Hello everybody and welcome to the Mindscape Podcast. I'm your host Sean Carroll and if you're familiar with my book, The Big Picture or any of the various talks that given on that book,

you'll know that one of my favorite factoids is that the average human lifespan is 3 billion heartbeats.

That's not a very profound fact if you just take the fact that the average human being lives for about 75 years and do your dimensional analysis to convert from years to heartbeats.

The number works out to be about 3 billion for a typical human being stretching from birth to death.

But it's an interesting fact because it brings home in a slightly more vivid way how short our life is.

3 billion is a big number but it's not unimaginably big and unlike years, heartbeats are going by all the time.

You've squandered several of your heartbeats already, listening to me talk right here.

I've learned this fact about the 3 billion heartbeats from today's guest Dr. Jeffrey West who's a distinguished professor at the Santa Fe Institute.

It's the context of a much more profound fact about biological organisms here on Earth.

If you take any particular kind of organism, let's say mammals, because human beings are mammals, they come in all shapes and sizes.

There's tiny little mice, there's big old blue whales or elephants, but there are relationships between the size of an animal, in mass for example, the number of kilograms the thing has, and other biological facts such as how long it lives.

Bigger animals, whales and elephants live for much longer than tinier things like mice or squirrels.

Meanwhile, there's another relationship between your size and your heart rate.

Big animals like whales and elephants have very slow heartbeats, tiny animals have very rapid heartbeats, and you can see where this is going.

These two facts exactly cancel out. The average number of heartbeats for mammals is approximately the same for tiny little mice or big blue whales or elephants.

It's not an exact relationship. In fact, the number for a typical mammal works out not to be 3 billion, but 1.5 billion, which maybe you could argue is roughly what we had, we human beings, back in the state of nature before we had pasteurized milk and Obamacare and things like that.

But the point is that something is going on that goes beyond mere biology. There's some reason why there's a relationship between how fast your heart beats, how massive you are, and how long you live.

That's what got today's guest, Jeffrey West, interested in biology, networks, and complex systems.

He started his academic life as a particle physicist, but he read about these scaling relations and he realized from his physics point of view, no one understood them.

No one knew why things were like that. So he and his collaborators developed a theory that explains why you get this relationship as an animal gets bigger, it lives longer, but its heart slows down.

The pace of life is slower for larger animals. These days he's extending that analysis not just to individual animals, but to cities or cultures or other kinds of networks that fill our daily lives today.

This kind of analysis is absolutely crucial for sustainability, for thinking about how we live in the world, for choosing how to manage our life here on Earth.

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