Life in a greenhouse world
What constrained the evolution of life during the very hot early Earth? Was a simple drop in temperature largely responsible for the emergence of cyanobacteria, a large and varied group of bacteria with chlorophyll that carry out photosynthesis in the presence of light and air with concomitant production of oxygen? Was it a reduction in carbon-dioxide levels?
Geochemist David Schwartzman of Howard University and Ken Caldeira of the Climate and Carbon Cycle Group at Lawrence Livermore National Laboratory have a different view. Looking at how feedback operates in Earth systems, they propose that the transition from a carbon-dioxide dominated greenhouse world to one dominated by methane actually did the trick.
Schwartzman and Caldeira will present findings of their research on Monday, October 28, at the annual meeting of the Geological Society of America in Denver, CO.
Its been argued that surface temperatures of 80-60 degrees centigrade kept the lid on evolution during the carbon-dioxide dominated greenhouse world of 3.8 to 2.5 billion years ago. Dominant forms of life were very simple, consisting of prokaryotes (cells without nuclei that reproduce asexually) and eucaryotes (more advanced cells with nuclei). Metazoa, the animal kingdom, did not emerge until 0.7 to 1.5 billion year ago, when temperatures apparently dropped below their upper limit.
“I have argued that primitive organisms emerged once their upper temperature limit was reached as the relatively high climatic temperatures of the Archean declined,” says Schwartzman. “It appeared likely that methane replaced carbon dioxide as the dominant gas in the greenhouse atmosphere of early Earth by about 2.8 billion years ago. So we began to look at the dynamics of methane dominance, reduced levels of CO2, reduced surface temperatures, and the appearance of cyanobacteria. The question that arose for me is, Is it a coincidence that the first good evidence for methane as a significant component of Earths atmosphere occurred at the same time as analogous evidence for the first cyanobacteria?”
Schwartzman and Caldeira followed up the proposal of Charles Dismukes and coworkers that now extinct bacteria were performing oxygen-based photosynthesis before cyanobacteria came onto the scene. In a CO2 dominated world, these early oxygenic photosynthesizers split bicarbonate instead of water as the source of oxygen. They apparently boosted organic productivity and caused greater methane production by methanogens living in the ocean.
“It takes far less methane to maintain climatic temperatures than it does carbon dioxide,” says Schwartzman.
As methane became dominant, CO2 levels dropped dramatically. Cyanobacteria then emerged and began oxygenic photosynthesis by splitting water as the source of oxygen. According to Schwartzman, only when atmospheric oxygen levels began to rise some 2.2 billion years ago did a CO2-concentrating mechanism emerge, an adaptation to declining CO2/02 ratios in the external environment.
Thus, global constraints on evolution appeared to have included carbon dioxide as well as oxygen levels in the atmosphere along with surface temperature. All the former have been strongly influenced by biological evolution in a complex set of feedbacks, an essential aspect of biospheric evolution. “The classical paradigm of evolution, that changes in the local environment lead to natural selection, should be rethought to include these feedbacks on a global scale. We hope that our hypothesis will be tested by looking more closely at the extant geologic record of the proposed transition as well as the insights from the study of photosynthesis and molecular biology of modern organisms,” says Schwartzman.
During the GSA Annual Meeting, Oct. 27-30, contact Christa Stratton at the GSA Newsroom in the Colorado Convention Center, Denver, Colorado, for assistance and to arrange for interviews: 303-228-8565.
The abstract for this presentation is available at: http://gsa.confex.com/gsa/2002AM/finalprogram/abstract_44609.htm
Post-meeting contact information:
David W. Schwartzman
Dept. of Biology
Washington, DC 20059
Director of Communications
Geological Society of America
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