Oh What a Big If

A team of scientists led by Armen Y. Mulkidjanian at the University of Osnabrück in Germany have invoked Charles Darwin’s “warm little pond” scenario to challenge one of the most prevalent views held in evolutionary biology— that life originated in the sea, not on land.  Using geochemical and phylogenomic data for support, their primary argument stems from the idea that the chemical environment of the ocean, even at hydrothermal vents, is too dissimilar to that of the internal cellular environment to have sustained life in the early stages of cell evolution. The authors contend that the chemistry of protocells would have been similar to that of the environment in which they were created, because these cells had not evolved sufficiently to contain sophisticated membrane barriers and pumps to compensate for disequilibrium between their internal and external environments.  In searching for environments on Earth that might better represent the chemical composition of cells, they discovered that the chemistry of condensed geothermal vapor forming pools at inland geothermal fields were a fit, especially with respect to the high K+ to Na+ ratio found in cells (and opposite to that present in seawater).

"Warm little ponds" at Yellowstone National Park.

Mulkidjanian et al. also present an alternative hypothesis— that the differences between the chemistry of the cell and of the marine environment are not related to the conditions in place which early cells were evolving, and that the cell’s requirements for certain ions evolved later, after transmembrane pumps allowed cells to exist in disequilibrium.  The authors reject this hypothesis however, on the inference that the most ancient proteins common to life require specific ions (e.g. potassium, zinc, phosphate, etc.) in concentrations that are predicted at the described geothermal pools.  Particularly interesting from a phylogenomic perspective, data analysis showed that the origin of GTPases (involved in protein synthesis) preceded the last universal common ancestor (LUCA).  These have highly conserved K+ binding sites, suggesting that K+ was required early in the evolution of life.  Additionally, the authors note that phosphates (which form the backbones of nucleic acids) are “four orders of magnitude greater than in seawater” in the cytoplasm of the cell.

Approximate total concentrations of key ions in modern sea water, the primordial ocean, and cell cytoplasm.

Several assumptions must be made for the arguments of Mulkidjanian et al. to hold up. The evidence presented is only acceptable if one presumes cells have not undergone dramatic changes to their chemical composition in the last 4 billion years of life (a notion that isn’t too hard to swallow given that the cells of all extant organisms use the same basic chemistry to function), if one believes the chemistry of modern cells should reflect the chemistry of the primordial environment where cells first evolved (what Mulkidjanian et al. cite as “the chemistry conservation principle”) and if the chemistry of the ancient sea (especially at hydrothermal vents) had not significantly changed since the early evolution of life, hence reflecting the low K+/Na+ ratios found in modern marine environments.  The authors point to a reference supporting the similarity of Archaean seawater to modern oceans, but do not elaborate on this conjecture.

With such a complicated and long debated topic as the origin of life, it goes without saying that a study such as the present one will generate quite a bit of controversy.  My initial evaluation upon reading the paper included being impressed and intrigued by the authors’ novel ideas and thorough reasoning, but I was also eager to read the refutations that were sure to have followed its publication.  Indeed, articles in Nature, Scientific American, and presumably in other reputable science and news publications offered commentary from some prominent scientists who study the origins of life.  Challenges included the unlikelihood that biological molecules could have remained in such unstable and transient environments as volcanic pools for a long enough period to evolve (Switek, 2012), that direct evidence of the described environments would be nearly impossible to ascertain from the fossil record, and that the acidity of these geothermal pools would have been too great to allow life (though this point is addressed by Mulkidjanian et al., who maintain that pH would have been closer to neutral in the absence of oxygen on the primordial Earth).  Though, some, like molecular biologist Jack Szostak of Harvard Medical School, agreed that protocells undeniably had leaky membranes, rendering the early ocean a hostile environment for cells due to its high levels of sodium (Biello, 2012).  However, Szostak does not agree that the K+/Na+ ratio of internal cells necessarily reflects the early environment.  It may simply be that cells functioned better under such ion concentrations, and thus selection for high K+ followed early evolution (Switek, 2012).  Perhaps future research might aim to elucidate the mechanisms by which leaky protocells may have maintained these favorable ionic conditions internally, despite the incompatible chemistry of the external environment.

References:

Biello, D.  February 15, 2012.  Did life’s first cells evolve in geothermal pools?  Scientific American.  Retrieved from http://www.scientificamerican.com/article.cfm?id=did-life-first-evolve-in-geothermal-pools

Mulkidjanian, A. Y., Bychkov, A. Y., Dibrova, D. V., Galperin, M. Y. & Koonin, E. V. (2012) Origin of first cells at terrestrial, anoxic geothermal fields.  Proc. Natl Acad. Sci. USA advance online publication http://dx.doi.org/10.1073/pnas.1117774109.

Switek, Brian.  February 13, 2012.  Debate bubbles over origin of life.  Nature News.  Retrieved from http://www.nature.com/news/debate-bubbles-over-the-origin-of-life-1.10024

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