Origin & Evolution of Life

Most Prokaryotes Trace to an Ancient Origin on Land

By Keith Cowing
February 18, 2009

Bacteria are not usually thought of as having a natural habitat like a mammal or insect, but indirect evidence has suggested that, if anything, most of the early evolution of bacteria was in the marine environment (oceans) and not on land. Surprisingly, NAI researchers from Penn State, Fabia Battistuzzi (now at Arizona State University) and Blair Hedges, found evidence that a large group of bacteria–two-thirds of all ~10,000 described species–trace their ancestry back to a life on land, not in the oceans. These bacteria have many useful adaptations, including the production of oxygen, which now may be tied to their land-loving lifestyle. Their article appeared in the February issue of Molecular Biology and Evolution.

The study involved evolutionary analyses of sequence data from hundreds of complete genomes. Members of the terrestrial group, which they call Terrabacteria, include the Gram positive phyla (Actinobacteria and Firmicutes) and two phyla with cell walls that differ structurally from typical Gram positive and Gram negative phyla: Chloroflexi and Deinococcus-Thermus. The large group of >6,000 species also includes the oxygen-producers, Cyanobacteria. Many members of Terrabacteria produce spores and have other adaptations for resistance to environmental stress. Earlier studies, including some by the same authors, found a similar phylogenetic pattern but community acceptance of any particular tree of prokaryotes has been slow, partly because ribosomal RNA trees have always differed from protein and genome trees. But the Penn State team revealed biases in the ribosomal RNA data, that when accounted for, produce a tree more similar to the genome tree and lending support for Terrabacteria. Their molecular clock estimates place the colonization of land deep in the Precambrian, about three billion years ago.

[Source: NAI Newsletter]

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