A spore is a bacterium’s survival capsule: dried almost solid, wrapped in protein armor and chemical processes shut down. Bacteria enter this state of hibernation when environmental conditions are unfavorable. What’s left is close to indestructible: It can survive boiling, desiccation, radiation and the vacuum of space. NASA once bolted Bacillus spores to the outside of a satellite and left them in orbit for nearly six years; the ones shielded from sunlight were able to come back to life upon returning to Earth.

Out in the dirt, bacteria have handed this spore-forming ability down for roughly 3 billion years. But if you put this bacterium in a comfortable laboratory flask and feed it well, within a few thousand generations it will lose the ability to make spores altogether.

Why would a trait this useful, conserved for 3 billion years, vanish in a few thousand generations? This paradox has bothered microbiologists like me for decades.

In our research, my colleagues Jay Lennon and William Shoemaker and I think the answer comes down to energy: We added up what it costs a bacterium to go dormant and wake up again, and the energy bill is large enough for evolution to push it to lose its spore-making genes if they aren’t being used.

This same arithmetic runs on any costly trait in any organism. Building the first complete energy budget of forming and sending out a bacterial spore also shows where a spore is most vulnerable – an important data point to fight the bacteria behind some of the hardest hospital infections to treat.

Cells run on a molecule called adenosine triphosphate, or ATP, which acts like a rechargeable battery. Because every job in a cell is paid for with ATP, we can treat this molecule like a real currency: It lets you directly compare the cost of producing a spore with the cost of producing a swimming........

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