Scientists from UCLA have discovered a deep-sea microorganism that's the same today as it was two billion years ago. It's an observation that actually bolsters Darwin's theory of natural selection, while offering the most extreme example of evolution's "null hypothesis."
Above: A 1.8 billion-year-old fossil-bearing rock. The fossils are virtually the same as fossils 500 million years older and to modern microorganisms. (UCLA Center for the Study of Evolution and the Origin of Life)
A blurb from the study's abstract, which appears in the latest edition of PNAS, puts it nicely:
An ancient deep-sea mud-inhabiting [1.8 billion-year-old] sulfur-cycling microbial community from Western Australia is essentially identical both to a fossil community 500 million years older and to modern microbial biotas discovered off the coast of South America in 2007.
Which is incredible when you think about it. This bacteria hasn't evolved for nearly half the history of Earth.
The microorganisms, which were preserved in rocks from Western Australia's coastal waters, are virtually indistinguishable from modern sulfur bacteria found in mud off the coast of Chile. The discovery lends credence to the "null hypothesis" of evolution, which states that, "if there is no change in the physical-biological environment of a well-adapted organism, its biotic components should similarly remain unchanged."
In other words, if an animal's environmental circumstances don't change, and the animal is already well-adapted to that environment, there's no need to fix what ain't broke. And indeed, the muddy, deep-sea environment in which these microorganisms live hasn't changed for three billion years.
"The rule of biology is not to evolve unless the physical or biological environment changes, which is consistent with Darwin," noted J. William Schopf in a UCLA release. "These microorganisms are well-adapted to their simple, very stable physical and biological environment. If they were in an environment that did not change but they nevertheless evolved, that would have shown that our understanding of Darwinian evolution was seriously flawed."
This discovery also shows how evolutionary drift, in such strict circumstances, doesn't work to produce adaptations. Like any community over time, these microorganisms are likely experiencing superfluous (or "neutral") mutations. But in this environment, mutations and genetic drift don't go very far, and t the organisms will revert back to their "optimal" mode.
The fossils date back to the Great Oxidation Event — about 2.2 to 2.4 billion years ago — when there was a substantial rise in our planet's oxygen. The process also generated a dramatic increase in sulfate and nitrate, which are the only nutrients the microorganisms would have needed to survive and which enabled the bacteria to thrive and multiply.
Read the entire study at PNAS: "Sulfur-cycling fossil bacteria from the 1.8-Ga Duck Creek Formation provide promising evidence of evolution's null hypothesis".
Images: UCLA Center for the Study of Evolution and the Origin of Lif