Human evolution: we are entirely improbable (Introduction)

by David Turell , Sunday, August 20, 2023, 19:58 (251 days ago) @ David Turell

A new discussion of probability:

https://inference-review.com/article/a-lonely-universe

"The fact that it took so long suggests that human-like intelligence is difficult to evolve and uncommon.8 This is the basic reasoning behind Snyder-Beattie et al.’s conclusion that intelligent life in the universe is rare.

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"PREVIOUS ANALYSES HAVE also looked at the emergence of life in conjunction with the emergence of human-like intelligence.9 Motivated by the assumption that four data points are better than two, Snyder-Beattie et al. have extended this earlier work with a Bayesian analysis of not only the timing of abiogenesis and the evolution of intelligence, but also the timing of two other major transitions: eukaryogenesis and the evolution of sexual reproduction. They conclude that intelligent life is rare in the universe because it took humans such a long time to evolve all four of the assumed prerequisites: abiogenesis, eukaryogenesis, sexual reproduction, and intelligence itself. Their Bayesian exploration of this result includes varying the timing of abiogenesis over a relatively wide range—between 4.3 and 3.5 billion years ago—and computing the effect of discovering that life emerged twice on earth.10 They found that their conclusion no longer holds if life emerged twice; or if abiogenesis occurred earlier, say, within ~10 million years of habitability; or if the habitable lifetime of the earth is 10 times longer than expected.

"Recent exoplanet studies strongly suggest that every star has some kind of planetary system and that earth-like planets are likely common in such systems.12 The earth may well be representative of a very large group of wet, rocky planets. But what about atmospheric composition, ocean volume, plate tectonics, spin period, orbital period, obliquity, the presence of a large moon, and the timing of large impacts? If the emergence and evolution of life are dependent on some of these additional details, the number of earth-like planets could be quite small.

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"...the Snyder-Beattie et al. result depends on the assumption that “intelligent life elsewhere requires analogous evolutionary transitions.” The validity of the Snyder-Beattie et al. result, among others,15 is dependent on the assumption that the major transitions that characterize our evolution happen elsewhere.

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"In our lineage, eukaryogenesis occurred about two billion years ago and the transition to sexual reproduction about a billion years ago. The transition to intelligence is much more recent and its timing depends on how intelligence is defined. The transition to human-like intelligence or technological intelligence occurred only about 100,000 years ago and is species-specific. The latter trait is strong evidence we should not expect to find it elsewhere.

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"IS IT REASONABLE to argue that among the features of life on earth, the most likely to appear in life elsewhere are those that have evolved independently many times, such as complex multicellularity, eyes, wings, and canines?20 Not really. These examples of convergent evolution have only occurred within a unique eukaryotic branch that represents a tiny fraction of the diversity of life on earth. The absence of these features elsewhere in the tree of life suggests that eukaryote evolution brought with it fundamental and deeply homologous adaptations necessary for the emergence of these features. If eukaryogenesis were a convergent feature of evolution, we would expect it to have evolved on multiple occasions among the hundreds of independent major lineages. It did not. As a result, we have no evidence that eukaryogenesis is likely to appear outside our own tree of life. The same can be said for the two other major transitions that occurred during the evolution of our lineage. These transitions did not occur in any of the hundreds of non-eukaryotic lineages. Eukaryogenesis, sexual reproduction,21 and human-like intelligence evolved only in our own lineage. (my bold)

"The evolutionary lineages in the universe closest to our own lineage are those found here on earth. And since none of them underwent the three major transitions that happened in our lineage, we have no reason to think they might occur elsewhere. Attempting to compute the probability of human-like intelligence elsewhere based on our lineage is akin to analyzing the evolution of the English language on earth and trying to use the timing of the Great Vowel Shift to estimate its timing on other planets. The quirky, contingent, and self-referential nature of biological evolution means that, like history, it does not lend itself to being modeled using the more deterministic probabilities that physicists and chemists are used to dealing with."

comment: this analysis stresses that the uniqueness of our single evolutionary line tends to negate the proposition life is elsewhere, especially life like human. This is absolutely parallel to Adler's position that the presence of unique humans proves God. It is all in your point of view, isn't it? The original work discussed:

https://www.liebertpub.com/doi/10.1089/ast.2019.2149