That is, it’s possible that life might have gotten its start on the surface of the Earth, where it found creative ways to survive and spread, including to deeper environments. But it’s also possible that life began underground, at some fortuitous juncture of rock and water—eventually also making its way to the surface and figuring out how to harness the sun’s energy. (On that note, photosynthesis-dependent surface life and radiolysis-dependent subsurface life have so far been found to have an ancient, shared ancestry—but some researchers are intrigued by the possibility that life could have evolved more than once on Earth, in a “second genesis.”)
That, in turn, has significant implications for the search for life on Mars, Saturn’s moon Enceladus, and exoplanets beyond our solar system. Given the prevalence of water and volcanic rock throughout the universe, “life could have started anywhere,” Fisk says.
Read: Microbes could thrive on Saturn’s icy moon
D’Hondt agrees. “From a literally universal perspective, it opens up the potential for sustaining life on all kinds of planets,” he says. “There could be life on other worlds that’s independent of photosynthesis,” thriving beneath the surface, out of sight.
Even more intriguing, perhaps life can evolve and adapt down there too—perhaps very differently than how it has on the surface. Given how slowly the cells beneath the ocean floor grow, could they also be adapting on much more gradual time scales?
A couple of preliminary studies, conducted in younger sediments, have not found evidence for such evolution—but those studies dealt with only one type of deep subsurface environment, and even then, they went back just 10,000 years or so. That might not be enough time for a slower kind of natural selection to have played a role. “What if you go back a million years or 10 million years?” Orsi asks.
Older sediments—as well as other sub-seafloor environments, such as the basalts examined by Suzuki and D’Hondt—might hold fresh insights. “When you expand the possibilities to greater and greater time scales” and contexts, Lloyd says, “then you have a wider array of possible drivers for evolution.”
Even now, scientists are continuing to push further, deeper. A paper published in Nature in March, spearheaded by Edgcomb, detailed the results from one of the most ambitious such pushes to date. They drilled nearly 800 meters below the seafloor at a location where the lower ocean crust bulges up closer to the surface. There, they sampled gabbro rocks, which are formed when magma cools more slowly: They’re typically found beneath basalt and considered a sort of window into the mantle, as well as into what an earlier Earth’s rock environment might have looked like. And in those rocks, the scientists reported trace numbers of cells—which once again seemed to be surviving on nutrients they gleaned from the flow of seawater. Although other researchers, including Orsi, have raised concerns about the possibility that those samples were contaminated, they also expect future analyses to uncover life down there.
No matter what, Edgcomb says, echoing the famous line from Jurassic Park, “life finds a way.”