New fossil evidence suggests a bacterial community has remained unchanged for two billion years.
In a finding published by an international team of scientists led by Prof. J. William Schopf at UCLA, researchers paradoxically argue that the apparent lack of evolution in a deep-sea microbial assemblage provides excellent evidence in support of the theory of evolution.
The paper, which appeared last month in the Proceedings of the National Academy of Sciences, presents evidence suggesting that a group of sulfur-consuming microorganisms found in deep ocean sediments is in 2 billion years of evolutionary stasis. The team discovered a 1.8-billion-year-old (1.8 Ga) fossilized bacterial community that is morphologically identical to a living community discovered in 2007 off the coast of Chile, as well as to another fossilized community discovered by the same group that is nearly 2.3 Ga.
These dates are no coincidence, the scientists argue, because they coincide with a complex evolutionary model called the Great Oxidation Event. This period in history, which occurred 2.2-2.4 Ga, caused a substantial rise in available oxygen atoms in the atmosphere, which began to form the precious oxygen-containing molecules critical to most life on our planet. During this oxidation event, bacteria known as sulfur-cycling organisms that primarily consume sulfate show up on the scene. Surprisingly, in the hundreds of millions of years since that origin, sulfur-cycling bacterial communities have changed very little.
Modern sulfur-cycling bacterial communities are found in deep-sea mudflats, where they metabolize and cycle sulfates from the seawater for energy. By studying the habitat, web-like fabric, and organismal morphology and composition of these living populations and comparing them to their fossilized specimens, Schopf and his colleagues concluded that sulfur-cycling bacterial assemblages have remained largely unchanged at least since the time that the fossils were formed.
So what does this mean? The theory of evolution states that organisms respond and adapt to changes in their environment over time. Since the deep-sea habitats in which these sulfur-cycling bacteria live has remained relatively unchanged over long timescales (nearly half of Earth’s existence!), the hypothesis of the current theory states that the organisms living in this habitat should not change much either. The findings of this study provide empirical evidence to support this hypothesis.
Although their study supports this aspect of evolutionary theory, the researchers explain that more evidence is needed to establish this claim as a fundamental part of evolutionary theory. Schopf has pioneered and is currently developing several microscopy and imaging techniques for examining microbial fossils. As these technologies progress, evidence supporting the theory of bacterial evolutionary stasis over long timescales should become increasingly available.