In the 1960s, almost all scientists believed that we were alone in the universe. The search for intelligent life outside of Earth was ridiculed; as well, it seemed, one could look for fairies or witches. Skeptics believed that the origin of life was due more to a chemical misunderstanding, an event so incredible that it would never happen twice. "The origin of life seems almost a miracle," Francis Crick said about it. "There are too many conditions to be met for this to happen." Jacques Monod echoed him; in his 1976 book "Chance and Necessity," he wrote: "Man knows, at last, that he is alone in the indifferent immensity of the Universe, in which he appeared due to chance."
Today the pendulum swung decisively in the other direction. Many eminent scientists claim that the universe is teeming with life, some of which must definitely be intelligent. Biologist Christian de Duve went so far as to call life a "cosmic imperative." What has changed in science? Obviously nothing. We wander in practically the same twilight, trying to comprehend the transition from non-life to life that we had under Darwin, when he wrote: “It is extremely unreasonable at the present time to think about the origin of life; with such success one could speculate about the origin of matter."
There is no doubt that SETI - a global initiative for the search for extraterrestrial intelligence - has received a powerful boost from the recent discovery of hundreds of extrasolar planets, or exoplanets. Obviously, there is no shortage of real estate in space. But this property can be inhabited only if life really appears.
We often talk about how likely it is that we will find intelligent life outside of Earth. This question doesn't make sense. Since we do not know the process that transformed the jumble of chemicals into a living cell, in all its mind-boggling complexity, it’s impossible to calculate the likelihood that this could happen. Quite different forces can be involved here - even solar storms, a recent study has shown. And we have not yet managed to collect this puzzle. The odds of an unknown process cannot be estimated. But astrobiologists, however, are more interested in the likelihood that microbial life will eventually gain intelligence. Although biologists cannot calculate it mathematically, they fully understand the process; this is Darwinian evolution. And yet it's like putting a cart in front of a horse - the biggest uncertainty surrounds the first step,where the microbes come from.
Carl Sagan once remarked that the origin of life may not be such a complicated process, and life did not arise immediately once the Earth became hospitable to life. We can actually trace the presence of life on Earth as early as 3.5 billion years ago. But Sagan's argument ignores the fact that we are a product of terrestrial biology specifically. If life on Earth did not appear early enough, humans would not be able to appear before the Sun gets too hot and roasts our planet to the crust. We are biased in our judgments and cannot get a statistically significant sample based on ourselves.
Another common argument is that the universe is so huge that life must be somewhere. What follows from this statement? If we restrict ourselves to the observable universe, we get about 10 to the 23 power of planets. This is a large number. But it pales in comparison with the probability of the formation of even a simple organic molecule by chance alone. If the journey from chemistry to biology is long and difficult, it may well be that only one planet in a trillion could have life.
The assumption that life is widespread is based on the implicit assumption that biology is not a product of random chemical reactions, but a product of directed self-organization that promotes a living state - something like a life principle at work in nature. There may be such a principle, but we have not found any evidence of its existence.
We may not need to look far for an example. If life does emerge with ease, as Sagan suggested, it could have arisen a second time - a third or a fourth - on our own planet. If life on Earth appeared many times, we would be surrounded by the descendants of microbes of a completely different genesis, forming a kind of shadow biosphere. But no one has seriously studied microbes, and there may be billions of species, so we don't know yet. You only need to find one "alien" microbe to answer this question.
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