The Case for Extraterrestrial Life
Ted S. Frost
While NASA nurses and coaxes and tweaks its R2D2’s on the dusty barrens of Mars, astrobiologists anxiously await signs of life. Despite the inhospitable nature of Mars, their hopes are high. Are they deluded?
First, the laws of physics and chemistry appear to be universal. Scientific principles should work everywhere. All our observations verify this.
Second, we have evidence of planets in other solar systems. But, of course, we gave up thinking we are the center of the universe a long time ago. We’re probably just a Joe Average planet in a Joe Average solar system in a Joe Average galaxy. Might not life be Joe Average, too?
Third, we know some of the elemental organic molecules involved in life’s chemistry, such as amino acids, exist abiotically, even in meteorites. These molecules include chemicals that could provide a catalytic function, such as cyanic compounds.
Fourth, since life has come into existence once, why not again, especially considering the size of the universe?
Fifth, life appeared very early on Earth. In 1995, Roger Buick, now of the University of Washington, uncovered evidence in Western Australia of stromatolites (colonies of photosynthesizing cyanobacteria)
3.5 billion years old. If stromatolites existed that long ago, life must have started earlier: scientists estimate about 3.9 billion years ago. Earth is only 4.5 billion years old. Isn’t the early appearance of life on Earth evidence that it is a natural phenomenon? Sixth, life exists in extreme environs. Bacteria and archea live happily in scalding black smokers and at Antarctica, at extreme pH, at extreme salinity, buried a kilometer or two underground, in anoxic and oxygenic atmospheres, and when exposed to radiation. Life survives millennia as dormant spores. Unlikely as a habitat might seem to us, it isn’t necessarily fatal. Which is probably why life began back at the end of the Hadean Era (4.5-3.8 GYr) when we were still being clobbered by bolides--meteors and comets.
Martian Landscape Image: NASA/JPL/Cornell University
Seventh, in the case of Mars, there are tantalizing indications that liquid water, absolutely essential to life, previously existed.
Eighth, even if life on Mars never was and situations favorable for creation of life are, indeed, rare, there are 7x1022 star systems. I refer you to the Drake Equation. http://www.pbs.org/lifebeyondearth/listening/drake.html
It sounds like creation of life is plausible, even likely. But there are still big questions regarding the advent and extent of life.
First, we still have no overt evidence life exists anywhere but here. It’s all speculation and theory.
Second, and this is a biggie, life seems wildly implausible. The more science pokes and probes into life’s mechanisms, the more mysterious its creation becomes. In the words of Alice in Wonderland: “Curiouser and curiouser.” Even the simplest of organisms - bacteria and archea - are complex to a degree that boggles the mind. And the more we learn about them, the more complex their processes prove to be.
For example, archea that produce Methane (CH4) when creating energy are considered among the most primitive and earliest of organisms. The chemical pathway for one of them (Methanobacterium thermoautotrophicum) in carrying out its respiration process is:
CO2 + 4H2 CH4 + 2H2O + energy
Simple enough-but this process entails a number of separate steps requiring complex enzymes, electron carriers, catalysts, and cofactors not shown by the formula because they are cyclically regenerated and not
permanently altered--complex chemicals with tongue-twisting names such as methanofuran, 7mercaptoheptanoylthreonine phosphate, tetrahydromethanopterin.
This is just one basic metabolism process of one ‘simple’ one-celled organism. Darwin’s theory of evolution explains things nicely once you have the first functioning self-replicating cells. But creation of those first cells remains an enigma wrapped in a mystery contained in a singularity.
When scientists try to envision a plausible scenario for creation of life, they bump into perplexing anomalies. For example, life uses ‘left-handed’ amino acids and ‘right-handed’ ribose sugars in its biological structures. Yet right-handed amino acids and left-handed ribose sugars are just as plentiful. Why and how did life choose to use one sort, instead of the other?
Why does all of life use exactly the same 21 amino acids, the same four letters in its DNA alphabet (T, A, G, C), and the same dictionary of three letter words (codons) for
biosynthesizing? If creation of life is such a ubiquitous thing, why don’t we see different languages and life-form recipes? It’s not as if there aren’t choices. There are many types of abiotic amino acids and sugars. How did life avoid dilution and destructive cross-reactions?
Life as we know it requires DNA, RNA, and proteins. DNA can reliably store hereditary information from generation to generation as well as mutate, which drives evolution. But DNA can’t replicate without help from RNA and proteins, and it can’t perform any metabolic functions. Proteins perform magical metabolism functions but can’t replicate without instructions from DNA and help from RNA. RNA can store information, albeit not terribly reliably, and can help both proteins and DNA replicate, but can’t replicate itself without help from proteins and can’t perform metabolic functions.
DNA and RNA and proteins need each other to do their life things. But all three are exceedingly complex, being composed of snarled-up tangles of atoms and molecules thousands of units long. It is inconceivable all three could spontaneously pop into existence at the same time and in the same location.
During the 1980’s, evolutionary biologists ran around all excited over discovery of a type of RNA called ‘ribozyme’ that could perform all by itself limited self-replication and protein catalytic feats. Perhaps life started as an RNA world, without DNA and proteins. But ribozyme’s in vivo (within a living organism) tricks were limited. Its in vitro (outside a living organism) feats were performed under carefully controlled laboratory conditions (T. Cech & S. Altman). And besides, spontaneous creation of RNA--complicated and chemically fragile-- is nearly as miraculous as spontaneous assemblage of the whole shebang.
Creating life ain’t easy.
If we are honest, despite our exciting fancies about extraterrestrials, we must admit the real possibility that life arose but once and that we are alone and unique in the cosmos… --Simon Conway Morris, paleontologist
Currently, evolutionary biologists and biochemists have no definitive answers. Scientists speculate that at first, much simpler chemicals and unknown processes were involved, gradually evolving into the complexities we see today. But they have no living examples and no specifics. It’s hard to evaluate a hypothesis dependent upon unknown substances and processes. Which causes frustrated scientists such as
G. Gonzales & J. Richards in The Privileged Planet, to conclude: “Earth-based origin of life research has reached an impasse.” Yet life DID happen. Could a second Genesis have happened on Mars? Does life evolve wherever favorable conditions exist? Or are our searches futile? Is the answer on Mars? Stay tuned, folks.