2014 Annual Science Report
University of Wisconsin Reporting | SEP 2013 – DEC 2014
Project 1C: Studies of Early-Evolved Enzymes in Modern Organisms May Reveal the History of Earth's Ambient Temperature Over Geological Time
By addressing a focused question — “Does the thermal stability of the reconstructed ancient enzymes of modern organisms provide evidence of the temperature of the environment in which the enzymes originated?” — this study asks a much broader question, namely, “can the biochemistry of extant life provide evidence of ancient environments?” In the geological record, there is virtually no mineralogical evidence to determine ambient surface temperature and data from other sources are ambiguous and contradictory. By analyzing the thermal stability of ancient reconstructed enzymes we hope that this work will pave the way to solve this fundamental problem and, by doing so, demonstrate a new way to understand the co-evolution of life and its planetary environment.
By addressing a focused question — “Does the thermal stability of the reconstructed ancient enzymes of modern organisms provide evidence of the temperature of the environment in which the enzymes originated? — this study asks a much broader question, namely, “Can the biochemistry of extant life provide evidence of ancient environments?”
To address these questions, we focus on enzymes and make the following five assumptions: (1) To function, enzymes must be adapted to their ambient environment. (2) All enzymes must therefore have functioned at the temperature prevailing in the environment in which they originated. (3) To maintain functionality over time, the active sites of enzymes remained unchanged as their ancillary parts (e.g., composition and folding of side chains) evolved to adapt to changing environmental conditions. (4) Thermal analyses of enzymes reconstructed to their original composition should therefore evidence the temperature of the environment in which they originated. (5) Such analyses of enzymes that originated at widely spaced intervals over geological time should yield telling evidence of Earth’s ambient surface temperature over the geological past.
In the geological record, there is virtually no mineralogical evidence (save the conversion of gypsum to anhydrite at ~58oC) to determine ambient surface temperature. And data from other sources are ambiguous and contradictory. The early “dim Sun,” ~30% less luminous than the present, suggests a cold early Earth. Yet models of Earth’s early atmosphere, comparisons with Venus, and carbon isotopic evidence from the geological record suggest that CO2 and CH4 offset the dim Sun. Silicon isotopic data from ancient cherts suggest temperatures of ~60o some three billion years ago whereas geological evidence reflects the presence of glaciations at ~2.8 Ga — a seemingly intractable dichotomy. Problems and inconsistencies are illustrated by comparison of Figs. 1, 2 and 3.
By analyzing the thermal stability of ancient reconstructed enzymes, this work may show a way to solve this problem. As shown in Figs. 1, 2 and 3, for our initial work we have identified several temporally widely separated enzymes amenable for study. To conduct this work we have teamed with the world’s expert on the reconstruction of ancient enzymes, Akihiko Yamigishi of the Japanese Astrobiology Group (and renowned for his experimental studies suggesting the thermophily of LUCA, life’s Last Universal Common Ancestor). In order to apply our analyses to the evolution of Earth’s surface environment, the first stage of our work is focused on enzymes limited to photic zone organisms (e.g., those involved in biosynthesis of cyanobacterial Rubisco, originating ~3000 Ma ago; vascular plant lignin, ~420 Ma:, and flowering plant triterpenoids, ~100 Ma) or obligate aerobes (e.g., metazoan collagen, ~700 Ma). And by arranging for Schopf and his graduate student Amada Garcia to spend weeks working in Yamagishi’s lab in Tokyo in 2015, this work can be expected to provide a firm indication of the efficacy of this approach to understanding planetary and biological co-evolution.
Though technically difficult, this project holds promise to yield high return. If our notion is confirmed, if our analyses of the biochemistry-molecular biology of modern organisms are shown to hold a key to understanding the co-evolution of Earth and life, this study will be of pivotal importance.
PROJECT MEMBERS:James Schopf
RELATED OBJECTIVES:Objective 4.1
Earth's early biosphere.
Environment-dependent, molecular evolution in microorganisms
Effects of environmental changes on microbial ecosystems