A section of a 1.8 billion-year-old fossil-bearing rock. (UCLA Center for the Study of Evolution and the Origin of Life)

Far beneath the ocean’s surface, buried in a layer of murky sludge, communities of ancient bacteria have existed virtually unchanged for nearly half of Earth’s history.

In what researchers call the “greatest absence of evolution ever reported,” these deep-sea creatures have taken a pass on the chaos of biological progress for the past 2.3 billion years. Leave it to other organisms to battle with the forces of natural selection — to mutate and change, to prove their fitness in order to survive. The dinosaurs tried it, and where are they now?

study published this week in the Proceedings of the National Academy of Sciences said these communities offer evidence of “extreme evolutionary stasis” — a total lack of evolution in response to a lack of change in the surrounding environment. Populated by sulfur-cycling bacteria that derive energy from processing dissolved sulfate in the surrounding water, the communities were found in nearly identical forms at two distinct points in the fossil record and still exist today. 

A section of a 2.3 billion-year-old fossil-bearing rock. The fossils (the dark areas) are essentially identical to ones 500 million years younger — and to modern microorganisms that live in seawater mud. (J. William Schopf/UCLA Center for the Study of Evolution and the Origin of Life)

The web-like clusters of filament-shaped microbes first appeared in a 2.3 billion-year-old chunk of fossilized mud from Western Australia. The same types of communities were also spotted in a second, 1.8 billion-year-old Australian rock — and in modern deep-sea environments off the coasts of Chile and Southern Africa.

“The microbes we see in the fossils are almost identical to what we see in the ocean now,” study co-author Malcolm Walter, a professor of astrobiology at the University of New South Wales, told The Washington Post in a telephone interview. “They have similar shapes and are doing similar chemistry.”

But the fact these particular organisms successfully avoided evolving for billions of years doesn’t disprove the theory of evolution — quite the opposite.

Darwin’s theory states that species evolve through natural selection in response to environmental changes — increased threats from predators, new competition from other animals, changes in access to water or air. But the inverse is also true: If there is no change in the environment of a balanced ecosystem, the organisms that constitute it should remain similarly unchanged — a principle dubbed evolution’s “null hypothesis.”

“These microorganisms are well-adapted to their simple, very stable physical and biological environment,” the study’s lead author, University of California at Los Angeles professor William Schopf, said in a university press release. “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.”

Deep-sea microorganisms are unchanged over more than 2 billion years, a new analysis of ancient fossil microorganisms revealed. (J. William Schopf/UCLA Center for the Study of Evolution and the Origin of Life)

Ensconced in their ocean-floor habitat, these bacteria were isolated from the traumas occurring elsewhere around the planet (earthquakes, meteor impacts, Nickelback). They were apparently aware of the null hypothesis and were not going to change their DNA unless they had to, thank you very much.

A couple of billion years later, it would appear they never had to.

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