In the muddy sediments beneath the deep sea, scientists have found ancient communities of bacteria that have remained virtually unchanged for 2.3 billion years.
Researchers say these microscopic organisms are an example of "extreme evolutionary stasis" and represent the greatest lack of evolution ever seen.
They may also, paradoxically, prove that Darwin's theory of evolution is true.
"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," said J. William Schopf, a paleobiologist at UCLA.
He calls it the null hypothesis required of Darwin's equation.
In a paper published this week in PNAS, Schopf and his colleagues describe three distinct communities of the deep sea microbes separated from each other in time by hundreds of millions of years.
The first is a fossilized community found in 2.3-billion-year-old rock in Western Australia. The second fossilized community was discovered in 1.8-billion-year-old rock, also from Western Australia. The third is a living community discovered in the last decade in sediments off the west coast of South America.
The researchers say that despite their vast age differences, the three communities look exactly the same, each exhibiting a telltale irregular weblike fabric, and a two-tier structure.
"In form, function and metabolism, they are identical," Schopf said.
It may seem unlikely that any organism can remain the same for 2.3 billion years, but Schopf said that for these deep sea microbes, the lack of evolution makes sense.
"Surface environments change all the time and when they change, the biology changes," he said. "But the muds underneath the ocean don't receive any signals from the above environment."
The microbes described in the study live 4 to 12 inches beneath the deep sea sediments, in one of the most stable environments on Earth. Their world is cold and dark -- an endless night that feels none of the effects of either ice ages or warming spells.
"There is no turning of sediments, things don't get stirred up, there is no oxygen at all -- they get no time signal, there is no change," said Schopf.
The microbes reproduce asexually, which keeps genetic changes to a minimum, and their simple ecosystem requires only nitrate and sulfur for energy.
"They are well adapted for their environment, and there isn't any competition," Schopf said.
So with no pressure to change, Schopf proposes that these organisms didn't.
"The rule of life is don't fix it if it isn't broken," he said.
Schopf said it is likely that these ancient organisms exist at the bottom of oceans throughout the world, but finding them is difficult and expensive, since it involves drilling into sediments at the bottom of the ocean.
He also said there may be other similarly static communities on our planet. The next place he'd like to look are microbes that live deep in the pore spaces of rocks half a mile beneath the surface of the Earth.
"I suspect that is an environment that hasn't changed much over the history of the Earth," he said.
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