GSI Seminar Series - Dan Mills 'Reassessing the “hard-steps” model for the evolution of intelligent life'

According to the “hard-steps” model (HSM), the evolutionary origin of humans depended on a small number of key intermediate “steps” that were inherently unlikely to occur during Earth’s habitable lifespan – thereby making humans an unlikely product of biological evolution. Likewise, the HSM predicts that human-like life beyond the Earth is “exceedingly rare.” The HSM is ostensibly supported by the claimed “difficulty” of evolutionary events required for human existence (for example, the origin of eukaryotic life, organisms with nuclei and mitochondria). However, the HSM takes the apparent difficulty of these evolutionary innovations for granted, ignoring alternate ways of evaluating the inherent probability of evolutionary events. Furthermore, the HSM model assumes that the Earth always has been hospitable to humans, as well as to each evolutionary step required to produce humanity. In this talk, I critically evaluate these core assumptions of the HSM and propose an alternative scenario: “hard steps” – as originally defined – simply do not exist, and the timing of many key evolutionary transitions necessary for human origins occurred when they did (and not much earlier) due to the sequential and unidirectional opening of different global-environmental “windows” of habitability over Earth history. In this light, the origin of humans was not inherently improbable, and did not take a long time relative to the opening of humanity’s respective habitability window. If true, human-like life may evolve in a more predictable or deterministic manner on suitable worlds beyond the Earth.

Daniel Mills is a microbial ecologist and historical geobiologist at the University of Munich. He reconstructs the evolutionary history of Earth’s biosphere using data collected from modern marine organisms and environments. He is broadly interested in applying knowledge of the Earth system to understand the likelihood and nature of extraterrestrial biospheres capable of detection from Earth.

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