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Physicists today seem to believe that human life on Earth is the consequence of a series of highly unlikely events.

For example, the universe needed to expand with a specific speed after the big bang, to allow for stars and planets to form; had it expanded faster or slower, matter would not have coalesced in star-sized clumps at stable distances.

Some interpret this as proof that there must have been some guiding force or fate that navigated all these improbabilities. They contrast the fact of human existence (probability = 1) with the slim probability of humans coming into existence and conclude that if something that is almost unlikely to happen, happens, it cannot have been chance.

Disregarding the physical theories and wether or not they are accurate, what does statistics have to say to that interpretation?

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    $\begingroup$ Your remark about "the slim probability of humans coming into existence" reminds me of the remark by Feynman <en.wikiquote.org/wiki/Richard_Feynman>: You know, the most amazing thing happened to me tonight. I was coming here, on the way to the lecture, and I came in through the parking lot. And you won't believe what happened. I saw a car with the license plate ARW 357. Can you imagine? Of all the millions of license plates in the state, what was the chance that I would see that particular one tonight? Amazing! $\endgroup$ – user20637 Dec 9 '14 at 13:53
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    $\begingroup$ @user20637 This Feynman remark says it all. It is too hard for us humans to really grasp randomness, even though we came up with the concept. So when we combine an essentially deterministic approach with a half-baked notion of randomness, everything becomes "Amazing!" since anything has a very slim chance of happening. $\endgroup$ – Alecos Papadopoulos Dec 9 '14 at 21:34
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    $\begingroup$ As recently as 40 years ago people were deploying identical arguments to suggest the earth was practically alone in the universe: that there were few planets anywhere and not likely any with conditions conducive to the evolution of life as we know it. I have been glad recently to follow the explosion of discoveries of exoplanets, because each new one helps expose the fallacy of trying to apply probabilistic arguments so speculatively. $\endgroup$ – whuber Jan 14 '15 at 18:25
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This line of thought needs to take into account that we are not discussing "unconditional" probabilities here (whatever that means: any probability which we assign will be conditional on some assumptions and models, and even if we don't state them, they won't go away). Instead, we are observing a sequence of events conditional on there being someone to observe them, namely us. This is the Anthropic Principle.

The problem with, e.g., Bayesian approaches to this is that we have little to no idea about the "unconditional" (in this context; see above) probabilities involved: even if P("the universe offers conditions that support life"|"life develops") = 1 (this is a tautology), we still have no idea about P("the universe offers conditions that support life") or P("life develops"), since this universe is all we have to work with. (Your friendly neighborhood physicist may have a different take on my last sentence.)

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I think the only thing that statistics can add to this conversation is the idea of the binomial distribution. Basically even if the probability of life on the planet is miniscule, due to the extremely large number of planets in our universe, at least one planet is going to have life on it with very high probability.

The fact that we got lucky to be on a planet that has life is not rare at all. Because if we were on a planet that didn't have life, we wouldn't know. You could put this in a conditional probability framework.

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First, your question makes the remark that we exist with probability = 1. While this is true, is irrelevant for the question: it's like thinking that the presence of a unique value (in a result of a random process for example) is relevant, not knowing anything of this process (i.e. the probabilities involved).

Probably the point would be to evaluate if human life is a consequence of highly unlikely events. This is related with the Fermi Paradox.

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