Life 4.1 Billion Years Ago?

Introduction

Zircons in Australia reveal graphite embedded within them that hints at carbon-based like on Earth 4.1 billion years ago.

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Abstract

Evidence of life on Earth is manifestly preserved in the rock record. However, the microfossil record only extends to ∼3.5 billion years (Ga), the chemofossil record arguably to ∼3.8 Ga, and the rock record to 4.0 Ga. Detrital zircons from Jack Hills, Western Australia range in age up to nearly 4.4 Ga. From a population of over 10,000 Jack Hills zircons, we identified one >3.8-Ga zircon that contains primary graphite inclusions. Here, we report carbon isotopic measurements on these inclusions in a concordant, 4.10 ± 0.01-Ga zircon. We interpret these inclusions as primary due to their enclosure in a crack-free host as shown by transmission X-ray microscopy and their crystal habit. Their δ¹³C_PDB of −24 ± 5‰ is consistent with a biogenic origin and may be evidence that a terrestrial biosphere had emerged by 4.1 Ga, or ∼300 My earlier than has been previously proposed.
Significance

Evidence for carbon cycling or biologic activity can be derived from carbon isotopes, because a high ¹²C/¹³C ratio is characteristic of biogenic carbon due to the large isotopic fractionation associated with enzymatic carbon fixation. The earliest materials measured for carbon isotopes at 3.8 Ga are isotopically light, and thus potentially biogenic. Because Earth’s known rock record extends only to ∼4 Ga, earlier periods of history are accessible only through mineral grains deposited in later sediments. We report ¹²C/¹³C of graphite preserved in 4.1-Ga zircon. Its complete encasement in crack-free, undisturbed zircon demonstrates that it is not contamination from more recent geologic processes. Its ¹²C-rich isotopic signature may be evidence for the origin of life on Earth by 4.1 Ga.

Authors

1.      Elizabeth A. Bell

o        ^aDepartment of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095;

2.      Patrick Boehnke

o        ^aDepartment of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095;

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3.      T. Mark Harrison

o        ^aDepartment of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095;

4.    Wendy L. Mao

o   ^bSchool of Earth, Energy, and Environmental Sciences, Stanford University, Stanford, CA 94305

http://www.pnas.org/content/112/47/14518.abstract