Ancient communities of bacteria found in ocean floor sediments off Western Australia that haven't changed in more than 2 billion years display the greatest lack of evolution ever seen, researchers say.

Paradoxically, the microscopic organisms living in "extreme evolutionary stasis" are seen as proof of Darwin's theory of evolution, they say.

"If evolution is a product of changes in the physical and biological environment, and there are no changes in the physical and biological environment, then there will be no evolution," says UCLA paleobiologist J. William Schopf.

It's the null hypothesis, the "exception that proves the rule," that Darwin's equation requires, he suggests.

For the study, Schopf and colleagues compared fossilized communities of sulfur bacteria off the coast of Western Australia, one 2.3 billion years old and another 1.8 billion years old, with a recently discovered living bacterial community off the coast of Chile in South America.

Despite the extreme difference in age between the fossils and the living community, they all appear exactly the same, Schopf says.

They are all identical in form, in function and in metabolism, he says.

"It seems astounding that [this] life has not evolved for more than 2 billion years -- nearly half the history of the Earth," Schopf says. "Given that evolution is a fact, this lack of evolution needs to be explained."

The explanation, he says, can be found in the similar and extremely stable environments the bacteria have lived in; the deep-sea bacteria in this study haven't changed for eons because neither have their surroundings.

Since the central tenet in Darwin's theory is that species evolve in response to the survival challenges brought on by a changing environment, the microbes' lack of evolution, linked to a lack of change in their environment, is consistent with the theory, Schopf says.

"Surface environments change all the time and when they change, the biology changes," he says. "But the muds underneath the ocean don't receive any signals from the above environment."

Where the microbes lived -- and still live today -- is one of the most stable environments to be found on Earth, the researchers point out.

The asexual reproduction of the microbes means genetic changes are minimal, there simple ecosystem provides nitrates and sulfur to meet all their energy needs, and with no competition and no real pressure to change, there's been no need to evolve, Schopf says.

"The rule of life is don't fix it if it isn't broken."