Our biological clocks, like those of other vertebrates, keep time thanks to the response to sunlight of the pineal gland. Stuck underneath our cerebral hemispheres, the lentilsized gland can’t detect light directly. Rather, it receives signals from our eyes via a tortuous circuit of nerve cells. Whatever pineal photosensitivity our ancestors once enjoyed lost out to a higher evolutionary priority: boosting the computing power of the mammalian brain.

But the pineal of lower vertebrates is both closer to the top of their heads and better equipped to detect light. In lizards, for example, the pineal corresponds to an eyelike organ that lies beneath the skin and between the eyes.

Intrigued by this light-sensing ability, biologists wondered whether it might be linked to mysterious functions that humans and other mammals no longer perform. Experiments ensued and, from the late 1960s onward, revealed that the pineal glands of certain species of fish...

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