Molecular foundations of Precambrian uniformitarianism

Uniformitarian assumptions underlie the oldest evidence for living organisms on Earth, the distinct isotope fractionation between inorganic and organic carbon. Aside from a handful of compelling deviations, the 13C/12C isotopic mean of preserved organic carbon ({delta}13Corg) has remained remarkably unchanged through time. RuBisCO is the principal carboxylase/oxygenase biomolecular component that is thought to primarily account for the generation of these distinct carbon isotopic signals. However, it is difficult to reconcile a mostly unchanging mean {delta}13Corg with several known factors that can affect the isotope fractionation of RuBisCO, such as atmospheric composition and the amino acid composition of the enzyme itself, which have each changed markedly over Earth history. Here we report the resurrection and genetic incorporation of a Precambrian-age, Form IB RuBisCO in a modern cyanobacterial host. The isotopic composition of biomass relative to CO2 ({varepsilon}p) in ancestral and control strains were much greater when grown under Precambrian CO2 concentrations compared to modern ambient levels, but displaying values within a nominal envelope of modern-day RuBisCO IB enzyme variants. We infer that these isotopic differences derive indirectly from the decreased fitness of the AncIB strain, which includes diminished growth capacity and total cell RuBisCO activity. We argue that to answer the greatest questions of deep-time paleobiology, ancient biogeochemical signals should be reproduced in the laboratory through the synthesis of the geologic record with experimentally-derived constraints on underlying ancient molecular biology. Significance StatementThe earliest geochemical indicators of microbes, and the enzymes that powered them, extend back almost 3.8 billion years on our planet. Paleobiologists often attempt to understand these indicators by assuming that the behaviors of modern microbes and enzymes are consistent (uniform) with those of their predecessors. This assumption seems uncomfortably at odds with the great variability of Earths environment and its highly adaptive microbes. Here we examine whether a uniformitarian assumption for an enzyme thought to generate these indicators, RuBisCO, can be corroborated by independently studying the history of changes recorded within RuBisCOs genetic sequences. We outline a new approach to paleobiology that informatively links molecule-level evolutionary changes with planet-level geochemical conditions in Earths deep past.