What do you think is the slowest and longest-living organism on Earth? Many might picture tortoises or ancient trees, but hidden below the surface of the Earth are tiny life forms that could outlive them all.
Let’s dive into the world of bacteria. You might think of them as quick to grow and die—like the pesky germs that can turn a mild throat tickle into a full-blown illness in a day. Bacteria like E. coli can double in under 20 minutes. However, not all bacteria act at this fast pace. Some, living deep underground, thrive in slow conditions, potentially remaining active for millions of years. These organisms, which I like to call “aeonophiles,” challenge our understanding of life.
Our grasp of deep microbial life began altering in the 1980s with advances in DNA technology. This allowed scientists to identify microbes directly in their environments without needing to cultivate them in labs. In 1998, researchers from UC Berkeley made groundbreaking discoveries in Yellowstone’s hot springs that revealed entirely new branches on the tree of life. The quest for knowledge about subterranean life exploded from there, revealing a vibrant ecosystem beneath our feet.
Interestingly, there are as many microbial cells residing in the Earth’s crust as there are on the surface. Scientists, including myself, have extracted samples from as deep as 5 km below the surface. These “intraterrestrials” live in minute spaces within rocks, finding homes in areas that to us seem solid and impenetrable.
One key observation is that many of these microbes exist under extreme food scarcity. Unlike surface organisms that find food easily, these bacteria may wait thousands of years between food sources. This slow pace of life has profound implications for how we define life itself.
Theoretically, an aeonophile can live indefinitely if it continuously repairs its body. This brings us to a fascinating idea: could these microorganisms experience something akin to evolutionary changes, even without typical reproduction? They may not be producing offspring in the environments we’re accustomed to observing, but it’s quite likely they’ve adapted to long cycles of dormancy and await significant environmental shifts before flourishing.
Take seasonal dormancy, for example. Some creatures emerge after a long winter, reproducing in spring. Aeonophiles, however, might be “waiting” for geological events that take place over millennia—like volcanic eruptions or tectonic shifts.
Addressing the question of how these organisms survive in extreme dormancy invites us to reflect on their timelines and influences in ways that differ from our understanding of life. Their existence opens up new ways to think about the purpose of life and how it interacts with energy and entropy.
From discovering aeonophiles, we also learn about the potential for life on other planets. If extraterrestrial organisms experience life at extraordinarily slow rates, we might struggle to detect them with our current methods. They might exist in the same state as the microbes below our feet—fully alive but unchanging on a visible timescale.
Ultimately, the study of these organisms teaches us that life is far more diverse and adaptable than we previously believed. The aeonophiles prompt us to reconsider our definitions of what constitutes life and how it persists in ways we are only beginning to comprehend.
