I'm Shankar Vedantam, here to tell you about a great mystery.

That mystery is you.

As the host of a podcast called Hidden Brain, I explore big questions about what it means to be human.

Questions like, where do our emotions come from?

Why do so many of us feel overwhelmed by modern life?

How can we better understand the people around us?

Discover your hidden brain.

Find us wherever you get your podcasts.

The blue water of a seemingly endless, massive lake glistens in the midday sun.

You're standing on the eastern shore of Lake Turkana in what's now Kenya, just over 1.5 million

years ago in the early Pleistocene epoch. Around you, others have come to drink from the lake.

There are double-tussed whartogs, relatives of those that roam Kenya today, along with wildebeest ancestors

and kudus, a kind of woodland antelope. And it's no wonder they're thirsty. The air is warm,

the temperature over 30 degrees Celsius as the sun beats down relentlessly. It's fine for the moment,

but you'll need to find somewhere with some cover soon before the hottest

part of the day sets in. And the presence of the kudos tells you that there must be trees nearby.

Sure enough, you watch as they finish drinking and bound away from the shore, heading towards

the edge of a patch of forest that you can just barely see when you squint.

Walking towards the shade of the trees, you notice a volcano in the distance, one of forests that you can just barely see when you squint. Walking towards the shade of the trees you notice a volcano in the distance, one of several

that sometimes blanket the landscape with thin layers of ash.

And while you're looking up at the towering mountain, you stumble over some oddly shaped rocks.

You pick one up, turning it over in your hands.

...