1. NASA Astrobiology Institute, University of Florida, Gainesville, Florida 32611-7200, USA
2. Department of Anatomy and Cell Biology, College of Medicine, University of Florida, Gainesville, Florida 32611-7200, USA
3. Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, USA
4. Present address: Department of Cell and Developmental Biology, University of North Carolina, Chapel Hill, North Carolina 27599, USA

Correspondence to: Eric A. Gaucher¹ Email: gaucher@ufl.edu
Inferred ancestral sequences have been deposited in GenBank under accession numbers AY305395–AY305397

Abstract

Features of the physical environment surrounding an ancestral organism can be inferred by reconstructing sequences^{1, 2, 3, 4, 5, 6, 7, 8, 9} of ancient proteins made by those organisms, resurrecting these proteins in the laboratory, and measuring their properties. Here, we resurrect candidate sequences for elongation factors of the Tu family (EF-Tu) found at ancient nodes in the bacterial evolutionary tree, and measure their activities as a function of temperature. The ancient EF-Tu proteins have temperature optima of 55–65 °C. This value seems to be robust with respect to uncertainties in the ancestral reconstruction. This suggests that the ancient bacteria that hosted these particular genes were thermophiles, and neither hyperthermophiles nor mesophiles. This conclusion can be compared and contrasted with inferences drawn from an analysis of the lengths of branches in trees joining proteins from contemporary bacteria¹⁰, the distribution of thermophily in derived bacterial lineages¹¹, the inferred G + C content of ancient ribosomal RNA¹², and the geological record combined with assumptions concerning molecular clocks¹³. The study illustrates the use of experimental palaeobiochemistry and assumptions about deep phylogenetic relationships between bacteria to explore the character of ancient life.