Palaeotemperature trend for Precambrian life inferred from resurrected proteins

Comparisons of genome sequence data in closely and distantly related modern organisms can be used for the computational reconstruction of ancient protein sequences that may have existed in related but now extinct types. These proteins can then be 'resurrected' in the laboratory. This has now been achieved for a group of 25 ancestral elongation factors from bacteria across an estimated span of 3 billion years. These ancient proteins display a near linear increase in thermostability travelling back in geological time, suggesting that the environment supporting ancient life was initially hot, then cooled progressively by about 30 °C during that period. This pattern is corroborated by the palaeotemperature trend inferred for the geologic record. Using phylogeny to reconstruct proteins inferred to have existed in the ancestry of organisms is a powerful and provocative way to discover the evolutionary processes of life. A protein from various stages in the history of life has been reconstructed to show that life originated in a hot environment, and has tracked the cooling of Earth through time. Biosignatures and structures in the geological record indicate that microbial life has inhabited Earth for the past 3.5 billion years or so1,2. Research in the physical sciences has been able to generate statements about the ancient environment that hosted this life3,4,5,6. These include the chemical compositions and temperatures of the early ocean and atmosphere. Only recently have the natural sciences been able to provide experimental results describing the environments of ancient life. Our previous work with resurrected proteins indicated that ancient life lived in a hot environment7,8. Here we expand the timescale of resurrected proteins to provide a palaeotemperature trend of the environments that hosted life from 3.5 to 0.5 billion years ago. The thermostability of more than 25 phylogenetically dispersed ancestral elongation factors suggest that the environment supporting ancient life cooled progressively by 30 °C during that period. Here we show that our results are robust to potential statistical bias associated with the posterior distribution of inferred character states, phylogenetic ambiguity, and uncertainties in the amino-acid equilibrium frequencies used by evolutionary models. Our results are further supported by a nearly identical cooling trend for the ancient ocean as inferred from the deposition of oxygen isotopes. The convergence of results from natural and physical sciences suggest that ancient life has continually adapted to changes in environmental temperatures throughout its evolutionary history.