The Roots and Early Branches of the Tree of Life
Researchers have traced the tree of life back to a single ancestral form
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 overlap, but it was previously unclear which of the six came first and how their development interweaved with environmental and biological changes.
Braakman and Smith 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 carbonfixation, 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 allowed early life to compensate for a lack of control over its internal chemistry, and form a template for the earliest major branches in the tree of life. For example, the first major life-form split came with the appearance of oxygen, causing the ancestors of bluegreen algae and most bacteria to separate from Archaea, which are 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."
Once early cells had more refined enzymes and membranes, there was greater control over metabolic chemistry. Minimization of energy (ATP) was used to create biomass, changes in oxygen levels and alkalinity directed life's unfolding.
The environment drove major divergences in predictable ways, in contrast to the common belief that chance dominated evolutionary innovation.
"Mapping cell function onto genetic history gives us a clear picture of the physiology that led to the major foundational divergences of evolution," explains Braakman. "This highlights the central role of basic chemistry and physics in driving early evolution."
With the ancestral form uncovered, and evolutionary drivers pinned to branching points in the tree, the researchers now want to make the study more mathematically formal and further analyze the early evolution of metabolism.
This work was supported in part by the NSF FIBR grant nr. 0526747 - The Emergence of Life: From Geochemistry to the Genetic Code. ES is further supported by Insight Venture Partners. RB is further supported by an Omidyar Fellowship. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
CITATION: Braakman R, Smith E (2012) The Emergence and Early Evolution of Biological Carbon-Fixation. PLoS Comput Biol 8(4): e1002455. doi:10.1371/journal.pcbi.1002455
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Original article: http://www.eurekalert.org/pub_releases/2012-04/plos-ftr041612.php