Alien Life and Environments
Are we alone? If not, then what may it be like? These are philosophical questions that tend to haunt. The notion that we are the only example of a successful life form in the galaxy seems like an unlikely statistic, as we discover more and more habitable planetary bodies and hear yet more evidence of life's ability to survive in extreme and unlikely environmental conditions. A new essay, published May 8 in the online, open-access journal PLoS Biology, examines what really constitutes life and the probability of discovering new life forms. Accompanying the article is an interview with the author in the latest edition of the PLoS Biology Podcast.
Extraterrestrial life is defined as life that does not originate from Earth. Referred to as alien life, or simply aliens (or space aliens, to differentiate from other definitions of alien or aliens) these hypothetical forms of life range from simple bacteria-like organisms to beings far more complex than humans.
The development and testing of hypotheses on extraterrestrial life is known as exobiology or astrobiology. Many scientists consider extraterrestrial life to be plausible, but no direct evidence has yet been found.
Thanks to a combination of ground- and space-based astronomical observations, the number of confirmed extrasolar planets will soon exceed 1,000. An increasing number of these will be said to lie within the habitable zone and even be pronounced as Earth-like. In time there will be observational data regarding the atmospheric composition of some of those planets. Such data may begin to show more direct evidence of alien life such as artificial short lived pollutants or just plain oxygen availability.
What, in fact, is the probability that a temperate, rocky planet will generate life? Science cannot say. There is simply not enough evidence. The probability assessment would be more meaningful if there were even one more genuine example of life, whether discovered in space, on Earth, or in a test tube.
Joyce discusses whether such life has some similar properties akin to Earth but not others. What if it could self-reproduce, directing the assembly of progeny of identical composition, but could not evolve new functions? What if it consisted of complex chemical processes within a cellular compartment but had no basis for maintaining heritable genetic information?
In principle, there are two pathways by which a new life form can arise: either directly from chemistry or spun off from some other biology. If life arises from chemistry, then it begins with zero heritable bits and organizes into a bit-generating system. Following an era of prebiotic chemistry, perhaps reaching a high level of chemical complexity, molecular memory arises. Molecules of variable composition begin to replicate, mutate, and evolve in a Darwinian manner.
If, instead, life arises from another life form, then it may have a privileged beginning, benefitting from a chemical environment that has been shaped by preexisting life. A new life form may grow on the spoils of prior life with no carryover of bits, or it may descend from prior life with some carryover of bits into a different genetic system.
"I think humans are lonely and long for another form of life in the universe," says Joyce, "preferably one that is intelligent and benevolent. But wishing upon a star does not make it so. We must either discover alternative life or construct it in the laboratory. Someday it may be discovered by a Columbus who travels to a distant world or, more likely in my opinion, invented by a Geppetto who toils at the workbench."
See the Alien Life for further information.
Star image via NASA.