At two miles below ground, the sun last touched the buried rock when carbon dioxide filled the sky, before the days of Earth's oxygen. Drops of water formed time capsules for early microbial life to survive the deep sub-surface, their methods and madness hidden from Earth's surface for millions of years. Despite accounting for about 10 percent of the planet’s total biomass, we know little of these organisms, which scientists have called “microbial dark matter."

Until recently, our understanding of microbes was limited to those that could grow in a lab. Advancements in genome sequencing and culture techniques have now brought light to the darkness, and from the shadows, microbial secrets emerged. Some survived on the buried remnants of photosynthesis, while others house tools unlike any life on the surface.

One species was Candidatus Desulforudis audaxviator, or CDA, a sulfur-breathing microbe that has spent the last several hundred million years in total isolation, its only companion the radioactivity spilling from its rocky confines. Now, researchers from the Bigelow Laboratory for Ocean Sciences have found that 165 million years ago, CDA abandoned the very engine of life on Earth: evolution.

Scientists originally discovered CDA in a South African gold mine, and later in both North America and Eurasia. This geographic separation let researchers study how CDA evolved after millions of years. The team used DNA sequencing tools to read the genomes from individual cells. Strikingly, the CDA genomes from all three continents were nearly identical.

While cross-contamination was obvious initial explanation, the team found no evidence of CDA spreading by air, land, or sea. Nor did the microbes stall as spores. All were actively respiring and replicating. After ruling out all of these possible reasons for their results, the researchers concluded that as the supercontinent Pangea split, between 55-165 million years ago, these microbes hit pause on evolution.

CDA is a living fossil, subverting evolutionary change yet surviving millions of years of changes to our planet, including a mass extinction. How CDA managed an evolutionary standstill — perhaps through a meticulous replication process – may have immense application in biotechnology. It may also upend our understanding of microbial evolution. What other secrets to survival might microbial dark matter be hiding?