Could There Be Life On Every Planets?
May 17, 2010 7 Comments
How complex could a extraterrestrial life be, this is the question which is mind boggling. If you are going to search for life on other planets, you must have to assume the basic prospects of life. Lately we have found multicellular creature lurking under harshest condition on planet Earth. Is life only the thing which is to be so complex and does complexity mean life?
The critical question is:Is life the only type of material complexity expected in other habitable zones, or is life only one example of many types of complexity? In other words, is or is not life an inevitable consequence of the evolution of matter? Given the proper conditions and enough time, is life a sure bet or is it quite rare?
Current research seeks to understand how complexity arises from simplicity. Much progress has been made in the past few decades, but a good appreciation for some of the most important chemical steps that led to life still eludes us. That’s because life itself is extraordinarily complex, much more so than galaxies, stars, or planets.
Consider, for a moment, the simplest known protein on Earth. This is insulin, which has 51 amino acids linked in a specific order along a chain. Probability theory can be used to estimate the chances of assembling the correct number and order of amino acids for such a protein molecule. Since there are 20 different types of amino acids, the answer is 1/2051, which equals ~1/1066. This means that the 20 amino acids must be randomly assembled 1066, or a million trillion trillion trillion trillion trillion, times before getting insulin. This is obviously a great many combinations, so many in fact that we could randomly assemble the 20 amino acids trillions of times per second for the entire history of the Universe and still not achieve the correct ordering of this protein. Larger proteins and nucleic acids would be even less probable if chemical evolution operates at random. And to assemble a human being would be vastly less probable, if it happened by chance starting only with atoms or simple molecules.
This is the type of reasoning used by some researchers—especially biochemists—to argue that we must be alone, or nearly so, in the Universe. They suggest that biology of any kind is a highly unlikely phenomenon. They argue that meaningful molecular complexity can be expected at only a very, very few locations in the Universe, and that Earth is one of these special places. And since, in their view, the fraction of habitable planets on which life arises is extremely small, the number of advanced civilizations now in the Galaxy must be even smaller. Of all the myriad galaxies, stars, planets, and other wonderful aspects of the Universe, this viewpoint maintains that we are among very few creatures to appreciate the grandeur of it all. If their arguments are correct, we could be alone in the Universe.
But does chemical evolution operate at random, that is, by chance and chance alone? Alas, there’s another point of view—one often preferred by astrophysicists. Several reasons suggest that the change from simplicity to complexity may not proceed randomly. The first reason is this: Of the billions upon billions of basic organic groupings that could possibly occur on Earth from the random combinations of all sorts of simple atoms and molecules, only ~1500 actually do occur. Furthermore, these 1500 organic groups of terrestrial biology are made from only ~50 simple organic molecules, including the known amino acids and nucleotide bases. This implies that molecules critical to life aren’t assembled randomly by chance. Apparently, the electromagnetic forces at work at the microscopic level remove some of the randomness by guiding the molecules into certain, specific linkages.
Direct laboratory experiments support this alternative view. Simulations that resemble conditions on primordial Earth are now routinely performed with a variety of energies and initial reactants (provided there’s no free oxygen). These experiments demonstrate that unique (or even rare) conditions are unnecessary to produce the precursors of life. Complex acids, bases, and proteinoid compounds are formed under a rather wide variety of physical conditions. And it doesn’t take long for these reasonably complex molecules to form—not nearly as long as probability theory predicts by randomly assembling atoms.
Furthermore, every time this type of experiment is done, the results are much the same. The oily organic matter trapped in the test tube always yields the same proportion of acids, bases and rich proteinoids. If chemical evolution were entirely random, we might expect a different result each time the experiment is run. Apparently, electromagnetic forces do govern the complex interactions of the many atoms and molecules in the soupy sea, substituting organization for randomness.
Of course, precursors of proteins and nucleic acids are a long way from life itself. But the beginnings of life as we know it seem to be the product of less-than-random interactions among atoms and molecules. That’s important to know. Just how nonrandom—that is, how common—life itself might be is unknown.
An important caveat deserves mention here. Even if life everywhere in the Universe is based on carbon chemistry and obeys the basic laws of biology familiar to us, we shouldn’t be foolish enough to think that organisms elsewhere would evolve to look like us anatomically. Life forms on other planets—even carbonaceous organisms operating in a watery medium—would likely experience a wholly different set of environmental and genetic changes. The mechanism of biological evolution, with its mutations, natural selection, and adaptations functioning over long durations of time, would guarantee little outward resemblance to life on Earth.
So what do we choose as a numerical estimate for the fraction of habitable planets on which life actually arises? Either the number is much smaller than 1 if chance has a big influence. Or the number is close to or equal to 1 if chance plays no appreciable role. The former view suggests that life arises naturally, though rarely, whereas the latter view maintains that life is virtually inevitable given the proper ingredients, suitable environments, and long enough periods of time. No easy experiment can distinguish between these alternatives.
What we really need is a laboratory where organic chemistry has been left alone for a few billion years. What transpires there could help us decide the degree of randomness inherent in the molecular reactions. Fortunately, some of the nearby planets or their moons provide us with just such a laboratory, and a most interesting period of exploration will unfold as our spacecraft probe them for signs of life. In the minds of some researchers, the discovery of life on Mars, Europa, Titan, or some other object in our Solar System would convert the origin of life from an unlikely miracle to an ordinary statistic—to a value equal to or near 1 for this term of the equation.
In addition to “randomness” not being fully operational, other grounds tend to bolster the prospects for extraterrestrial life. One of these is that aliens could be based on something other than the carbon atom. Life “as we know it” is carbon-based life, operating in a water-based medium, with higher forms metabolizing oxygen. Yet once again, are we being chauvinistic by thinking that other types of biology are impossible? Perhaps so, but we’ve also noted several reasons why carbon-based life has more strength, diversity, and adaptability than any other.
Can we make an objective judgment of this factor in the Drake equation independent of our own prejudices? After all, chemists study Earth chemistry, not general chemistry. And biologists study the only kind of biology they know. Perhaps the alternatives haven’t yet been sufficiently investigated. At any rate, should biochemistries exist other than the carbon-in-water type, then the prospects for extraterrestrial life increase greatly. What we need now is ,just changing our searching tactics and be open minded.