The Quirky, Contingent, And Self-Referential Nature Of Biological Evolution Is Rare

inference-review |  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.¹¹

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.¹³

Once life has emerged from prebiotic chemistry, the strongest selection pressures on the evolution of a species come from other life forms: conspecifics, parasites, predators, diseases, viruses, and ecosystem variability. This self-referential nature of biology makes evolution a historical science characterized by the quirks of contingency. This characterization of evolution remains controversial.¹⁴ Our ability to extrapolate crow–puzzle experiments to crows on other planets depends on the existence of extraterrestrial crows. Similarly, 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.¹⁶

There is little evidence in the history of life on earth to support this assumption. Although abiogenesis is a transition shared by the lineages of all known life on earth, diverging lineages over the next four billion years are punctuated by their own evolutionary transitions. After diverging from other life forms, transitions within our own eukaryotic lineage include eukaryogenesis, sexual reproduction, and intelligence. A general feature of these transitions in the tree of life is that the closer a transition is to the end of a branch, the more recent, specific, and uncommon it is.¹⁷ 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.¹⁸