Beneath the Earth's surface lies a hidden realm of microscopic life, a world of "intraterrestrials" that have been "sleeping" for millions of years. These organisms, buried deep within the oceanic seafloor sediments, survive in some of the planet's harshest conditions, and scientists are eager to uncover their secrets. In this article, we explore the fascinating concept of evolution among these ancient microbes, which can remain dormant for hundreds of thousands to millions of years, and the mysteries they hold. But here's where it gets intriguing: these intraterrestrials might be waiting for something that only happens thousands of years later, a phenomenon that challenges our understanding of evolution and survival. How can an individual evolve to stop growing for thousands of years? Recent research suggests that microbes buried deep in the oceanic seafloor sediments may be doing just that. These intraterrestrials, small microorganisms living inside Earth's crust, are adapted to survive in conditions that would be inhospitable to most life forms. To understand this evolutionary puzzle, we must consider the unique experiences of these organisms. Unlike animals, which are influenced by daily and yearly cycles, these microbes are buried so deep that they cannot detect the sun or seasonal changes. Instead, they respond to longer geological rhythms, such as the opening and closing of oceanic basins through plate tectonics, the formation and subsidence of new island chains, and the slow formation of cracks in the Earth's crust. These events, which occur over millions of years, are not evolutionary drivers for individual organisms but rather for species. For instance, Darwin's finches evolved new beak shapes due to the isolation on an island with a specific type of seed, a process that occurred over the geological timescale of island formation. However, individuals within a species can also adapt to their environment. An Arctic fox's fur changes from white to brown with the melting of snow each spring, and many people wake up at the same time daily. But what about anticipating changes over much longer timescales, like ice ages? It's challenging to imagine an individual finch evolving the ability to swim because it foresaw its island subsiding into the sea in 100,000 years. Similarly, a beetle in the Gobi Desert might not reproduce when it eats an Amazon rainforest seed because it was born millions of years ago when South America and Africa were closer together. These scenarios are illogical for animals, but they may be plausible for intraterrestrials. An individual living for a million years might be evolutionarily predisposed to rely on slow geological events like island subsidence, just as we are predisposed to wait for the sun to rise. To fully comprehend intraterrestrials, we may need to redefine what constitutes an evolutionary cue. Living for millions of years raises intriguing questions about adaptation and evolution. Can microbes be adapted to avoid cell division for thousands of years, and how does evolution work for organisms that seemingly never produce offspring? These microbes, buried deep in the oceanic seafloor sediments, may be evolutionarily adapted to survive in a dormant state for thousands or millions of years, rather than just persisting due to the lack of special adaptations. The fact that these microbes are rarely found elsewhere, such as in normal seawater, suggests that they are specially adapted to their environment. Their adaptations enable ultraslow metabolisms and cell divisions, indicating that they are evolutionarily poised for long-term nongrowth. However, this poses a problem according to Darwin's theory of natural selection, which relies on growth and reproduction for adaptation. How can natural selection occur in populations that don't reproduce? This is where the concept of short-term seasonal dormancy comes into play. Dormancy during winter provides an evolutionary advantage, as dormant organisms have larger populations when conditions are favorable for growth in the spring. These organisms can pass their dormancy genes to a larger population of progeny, ensuring their survival. Extending this model to dormancy lasting for thousands of years in marine sediments, we must consider what these intraterrestrials might be waiting for. What event could pull them out of dormancy after being buried hundreds of meters deep in the Earth's crust? Imagine a scenario where human lives only lasted about 24 hours. This thought experiment highlights the challenge of understanding life in the subsurface. Are we like day-lifespan humans contemplating a tree, or are long-lived intraterrestrials waiting for cues we can't recognize due to our short lifespans? What is the point of living for hundreds of thousands of years? There must be a reason for these intraterrestrials' longevity. Evidence suggests that long-term dormancy has a selective advantage. When starved, some bacteria, like Escherichia coli, enter a state of long-term dormancy, where they remain alive and metabolizing but grow much slower. These "old geezers" outperform fresh, fast-growing E. coli 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, allowing them to take advantage of new situations. These microbial "monks" are accustomed to deprivation while others perish. Life on geological timescales offers a plausible explanation for these microbes' longevity. Seasonal cycles are too rapid, but geological processes like island subsidence, floods, droughts, and storms occur on hundred- to thousand-year cycles. Submarine landslides, earthquakes, tsunamis, and volcanic eruptions can expose intraterrestrials to new food sources, coaxing them out of dormancy after hundreds of thousands of years. It's counterintuitive to think that a microbe is adapted to wait for a volcanic eruption, but Earth's history shows that such events are reliable over long periods. Even more slowly, intraterrestrials might be adapted to glacial cycles or tectonic plate movements. As new seafloor forms at mid-ocean ridges, the existing seafloor is pushed away, eventually jamming into a continent. Some sediments and intraterrestrials will be subducted, only to be returned through cracks and fissures in the overriding plate. This process could be what these intraterrestrials are waiting for, a chance to resurfacing and recommence growth. The evolutionary payoff for waiting for millions of years in deep marine sediments would be to return to the upper seafloor, where food is more abundant. Individuals with the best adaptations to dormancy would have a growth advantage once they return to the surface, ensuring the stability of these adaptations in the communities. Could this be the intraterrestrials' version of summer?
