1. Foundation for Applied Molecular Evolution, Gainesville, Florida 32601, USA
2. DNA2.0, Inc., Menlo Park, California 94025, USA
3. Department of Biochemistry and Molecular Biology, University of Florida, Gainesville, Florida 32610, USA

Correspondence to: Eric A. Gaucher¹ Correspondence and requests for materials should be addressed to E.A.G. (Email: egaucher@ffame.org).

Biosignatures and structures in the geological record indicate that microbial life has inhabited Earth for the past 3.5 billion years or so^{1, 2}. Research in the physical sciences has been able to generate statements about the ancient environment that hosted this life^{3, 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 environment^{7, 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.

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