Giving thanks isn't just a holiday tradition. It's part of how humans evolved

It's the season of giving thanks—and it turns out humans have been doing it for a long, long time.

As more researchers dig into the science of gratitude, they've found the feeling likely played a key role in helping our ancestors band together and survive.

That legacy continues today, as being in the mood for gratitude shapes who we are as a species and how we connect with the people around us.

"This is something that is part of our human DNA," said Sarah Schnitker, a psychologist at Baylor University. "It is a glue, in a sense, that holds us together."

How we got grateful

Humans are social animals. That's how we've lasted so long; not by being the biggest or the strongest, but by figuring out how to work together.

A key part of building relationships is the idea of reciprocity: "If you like me and do nice things for me, then I like you and do nice things for you," said Michael Tomasello, a developmental psychologist at Duke University.

The animal kingdom has some parallel give-and-take behaviors, said Malini Suchak, an animal behavior researcher at Canisius University. In experiments with capuchin monkeys and chimpanzees, Suchak found the primates became more willing to help out a partner if that individual had helped them in the past.

Some scientists think the feeling of gratitude evolved to keep the helping exchanges going. In other words, if you were helped, you'd feel like you should repay the debt with a good deed in return, said Jenae Nelson, who researches gratitude at Baylor and Harvard universities.

"This give and take—this is very, very primal and very important to a cooperative society," Nelson said. "Otherwise, you just get a culture of takers."

In animals and in humans, these aren't always one-to-one transactions. Sometimes, an ape that gets groomed by another will later back that partner up in a fight, Suchak said, showing that reciprocity might not be about keeping exact scores, but forming broader emotional ties.

Though we can't "speak chimp" well enough to know if they're actually saying thanks, Suchak added, it makes sense that some form of this social debt showed up early in our lineage.

"It didn't just pop out of nowhere when humans evolved," Suchak said.

And how we stayed grateful

Thousands of years later, gratitude has taken root in humans.

Studies have found gratitude may show up in a few spots in our genes and brains—including ones linked to social bonding, feeling reward and seeing other people's point of view.

And the feeling emerges early on: Children as young as 2 and 3 demonstrate they want to return favors, said Amrisha Vaish, who studies moral development at the University of Virginia. At age 4, children also show a tendency to "pay it forward," she added.

In a study, Vaish found that when kids got help completing a task—in this case, finding a key to unlock a box of stickers—they were more likely to share their sticker reward with a new person.

It's that kind of behavior that shows gratitude is more than simple exchange, Schnitker said. It can make us more generous with other people in general—even if they didn't help us first.

Giving thanks might be good for you, too: A 2016 study found that people who wrote letters of gratitude reported better mental health and saw changes in their brain activity—even months down the line.

But Nelson pointed out that recognizing the giver, not just the gift, is key.

So, if Thanksgiving has you in a mood for gratitude, she suggested focusing on thanking the people in your life, rather than just making "gratitude lists" of the stuff you have. This is more in line with why the feeling evolved in the first place, she said.

"It's not just about stuff and materialism," Nelson said. "It's about relationships, and the things that people do for you, and then the things that you can in turn do back for other people."

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