It’s pleasing when the world finds a new use for something old and familiar. Pie pans gave birth to Frisbees; botulinum toxin, once known mostly for poisoning people, is now better known as Botox and gets rid of wrinkles. And Lady Gaga has taught the world how to use a large silver lobster as a hat. But evolution has come up with a bigger innovation than any of these.
The discovery concerns the mammalian hormone oxytocin, which evolved around 500 million years ago from an ancestral version still found in fish and amphibians. The hormone evolved to play a key role in what makes mammals mammalian.
Other newborn animals typically fend for themselves: Crocodiles, for example, are catching insects soon after birth. But mammals develop slowly, and mothers have to feed their newborns. Oxytocin evolved to make this possible, prompting mothers who are nursing to produce more milk as their babies demand it.
Evolving the means to nurse the young was only half the battle. You also have to want to take care of them and to invest zillions of calories in generating milk and fending off predators. And you need to be able to recognize your offspring in a crowd, so you don’t waste your energy helping others to leave behind copies of their genes.
Oxytocin helped to solve both problems. Around the time of birth, female mammals release oxytocin in some brain regions, and the hormone allows them to register and recall their offspring’s smell, appearance or voice. Oxytocin rewards such maternal behavior with feelings of well-being.
More oxytocin innovations emerged. In the eons since mammals proliferated on earth, some primate and rodent species independently evolved pair-bonding (that is, sexual and/or social monogamy). In the brain, oxytocin is heavily involved in this as well. And as primates developed complex you-scratch-my-back-I-scratch yours relations, evolution adapted oxytocin to mediate the formation of trust and altruistic feelings toward fellow members of one’s group.