Isotopes Beyond PET

Being in a lab that uses short-lived radioactive isotopes to study the human brain, we sometimes forget the utility of long-lived isotopes in other scientific fields. Reading the recent report of Bell et al. that indicates a potential for life developing on Earth as early as 4.1 Billion years ago,1 I was reminded of the vast historical knowledge humans have gained through isotopic measurement. In this study, zircons from the Jack Hills in Australia, known for containing the oldest terrestrial-formed material on Earth,2 were uranium-lead dated (4.1 Ga) and analyzed for the presence of partially disordered graphite (i.e. carbon, the atom of life). Zircons with graphite were analyzed for the presence of > 40 nm cracks—thanks to synchrotron transmission X-ray microscopy—to avoid regions that may have gathered carbon-based material after zircon formation. Crack-less regions were analyzed for evidence of biogenic carbon through 12C/13C isotopic ratio measurement.3 The 12C/13C ratio found in carbon-based inclusions in the Jack Hills zircon was consistent with the known biogenic carbon signature, suggesting that life may have had its start as early as 4.1 billion years ago, 300 million years earlier than previous reports.4 If earlier development of planetary life is the norm, it would increase current estimates of the prevalence of life throughout the universe. Bring on the aliens!
~GV
1) Bell, E. A. et al. (2015). "Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon". PNAS, doi: 10.1073/pnas.1517557112.
2) Wilde, S. A.; et al. (2001). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago." Nature 409 (6817):175–178.
3) Schopf JW, Kudryavtsev AB (2014) Biogenicity of Earth’s earliest fossils. Evolution of Archean Crust and Early Life, Modern Approaches in Solid Earth Sciences, eds Dilek Y, Furnes H (Springer, Dordrecht, The Netherlands), Vol 7, pp 333–349.
4) Mojzsis SJ, et al. (1996) Evidence for life on Earth before 3,800 million years ago. Nature 384 (6604):55–59.