Beneath Earth’s surface exists a hidden kingdom of microscopic life, organisms that survive in some of the planet’s most extreme conditions. These “intraterrestrials,” as they’ve been dubbed, are the focus of a growing field of research attempting to understand how life can not only exist, but potentially thrive, in environments devoid of sunlight and reliant on geological processes for sustenance.
Recent work, detailed in Karen G. Lloyd’s Intraterrestrials: Discovering the Strangest Life on Earth, explores the evolutionary implications of organisms capable of remaining dormant for hundreds of thousands, or even millions, of years. The central question isn’t simply how these microbes survive, but what they might be “waiting for” to re-emerge into an active state.
The conventional understanding of life is predicated on relatively rapid growth, and reproduction. Evolution, as traditionally understood, operates on populations of organisms with varying traits, where beneficial mutations are passed on through successive generations. But what happens when generations span millennia? How does natural selection function when an individual organism essentially pauses its life cycle for an incomprehensibly long period?
Researchers are discovering that the deep subsurface—the cracks and sediments within Earth’s crust—may harbor over half of all microbial cells on the planet. This challenges the assumption that biodiversity decreases with distance from the sun and surface energy sources. As Emil Ruff, a microbial ecologist at the Woods Hole Marine Biological Laboratory, explains, “It’s commonly assumed that the deeper you go below the Earth’s surface, the less energy is available, and the lower is the number of cells that can survive.” However, studies are revealing that subsurface environments can exhibit diversity equal to, or even exceeding, that found on the surface.
This raises a fundamental question: are these microbes merely persisting in a state of suspended animation, or are they actively adapted to this long-term dormancy? Lloyd argues that simply surviving for such extended periods likely requires specific adaptations. It’s not a passive consequence of a slow metabolism, but an evolved strategy. Evidence suggests that subsurface microbes possess enzymes specifically tailored to the substrates available in their environment, indicating a degree of specialization.
The challenge lies in reconciling this long-term dormancy with Darwinian evolution. Natural selection requires reproduction and the transmission of genetic mutations. How can an organism evolve without producing offspring? Lloyd draws a parallel to seasonal dormancy in surface organisms. Just as plants and animals enter a dormant state to survive unfavorable conditions, subsurface microbes may be waiting for geological events that create new opportunities for growth.
But the timescales involved are vastly different. While seasonal dormancy lasts months, intraterrestrial dormancy can span hundreds of thousands of years. What, then, constitutes an evolutionary cue for these organisms? The answer, researchers believe, lies in geological rhythms – the opening and closing of oceanic basins, the formation of island chains, or shifts in fluid flows within the Earth’s crust. These events, while slow from a human perspective, could represent significant opportunities for microbial growth and reproduction.
Consider the process of plate tectonics. As seafloor spreads at mid-ocean ridges, older seafloor is subducted—pulled beneath continental plates. Microbes embedded within these sediments could be carried deep into the Earth, only to be resurfaced through volcanic activity or faulting. Lloyd proposes that this cycle of subduction and resurfacing could be the very event these organisms are “waiting for”—a return to more favorable conditions where they can resume growth and reproduction.
This perspective suggests that intraterrestrials aren’t simply surviving in a harsh environment; they’re actively participating in a geological cycle, evolving to exploit the rare opportunities presented by Earth’s dynamic processes. The concept of “growth advantage in stationary phase” (GASP), observed in laboratory studies of Escherichia coli, supports this idea. Dormant cells can outcompete actively growing cells when resources are scarce, suggesting that long-term dormancy can be a selective advantage.
The study of these deep subsurface microbes isn’t just about understanding life on Earth. It also has implications for the search for life elsewhere in the solar system. If life can thrive in the extreme conditions found deep within our planet, it raises the possibility that similar life forms could exist beneath the surfaces of Mars or other icy moons. As Tullis C. Onstott details in his book, Deep Life: The Hunt for the Hidden Biology of Earth, Mars, and Beyond, the quest for subsurface life is a crucial step in understanding the potential for life beyond Earth.
The discovery of these resilient microbes forces a re-evaluation of what constitutes an evolutionary cue and challenges our assumptions about the limits of life. These aren’t organisms living on a human timescale; they’re operating on geological timescales, waiting for events that unfold over millennia. Understanding their adaptations and strategies for survival could unlock new insights into the fundamental principles of life itself.
