Beneath the Earth's surface, a hidden world of microscopic life awaits discovery. These 'intraterrestrials' have survived in some of the planet's harshest conditions for millions of years, and scientists are eager to uncover their secrets. In the book 'Intraterrestrials: Discovering the Strangest Life on Earth' (Princeton University Press, 2025), author Karen G. Lloyd explores the idea of evolution among these long-dormant life forms and what they might be waiting for to 'wake up'.
The Long Sleep of Intraterrestrials
But how can life evolve to stop growing for thousands of years? Recent research suggests that microbes buried deep in oceanic seafloor sediments may be doing just that. These organisms, known as intraterrestrials, are small microorganisms living inside Earth's crust worldwide. To understand this evolutionary conundrum, we must consider what these organisms experience in their lifetimes. Since they are buried so deep, they cannot detect the sun or notice seasonal changes. However, they might be attuned to longer geological rhythms, such as the opening and closing of oceanic basins through plate tectonics, the formation and subsidence of island chains, or new fluid flows caused by slow cracks in Earth's crust.
Evolution in the Long Term
For instance, Darwin's finches evolved new beak shapes due to isolation on an island with specific seeds to eat, a process that occurred over geological timescales. However, individuals within a species can also adapt to their environment. An Arctic fox's fur changes from white to brown with the melting snow each spring. Many people wake up at the same time daily, regardless of an alarm. Daily and yearly rhythms are intuitive for humans and animals to track. But what about longer timescales, like ice ages? It's hard to imagine an individual finch evolving the ability to swim because it anticipated its island's subsidence into the sea in 100,000 years.
Dormancy and Adaptation
The fact that living cells can exist in a nongrowth state for long periods raises two questions. Can microbes be adapted to avoid cell division for thousands of years, and if so, how does evolution work for an organism that seemingly never produces offspring? Short-term seasonal dormancy provides a model for this. Dormancy during winter offers an evolutionary advantage, as dormant organisms have larger populations when conditions are favorable again in spring. These organisms can pass their dormancy genes to a larger population of progeny in the spring and summer, ensuring their survival.
Waiting for the Next Big Thing
What if intraterrestrials are waiting for something that only happens thousands of years later? They might be adapted to geological processes like island subsidence or volcanic eruptions, which occur on long cycles. For instance, individual microbes might be adapted to events with even longer periods, like glacial cycles or tectonic plate movements. As new seafloor forms at mid-ocean ridges, existing seafloor is pushed away, eventually jamming into a continent in the slowest-motion train wreck. Some sediments and intraterrestrials may be dragged down to the mantle, only to be returned through cracks and fissures in the overriding plate.
The Evolutionary Advantage of Long-Term Dormancy
There is evidence that long-term dormancy has a selective advantage. When starved, some bacteria like Escherichia coli enter a state of long-term dormancy, where they are alive and metabolizing but not growing. These 'old geezers' can outcompete fresh, fast-growing batches when both are starved. This growth advantage in stationary phase (GASP) may explain why intraterrestrials live so long. They might be waiting for something that only happens thousands of years later, ensuring they are the ones to take advantage of the new situation.
The Intraterrestrial's Wake-Up Call
So, what are these microbial monks waiting for? Seasonal cycles are too fast, but geological processes like volcanic eruptions or tectonic movements occur on long cycles. These events could coax intraterrestrials out of dormancy after hundreds of thousands of years. The evolutionary payoff for waiting in deep marine sediments would be to return to the upper seafloor with more nutritious food, where they can pass their genes to future generations. Is getting tossed back up into surface sediments an intraterrestrial's version of summer?
