On the Origin and Evolution of Life in the Galaxy

TL;DR

This paper uses a stochastic model of critical evolutionary steps to explore the origins and distribution of life in the galaxy, suggesting that life is scarce extraterrestrially due to limited time for evolution, but migration could extend habitable timescales.

Contribution

It applies a critical step model to both Earth and extraterrestrial origins, providing new insights into the timing and likelihood of life elsewhere in the galaxy.

Findings

Humans emerged after 4 critical steps, fitting the model.

Adding earlier steps suggests habitability began around 6 billion years ago.

Extraterrestrial life is likely scarce due to insufficient evolutionary time.

Abstract

A simple stochastic model for evolution, based upon the need to pass a sequence of n critical steps (Carter 1983, Watson 2008) is applied to both terrestrial and extraterrestrial origins of life. In the former case, the time at which humans have emerged during the habitable period of the Earth suggests a value of n = 4. Progressively adding earlier evolutionary transitions (Maynard Smith and Szathmary, 1995) gives an optimum fit when n = 5, implying either that their initial transitions are not critical or that habitability began around 6 Ga ago. The origin of life on Mars or elsewhere within the Solar System is excluded by the latter case and the simple anthropic argument is that extraterrestrial life is scarce in the Universe because it does not have time to evolve. Alternatively, the timescale can be extended if the migration of basic progenotic material to Earth is possible. If extra transitions are included in the model to allow for Earth migration, then the start of habitability needs to be even earlier than 6 Ga ago. Our present understanding of Galactic habitability and dynamics does not exclude this possibility. We conclude that Galactic punctuated equilibrium (Cirkovic et al. 2009), proposed as a way round the anthropic problem, is not the only way of making life more common in the Galaxy.