Along with the microbes, the birds, the bats, there's another creature on Earth that has

had an unusual evolutionary trajectory. That would be the primates, the monkeys, the apes, and of course us, one branch of the great apes.

We often imagine that we are wildly different from our primate cousins, but in many, many deep ways, we're not very different at all.

Primates all have culture, they have deep personal relationships, primates are tool users.

And there's one tiny island where these similarities and differences have really been made clear.

It's called Monkey Island, or Kio Santiago, in Puerto Rico.

And for more than 80 years, scientists have logged these monkeys' births and deaths,

their social interactions, their parenting styles, and we've learned

a ton about ourselves in the process, how primates deal with stress, how we pass it through generations.

On this last episode of Creature Week and episode 15 of our

Advent calendar, we head to Puerto Rico to watch our primate cousins and realize just

how deeply our fates are intertwined. That's next.

It's next.

It's 7 a.m. and there's a strong wind blowing in Punta Santiago off the eastern coast of Puerto Rico.

A group of 15 researchers meet at a dock and squeeze onto a boat.

The sun rises as the boat's captain starts the engine

and sails off to Kaya Santyago,

an island just around a mile offshore.

It's a place only researchers are allowed to go. And it's

beautiful. The water is crystal clear. The breeze passing by is a perfect wake-up call

and later in the day the temperature will rise to 80

degrees but even so everyone on the boat is covered up wearing long sleeves

...