquitous. If you've concluded that life _must_ be so hardy that it's ubiquitous in the universe, this book may temper that conclusion. A good book to read before _Rare Earth_ is, _Other Worlds: The Search for Life in The Universe_, by Michael D. Lemonick (1998, Simon & Shuster). Lemonick is _not_ a real live scientist, but he does a pretty good job of presenting the other side of the argument.

At some point, there may have been microbial life on this planet that was so diverse that some of it wouldn't have been "life as we know it", but the odds of it surviving predation by other, more successful, life forms, regular bombardment from comets, asteroids, and other space junk, and extreme climate shifts, seem pretty small. Then again, tube worms living completely without sunlight at volcanic vents on the ocean floor seemed impossible not too long ago. We can theorize and philosphize all we want about the extremes of life, but we won't know the truth of it until we find it.

Joan said...

There is abundant evidence that "alien" life existed on our own planet in the past. Stephen Jay Gould's book, Wonderful Life, is both a history and analysis of the Burgess Shale, a tremendous fossil record of 5-eyed little critters and more -- apparently they evolved only in this one place, and never made it out -- but it is an utterly fascinating account.

Ann Althouse said...

To count as "alien life" in the sense I'm interested in and the op-ed is about, it needs to have developed from a different original cell than everything else.

Rob Bignell said...

There's a great new astrobiology blog, run by newspaper editor Rob Bignell, at http://alienlifeblog.blogspot.com/. It includes roundups of the latest news from the various scientific fields that form astrobiology and information about SETI.