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At a time when microscopic phenomena are the cause of fear and loss, it is surprising and enlightening to read about microbial discoveries that could help answer some of the eternal questions about life, the universe, and everything.

The Last Place on Earth We’d Ever Expect to Find Life

Microbes are turning up deep beneath the ocean floor, a sign that life might have fewer limits than scientists once thought.

Microbial life, almost unbelievably resilient, abides in boiling hot springs and bone-dry deserts, in pools of acid and polar ice, kilometers up into the sky and kilometers below the ocean floor. And while scientists are eager to uncover microbes in even less familiar territories beyond our solar system, it’s the last Earth-bound frontier on that list—the deep subsurface—where they’re now making exciting progress in their efforts to probe life’s extreme adaptability.

Lightless, barren of essential nutrients, and crushed under inconceivable pressures, the deep subsurface seems unrelentingly inhospitable, yet it is shaping up to be one of Earth’s biggest habitats. Moreover, its strangeness is forcing scientists to reckon with biological systems that operate on completely different energy sources and time scales from those that we surface dwellers are accustomed to.

Scientists have spent decades studying how and where microbes persist and even thrive beneath the oceans, far removed from the sun. Most of that work has focused on marine sediments, the tightly packed mud and detritus that in places extends for kilometers beneath the water. But there’s also the volcanic rock below that, the crust itself. The life in those rocks is much more difficult to access and analyze, and samples are scarce.

“We do not have a map of the deep subsurface microbial landscape right now,” Karen Lloyd, a microbiologist at the University of Tennessee, Knoxville, says. As a result, she notes, researchers studying its diverse range of environments can’t even make simple generalizations equivalent to “forests have trees and fish swim in the oceans.”

But a handful of new discoveries has finally opened a window on that landscape and the microbes that inhabit it. It has also offered a glimpse into the very origin and evolution of life, both on this planet and possibly elsewhere in the universe.

Earth’s crust has largely been the purview of geologists. The first indication that it might also be of interest to microbiologists came in 1926, when researchers reported the presence of bacteria in the deep oil wells of Illinois. Such findings, however, wouldn’t be taken seriously for decades: Contamination in the samples seemed far more likely than the possibility that anything could live so disconnected from the sun-driven photosynthesis that supports life everywhere else on the planet. (It didn’t help that in the 1950s, a pair of microbiologists performed a series of experiments that led them to pin the bottom of the biosphere to sediments just a few meters below the seafloor.)

In 1977, that all began to change. Scientists boarded a submarine to explore a spreading ocean ridge between two tectonic plates near the Galápagos Islands, and discovered the first hydrothermal vents. There, the Earth belched out billows of black smoke, as scalding-hot, mineral-rich fluids spewed from cracks in the rock and mixed with the extremely cold seawater. Around those vents, an entire ecosystem flourished, including tube worms, giant clams, and eyeless shrimp—and, it turned out, lots and lots of microbes that sustained them. “There, in these dark, deep parts of the ocean where they didn’t expect to find life, certainly they did,” Barbara Sherwood Lollar, a geologist at the University of Toronto, says. And it was “literally life as we didn’t know it.”

For the first time, scientists realized there could be ecosystems on Earth that did not depend on the sun. The microbes were powered not by solar energy, but by minerals and chemicals released at the vents. They challenged what life could be, and where its limits lay.

In the 1980s and ’90s, scientists began to uncover further evidence of an unlit but populated kingdom: Rocks beneath both the continents and oceans exhibited weathering that seemed unlikely to be a result of abiotic reactions alone. Drilling projects on land and at sea revealed living cells and DNA sequences in all sorts of environments, leading researchers to speculate that subsurface microbes constituted a hidden, pervasive majority, greatly outnumbering the microbial cells found in the world above. Current estimates, in fact, put the number of subsurface microbes on the order of 10³⁰ cells, an order of magnitude higher than the number of microbes thought to dwell in soil or the open ocean. “The deep biosphere,” Virginia Edgcomb, a marine microbiologist at the Woods Hole Oceanographic Institution in Massachusetts, says, “is extensive. What captivated the attention of microbiologists was that [potential] extent.”

And everywhere they’ve looked in that extensive realm since—no matter how deep or seemingly nutrient-poor—they’ve found life…

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