The Timing of Evolutionary Transitions Suggests Intelligent Life Is Rare

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

Abstract

It is unknown how abundant extraterrestrial life is, or whether such life might be complex or intelligent. On Earth, the emergence of complex intelligent life required a preceding series of evolutionary transitions such as abiogenesis, eukaryogenesis, and the evolution of sexual reproduction, multicellularity, and intelligence itself. Some of these transitions could have been extraordinarily improbable, even in conducive environments.

I seem to recall someone like Nick Lane argues that the origin of eukaryotes was essentially a singular event in life’s history, or at least very rare, and as a consequence even if life exists elsewhere in the universe, it’s only very rarely going to evolve into very complex multicellular life like plants and animals.

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For the math geeks among us…

Most of these “evolutionary transitions” are poorly understood and have not been very well studied by the scientists of likelihoods. And using their model, these scientists say that Earth’s series of Goldilocks lottery tickets are more likely to have taken far longer than they really did on Earth.

https://doi.org/10.1089/ast.2019.2149

Abstract

It is unknown how abundant extraterrestrial life is, or whether such life might be complex or intelligent. On Earth, the emergence of complex intelligent life required a preceding series of evolutionary transitions such as abiogenesis, eukaryogenesis, and the evolution of sexual reproduction, multicellularity, and intelligence itself. Some of these transitions could have been extraordinarily improbable, even in conducive environments. The emergence of intelligent life late in Earth’s lifetime is thought to be evidence for a handful of rare evolutionary transitions, but the timing of other evolutionary transitions in the fossil record is yet to be analyzed in a similar framework. Using a simplified Bayesian model that combines uninformative priors and the timing of evolutionary transitions, we demonstrate that expected evolutionary transition times likely exceed the lifetime of Earth, perhaps by many orders of magnitude. Our results corroborate the original argument suggested by Brandon Carter that intelligent life in the Universe is exceptionally rare, assuming that intelligent life elsewhere requires analogous evolutionary transitions. Arriving at the opposite conclusion would require exceptionally conservative priors, evidence for much earlier transitions, multiple instances of transitions, or an alternative model that can explain why evolutionary transitions took hundreds of millions of years without appealing to rare chance events. Although the model is simple, it provides an initial basis for evaluating how varying biological assumptions and fossil record data impact the probability of evolving intelligent life, and also provides a number of testable predictions, such as that some biological paradoxes will remain unresolved and that planets orbiting M dwarf stars are uninhabitable.

The references in that paper led me to this one:

I’m not sure it supports the conclusions of the first paper. I’m not sure the idea of constructing a probability distribution from one data point is that great either.

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For me, the first paper represents a fundamentally misguided use of Bayesian analysis. The idea is that the transitions needed for intelligent life could be really probable or really improbable. Because our observation of intelligence (such as it is) on Earth is conditional on those transitions having occurred, that observation does little to tell us about the probability of the transitions – we could be a rare outlier or a common occurrence. And (here’s the key step) because there are a lot more long times than short times (measured in the stellar lifetimes), then the posterior distribution of the transition probabilities favors low probability/slow transitions. For me, the proper Bayesian inference is that the observation of intelligence here tells us very little about whether intelligent life is common or rare, since Bayesian thinking is all about our level of knowledge. The conclusion, ‘therefore intelligent life is rare in the universe’ is a frequentist conclusion, not a Bayesian one.

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Or this math could be an obvious, definitive rebuke of current evolutionary theory since it is not required to be true in order for intelligent life to be here