SCIENCE AT THE SHINE DOME canberra 6 - 8 may 2009

Symposium: Evolution of the universe, the planets, life and thought

Friday, 8 May 2009

Professor Euan Nisbet
Royal Holloway, University of London, UK

Euan Nisbet was educated at the Universities of Zimbabwe and Cambridge, where he won the Harkness Prize in 1970, before PhD research at Darwin College. He is a distinguished fellow of the Geological Association of Canada, and a life member of the Geological Society of Zimbabwe. He has given both the William Smith and Fermor Lectures of the Geological Society of London and spoke on early life as an invited panellist at the US National Academy of Sciences' 129th annual meeting.

Euan studies the early Earth, working both on the development of the continents and mantle, and also on the habitat and evolution of life in the Archaean (prior to 2.5 billion years ago). Currently his geological work focuses on the interaction between early geology and biochemistry, seeking to understand the history of the air as a biological construction. He also studies the modern and glacial atmosphere, focusing on methane, and leads the Greenhouse Gas Activity in the European Union's GEOmon global atmospheric monitoring program.

Darwin the geologist: His relevance to 21st century Earth science

The first persuasive signs of life on Earth are in ~3.8 billion year (Ga) old sediments from Greenland. The original home of life is unknown, but early life may have depended on chemical contrasts near volcanic vents. By about 3 Ga ago, evolution had produced a diverse microbial ecology. Under a faint young Sun, stable liquid oceans were present. The implication is that a potent atmospheric greenhouse increment, probably from biogenic methane, was manipulated and sustained by life, to the extent that the air was a biological construction created by evolution.

Isotopic evidence suggests oxygenic photosynthesis evolved about 2.9 Ga ago. Oxygen emission would have challenged a methane greenhouse, and glacial episodes may have resulted. By 2.7 Ga ago, there is abundant evidence of microbial life. Presumably the ocean surface hosted cyanobacterial picoplankton. Although oxygen-rich photic-zone water may have existed in the Archaean, 33S evidence suggests the air did not become permanently oxic until roughly 2.3 Ga ago, in the Proterozoic. Since then, CO2 must have become the managing greenhouse gas.

Today, oxygenic rubisco-I organisms control redox segregation, and hence the atmospheric burdens of CO2 and CH4 (methane) while nitrogenase-using cells set the air pressure and the greenhouse broadening. It has been suggested that rubisco is the best of all possible enzymes, exquisitely tuned by evolution to maximise productivity while managing the greenhouse to sustain liquid oceans. That planetary management is now challenged, as we return segregated reduced carbon from the sediment to the air.