'Maybe they're waiting for something that only happens thousands of years later

 Beneath Earth's surface lies a kingdom of undiscovered microscopic life. These "intraterrestrials" survive in some of the harshest conditions on the planet — and scientists are hunting for these microbes.

In this excerpt from "Intraterrestrials: Discovering the Strangest Life on Earth" (Princeton University Press, 2025), author Karen G. Lloyd, a microbial biogeochemist at the University of Southern California Dornsife, examines the idea of evolution among life that can survive for hundreds of thousands — if not millions — of years in a dormant state and what it might be waiting for to "wake up."

"Intraterrestrials" is short-listed for the Pen/E.O. Wilson Literary Science Writing Award.

How does one evolve to stop growing for thousands of years? Recent work suggests that microbes buried deep in oceanic seafloor sediments may be doing just that. Such organisms can be referred to as intraterrestrials, small microorganisms living inside Earth's crust all around the globe. To answer this tough evolutionary question, first we have to think about what these organisms would experience in their lifetimes. These slow organisms wouldn't be concerned about the length of a day. They're buried so deep that they can't detect the sun anyway. They probably wouldn't even notice a change in season.

However, they might care about other, longer geological rhythms: the opening and closing of oceanic basins through plate tectonics, the formation and subsidence of new island chains, or new fluid flows brought on by slow formation of cracks in Earth's crust. The biology I was taught in school considered these events to be evolutionary drivers for a species, not an individual.

Living for millions of years

The fact that living cells likely exist in a nongrowth state for very long timescales raises two important questions. Can a microbe be adapted to avoid cell division for thousands of years or longer, rather than having it just happen by accident? And, if so, how does evolution work for an organism that seemingly never produces offspring?

Let's tackle that first question by stating it this way, in order to help us place this finding in the context of Darwinian evolution. Are these microbes evolutionarily adapted to hang out in this undead, dormant state for thousands or millions of years, or do they just persist because cells don't need any special adaptations to stay alive for so long?

To me, living for hundreds of thousands of years seems unlikely to happen without adaptation. Too many physiological changes are required to support this lifestyle for it to be a side effect of a "normal" fast-paced life. Furthermore, if this lifestyle is accidental, then their main growth-supporting lives must occur in some other environment. But we rarely see the types of microbes we find in the subseafloor elsewhere. It's not as if they were normal seawater microbes happily swimming around, dividing and growing when they fell to the seafloor and forgot to die.

On the contrary, most of this highly diverse group of microbes seem to exist only in marine sediments. Given this, they may be just as selected for in marine sediments as parrots are in a rainforest. Indeed, we find that at increasing depths in marine sediments, microbes make enzymes with a higher specificity for the type of substrates that are available in the subsurface, suggesting that they are specially adapted for this environment.

Subsurface microbes also have adaptations that enable ultraslow metabolisms and cell divisions. This suggests that they are somehow evolutionarily poised to be in a long-term nongrowing state. But here we have a problem. According to Darwin's theory of natural selection, these cells must grow and make new progeny to evolve. Natural selection works because, during reproduction, organisms experience mutations. And when an organism has a mutation that is beneficial, the mutation increases the organism's fitness, so the organism's progeny outcompete those of the nonmutated organisms, resulting in more progeny that have the mutation. These further generations continue to do better than the nonmutated lineages, and eventually the mutation spreads throughout the population.