We see evolutionary arms races play out all the time.

Take COVID, for example.

A strain would run through the population.

We built immunity from vaccines or getting it.

And then another strain mutated and adapted to get around our defenses would come

along and be successful and the cycle would start all over again. That's evolution, and there's

obviously nothing new about it. Bacteria have been in this kind of evolutionary arms race with

viruses for as long as they've existed, billions of years.

Evolution is a mad scientist, but it's also very demanding and, in the end, pragmatic.

Mess around just a little bit, and you might go extinct.

And when evolution finds something that works, it tends to run with it.

Thank you. something that works, it tends to run with it. Welcome to the Quanta podcast where we explore the frontiers of fundamental science and math.

I'm Samir Patel, editor-in-chief of Quantum Magazine.

It seems like the more we learn about the way that evolution operates,

especially at the molecular and cellular levels, the stranger the story gets.

In a recent piece in Quanta called The Ancient Weapons Active in Your Immune System today,

science journalist Vivian Callier explores a surprising evolutionary connection

across billions of years. And on the occasion of our 50th episode, she's here with us to talk

about it. Welcome to the show, Vivian. Thanks so much for having me. So Vivian, what's the big

idea? So the big surprise here was that some of these bacterial defenses against phages exist in our cells too.

And they really haven't changed that much, even though we've been separated from bacteria for billions of years, evolutionarily speaking.

And the reason that this is surprising is that arms races tend to drive the evolution of

...