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Origin & Evolution of Life: April 2012


A study published in PLoS Computational Biology maps the development of life-sustaining chemistry to the history of early life. Researchers Rogier Braakman and Eric Smith of the Santa Fe Institute traced the six methods of carbon fixation seen in modern life back to a single ancestral form.

Carbon fixation - life's mechanism for making carbon dioxide biologically useful - forms the biggest bridge between Earth's non-living chemistry and its biosphere. All organisms that fix carbon do so in one of six ways. These six mechanisms have overlaps, but it was previously unclear which of the six types came first, and how their development interweaved with environmental and biological changes.

The authors used a method that creates "trees" of evolutionary relatedness based on genetic sequences and metabolic traits. From this, they were able to reconstruct the complete early evolutionary history of biological carbon-fixation, relating all ways in which life today performs this function.

The earliest form of carbon fixation identified achieved a special kind of built-in robustness - not seen in modern cells - by layering multiple carbon-fixing mechanisms. This redundancy allowed early life to compensate for a lack of refined control over its internal chemistry, and formed a template for the later splits that created the earliest major branches in the tree of life. For example, the first major life-form split came with the earliest appearance of oxygen on Earth, causing the ancestors of blue-green algae and most other bacteria to separate from the branch that includes Archaea, which are outside of bacteria the other major early group of single-celled microorganisms.

"It seems likely that the earliest cells were rickety assemblies whose parts were constantly malfunctioning and breaking down," explains Smith. "How can any metabolism be sustained with such shaky support? The key is concurrent and constant redundancy."

Undergraduate students, graduate students, and post docs are invited to apply to the 2012 Sao Paulo School of Advanced Science, held from 19-31 August, 2012 on Ilhabela, an archipelago 200km from Sao Paulo, Brazil. The school will be organized around the theme of evolution, addressing topics such as paleontology, phylogenetics, homology, and character evolution, and will feature instructors from both North and South America. For more information: http://www.ib.usp.br/zoologia/evolution/index.html

Geological background of the samples analyzed in this study. Panel A shows the geological map at Marble Bar and the location of the ABDP-1 drill core. Panel B shows the simplified stratigraphic column of the lower part of the Pilbara Supergroup, with ages constrained by zircon U-Pb geochronology.

Astrobiologists from NAI's team at the University of Wisconsin, Madison have recently published a study of drill cores obtained through the NAI-funded Archean Biosphere Drilling Project which sampled the 3.4 billion year old Apex Basalt from the Pilbara Craton in Western Australia. Their innovative approach directly dates oxidation products of the ancient rock, and they show that oxidation occurred in the Phanerozoic during deep weathering. Their results indicate that oxidation of the Apex Basalt did not occur in the Archean, and therefore cannot be used to infer an oxygenated atmosphere at that time. Their paper appears in Earth and Planetary Science Letters.