Evolution is a scientific fact, as established as the existence of gravity. So imagine the surprise after researchers recently discovered organisms which haven't evolved in more than 2 billion years — roughly half the age of the Earth itself. 

But the discovery does not undermine Darwin's theory of natural selection, say scientists. Rather, it affirms it, according to ABC News.

“If evolution is as we understand it, organisms adapt to a changing environment,” explained J. William Schopf, a UCLA professor and lead author of the study. “But if you take away any changes then there should be no evolution.”

The organisms, sulfur bacteria which feast on sulfate and nitrate compounds that sink to the bottom of the ocean, live so far beneath the muddy ocean floor that they remain almost entirely insulated from the rest of the world. Since their environment has remained virtually unchanged and undisturbed for billions of years, they haven't changed either. According to Darwin's theory of natural selection, an organism only changes in response to a changing environment.

The discovery is the greatest absence of evolution ever reported. It represents a sort of backwards, however enlightening, way of proving Darwin's grand idea.

“These microorganisms are well-adapted to their simple, very stable physical and biological environment,” Schopf said. “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.”

For the study, modern sulfur bacteria were compared to fossils discovered in rocks from Western Australia that dated from 1.8 to 2.3 billion years ago. Researchers found nothing to distinguish the modern samples from the ancient ones, at least in appearance. Because genetic material is not preserved in fossils, it is impossible to say whether the organisms were also genetically identical to their ancient counterparts, although this would be unlikely.

Darwin's theory of natural selection came before the advent of modern genetics, so it only pertains to the selective pressures which influence an organism's form or appearance. But genetic changes can happen even if the form of an organism remains unaltered. Mutations are certain to have occurred over the billions of years, for instance, even if the environment remained the same.

So technically it's a bit of hyperbole to say these sulfur bacteria have not been evolving at all. Of course they have been evolving, at least on the genetic level, as have all organisms. But because their environment has stayed so consistent, the same selective pressures exist now as they did billions of years ago. Thus, natural selection has worked to keep the basic form of these organisms intact in spite of inevitable genetic change. That's what is so remarkable about these bacteria, and it's why their existence provides such remarkable proof for Darwin's theory of natural selection.

Related on MNN:

Bryan Nelson ( @@brynelson ) writes about everything from environmental problems here on Earth to big questions in space.

Organisms discovered that haven't evolved in more than 2 billion years
The biological oddity does not raise questions about Darwin's theory of natural selection — rather, it affirms the theory, say scientists.