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Project Progress

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 four assumptions:
(1) To function, enzymes must be adapted to their ambient environment and, to remain functional over geological periods of time, the ancillary parts (e.g., composition and folding of side chains) of early evolved enzymes would have to have evolved to adapt to slowly changing environmental conditions.
(2) All enzymes must therefore have been functional at the temperature prevailing in the environment in which they originated.
(3) Analyses of the thermal stability of enzymes reconstructed to their original composition should thus evidence the temperature of the environment in which they originated.
(4) Therefore, comparisons of photic zone enzymes that originated at widely spaced intervals over geological time should document Earth’s ambient surface temperature over the geological past.

In the geological record, there is virtually no mineralogical evidence (except the conversion of gypsum to anhydrite at ~58oC) by which to determine ambient surface temperature. Data from other sources are ambiguous and contradictory. The early “dim Sun,” ~30% less luminous than at present, suggests a colder 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 (δ¹⁸O and δ³⁰S-) from ancient cherts suggest temperatures of ~60o some three billion years ago (Knauth and Lowe, 1978, 2003; Robert and Chaussidon, 2006) whereas geological evidence reflects the presence of glaciations at ~2.8 Ga — a seemingly intractable dichotomy. The isotopic data have therefore been largely ignored, regarded widely as reflecting post-depositional metamorphic resetting of the isotopic signal.

By analyzing the thermal stability of ancient reconstructed enzymes, this work may show a way to solve this problem. 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; Akanuma et al., 2013). For this initial work we have selected nucleoside diphosphate kinase (NKD), the enzyme that converts ATP (adenosine triphosphate) to ADP (adenosine diphosphate) present in all organisms. In order to apply our analyses to the evolution of Earth’s surface environment, this first stage of our work focuses on enzymes only of photic zone organisms (viz., those of phototrophs such as cyanobacteria and vascular plants).

During 2015, Schopf’s graduate student, Amada Garcia, spent 3 months working in Yamagishi’s lab in Tokyo. Results to date suggest that at ~3000 Ma (for the origination of Cyanobacteria) Earth’s surface temperature was ~65º C, and at ~2100 Ma (for the origination of nostocalean Cyanobacteria), ~45º C – data consistent with Si-isotope measurements in ancient cherts (Figure 1).

Ms. Garcia will return to Japan in April 2016 to analyze and determine the temperature stability of the reconstructed NKD of early vascular plants (~410 Ma). 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 and our analyses of the biochemistry-molecular biology of modern organisms are therefore shown to hold a key to understanding the co-evolution of Earth and life, this study will be of pivotal importance, providing a major advance in efforts to unravel the history of life.

References Cited

Akanuma S. Nakajima, Y, Yokobori, S, Kimura, M, Nemoto, N, Mase, T, Miyazono, K, Tanokura, M, Yamagishi, A (2013) Experimental evidence for the thermophilicity of ancestral life. Proc. Natl. Acad. Sci. USA, 110:11067-11072.

Knauth, LP, Lowe, DR. (1978) Oxygen isotope geochemistry of cherts from the Onverwacht Group (3.4 billion years). Transvaal, South Africa, with implications for secular variations in the isotopic composttion of cherts. Earth Planet. Sci. Lett. 41:209-222.

Knauth, LP, Lowe, DR (2003) High Archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3.5 Ga Swaziland Supergroup, South Africa. Geol. Soc. Am. Bull. 115:566-580.

Robert, F. Chaussidon, MA (2006) Paleotemperature curve for the Precambrian oceans based on silicon isotopes in cherts. Nature 443:969-972.