Origin and Ancestor: Separate Environments
G.Arrhenius*, J.Bada, G.Joyce and A. Lazcano

*Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0220, U.S.A.

ABSTRACT

Nicholas Galtier et al. (Reports. 8 Jan. p.220) argue on the basis of calculated values of the guanine plus cytosine (G+C) content of ribosomal RNAs that the last common ancestor of extant life on Earth was not a hyperthermophile. They correctly point out that this neither supports nor invalidates claims that life originated at high temperatures. They clearly state that the environment in which life began may not have been the same as the one in which the last common ancestor thrived.

This thrust of the report by Galtier et al. is accurately reflected in the accompanying item in This Week in Science (8 Jan., p.143). However, in Gretchen Vogel News of the Week article ORNA study suggests cool cradle of life (8 Jan., p.155) the two environments have been conflated.

It is important to recognize that the arguments for and against a thermophilie last common ancestor are almost irrelevant to discussions of the temperature at which life originated. The last common ancestor seems to have been an organism with a biochemistry about as complicated as that of a contemporary bacterium, and it must, therefore, have had a complex evolutionary history. During the course of their evolution, the predecessors of the last common ancestor may have adapted one or more times to changes in the temperature of their environment. Therefore, it is broadly accepted that the nature of the last common ancestor, whether or not it was a thermophile, may not provide evidence about other cradle of life.

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