jueves, 18 de noviembre de 2010
Evolution May Go Wild on Violent Exoplanets
In the coming decades we will begin to characterize planets in the habitable zones around nearby stars. The problem is that just because they are habitable we will still be hard-pressed to understand if the chemistry of their atmospheres comes only from microorganisms, or six-legged giraffes, or even sentient beings.
A huge influence on planet's ability to evolve complex life is not just location, location, location, but rather environment, environment environment. The difference? Some planetary systems may have gas giant planets that accelerate the rate of comet or asteroid impacts on the surface of their terrestrial siblings.
Other Earth-like worlds might orbit petulant young red dwarf stars that spit out searing flares that pound the planets.
But Earth history has shown that one species’ Armageddon may be another species’ Genesis. Take the devastating asteroid impact that triggered a mass extinction 65 million years ago, toppling the mightiest predators that ever-walked Earth, the dinosaurs. Surviving dinosaurs evolved to take to the air and mammals rose from the shadows to dominate our planet.
Even more dramatically, a biological arms race -- the Cambrian Explosion -– took place on the cusp of Earth’s near-death experience as a "snowball Earth" about 500 million years ago.
Based on these discoveries, the emerging view is that long periods of ecological stability are punctuated by catastrophes, both Earth-made and extraterrestrial. It seems that when all hell is breaking loose on a Earth major evolutionary changes happen rapidly.
Aside from our new view of the devastating threats from space debris, this kind of "kick in the pants" model for evolution is not new.
Last year was the 150th anniversary of Darwin's landmark publication, On the Origin of the Species, which laid the foundation for modern biology.
Darwin's idea was that most evolution was accomplished very gradually by competition between organisms that became better adapted to relatively stable environment. But 28 years earlier than Darwin's publication the Scottish horticulturalist Patrick Matthew published his own ideas about the process of natural selection:
"There is a natural law universal in nature, tending to render every reproductive being the best possibly suited to its condition . . . it is only the hardier, more robust, better suited to circumstance individuals, who are able to struggle forward to maturity, . . . from the strict ordeal by which Nature tests their adaptation to her standard of perfection and fitness to continue their kind by reproduction.'
In other words, Matthew thought that it took no less than global catastrophes to shock evolution out of a laid-back state and spur diversity and competition among organisms. When the going gets tough only the tough get going -- to borrow the phrase from Billy Ocean's 1985 pop song.
Matthew’s musings were simply published in the appendix to his 1831 book with the utterly forgettable title: Naval Timber and Arboriculture.
Writing in a recent issue of Historical Biology, New York University geologist Michael Rampino concludes: "Others have said that Matthew's thesis was published in too obscure a place to be noticed by the scientific community, or that the idea was so far ahead of its time that it could not be connected to generally accepted knowledge. As a result, his discovery was consigned to the dustbin of premature and unappreciated scientific ideas."
Oh, only if the Internet existed in 1831, Matthew could have blogged away on his ideas.
But this is a very salient idea today as it applies to the rough-and-tumble planetary systems we are finding. Those terrestrial planets though to undergo intense periods of bombardments might be a hotbed of evolution gone wild. Or perhaps there is a "catastrophe rate limit" beyond which life suffers arrested development.
Age of the Exoplanet
This is one of the biggest motivations I can image for undertaking a many-centuries-long program of understanding the evolutionary history of nearby Earth-like planets. This would require extraordinarily sophisticated survey robots that would scrutinize a planet’s biosphere. Other robots would have to probe a planet’s geologic past, and the architecture of the home planetary system.
Therefore, simply finding circumstantial evidence of microbial activity on other worlds will become intellectually dissatisfying to future scientists.
Perhaps on the millennial celebration of Darwin’s work -- or instead Matthew’s work -- our distance descendants will have a truly universal concept of how life evolves in a range of planetary environments.
This is a monumental task only dreamt of in science fiction stories. But little might Matthew have imagined that his ideas would be potentially be applicable to worlds whirling around the distant stars.