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Again, when the spore encounters favorable conditions, it will germinate, as this Acetonema longum spore is doing. Unlike the Bacillus subtilis you saw earlier, germinating diderm spores do not shed their outer membrane. Instead, they remodel their cortex back into a thin cell wall (compare to the mature spore on the previous page). This process is a good reminder of the similarity of monoderm and diderm cell walls; they may look very different, but their fundamental architecture is the same, just with more sheets of material in monoderms (and spores). After the remodeling, the cell sheds its protein coat, as you see here, and starts to grow.
Some archaea also have resistant forms similar to bacterial spores. For instance, when water levels drop in its salty environment, rod-shaped Halobacterium salinarum uses a variant division process to produce three or four hardy spherical cells that can survive dormant in halite crystals for tens of thousands of years, awaiting water to revive.
Sporulation may seem like a highly specialized function practiced by relatively few species, but there is reason to think that it may have evolved long ago and been key to the success of life on Earth. Conditions today are extremely clement compared to what they likely were a few billion years ago, when the last common ancestor of all modern cells (called LUCA, for Last Universal Common Ancestor) may have lived. It is quite possible that LUCA was a spore, the only form of life hardy enough to survive conditions volatile and violent enough to kill off the spore’s ancestors. If so, most modern lineages of bacteria and archaea simply lost the ability to form spores when it was no longer essential to their survival. It is further possible that a LUCA spore was formed by a cell somewhere else in the solar system and delivered to Earth on one of the asteroids that bombarded the planet in its infancy. We may never know, but the possibilities are fun to consider.