- Throughout your life, you go through many remarkable phases of metamorphosis, and none of them are as uncomfortable as when you morph from the adorable pudgy caterpillar of childhood and spread your wings as an awkward teenage butterfly.

Not only are our bodies flooded with signals to become sexually mature, hair starts growing in strange places, voices change, your emotions are all over the place, bodies are growing in every which direction, and so many secretions.

All of these changes, as well as dozens of others that you experience on timescales ranging from seconds to years, are mediated by one of the strangest systems in that already peculiar sack of meat you call a body, the endocrine system and its hormone messengers.

To understand why this chemical soup exists, we first need to think about the Mongol Empire, wait, what?

(upbeat music) Hey, smart people, Joe here.

So the Mongols, Kublai Khan, in the mid 1200s, reigned over the largest empire in history up to that point, and he had a problem.

Let's say Kublai, Mr. Khan, Emperor K had an idea.

How do you get it to the farthest reaches of your empire and make sure that every distant feudal lord works together for the good of the whole?

Not to mention, how do you keep stability in your vast empire before the age of lightspeed communications?

He sent emissaries to deliver his message and make sure that it was carried out.

Your body has the same problem.

It has to send instructions to its distant reaches to coordinate stability, peace, and responses to the external environment, even the occasional invader.

But instead of emissaries and ambassadors, your body sends out tiny chemical messengers called hormones.

And instead of roads, they travel through your circulatory system to find the cells or organs that need to heed their message.

Hormones tell us many things, including when we're hungry, when to sleep, when danger is near, and even when to grow into a new body.

This problem of how to send messages over long distances and across hugely varying timescales is one that's likely even older than multicellular life.

This is a cellular slime mold.

They start life as individual amoeba blobbies, but when their environment gets stressful, like when there's not enough food, they secrete a chemical signal that tells them to pile up with their neighbors until they slither together like a single slimy slug.

Thousands of amoebas act together like one, like minions to their chemical messenger overlord, eventually even morphing into weird tree-like structures that allow a few lucky individuals to escape as spores to better and more fruitful lands.

Now you might not think you have a lot in common with a slime mold, but they're faced with the same problem your body is.

How do you get many individual cells to communicate, coordinate, and cooperate over long distances and even undergo major changes?

The answer, secreted chemical messages.

I mean, think about it, that chemically-induced slime mold metamorphosis, isn't that unlike what happens during puberty?

I mean, you're just living your life and then bam, your body is flooded with chemicals, setting off a series of weird and wonderful changes.

- [Narrator] This gradual and irregular readjustment of the glandular system is almost certain to make Johnny nervous, excitable, and a little emotionally unstable.

- This is the central problem that all multicellular creatures have to solve.

Bodies need a coordinated communication system so that all of their parts know what to do and when to do it, whether it's to induce changes or to keep the body nice and stable in the face of environmental challenges.

In humans, that means relaying messages between 30 trillion cells spread among dozens of different interacting systems, a vast metropolis of intermingling cellular mishmash.

Wait, isn't that what the nervous system's for, I mean, taking information from one cell and sending it to different parts of the body?

For most of medical history, we thought that every process from fertility, to appetite, to bodily temperature was controlled by our nervous system.

But in the mid 1800s, that way of thinking started to change, thanks to some castrated chickens.

(chicken cackles loudly) For hundreds of years, people knew that castrated male chickens stayed tender and delicious instead of becoming not-so-tasty and not-so-friendly roosters.

But in 1849, for some reason, German zoologist, Arnold Berthold, decided to put testicles back into some of those castrated male chickens.

And he noticed that they matured into normal roosters.

The transplanted testicles had only connected to the bloodstream and not the nervous system.

That told him that the grow sexy rooster feathers signal was carried in the blood and not the nerves.

Around the same time, another German doctor was busy taking people's temperatures, 25,000 people's temperatures, adding up to millions of total readings.

When he averaged those millions of temperatures out, he got 98.6 degrees Fahrenheit, which is what most people think of as normal body temperature.

But the really interesting thing he found was that, two individuals may have different normal body temperatures, varying by a degree or more.

But within an individual healthy body, temperature was kept remarkably stable.

They just didn't know what was doing it.

It took more than 50 years for scientists to figure out how these huge bodily changes, like the roosters and their testes, and the way that bodies keep themselves from changing, like with temperature, are controlled.

And it turns out, they're mostly controlled by the same thing, hormones.

All of these hormone messengers and the tissues that make them, control them, and shuttle them throughout the body are known as the endocrine system.

It was the last major body system identified by doctors because those chemical messages and the parts of cells that sense them are all basically invisible.

Today we know of more than 50 hormones, manufactured by at least nine lumps of tissue that are spread through your body called glands.

Now, once hormones are sent into the bloodstream, they travel around until they reach the right target, usually by latching onto a specific receptor on a certain type of cell, and they deliver their chemical message.

Hormones help us respond to changes in blood sugar, prepare our body to flee danger, tell us when to store or burn extra energy, they work in concert to prepare bodies to grow other bodies inside of them, and as every former teenager knows, even influence our decision-making and mental state.

(Joe sighs loudly) Ah, who am I kidding, your brain stays a mess, kids.

You just get better at dealing with it.

But our nervous system is also really good at passing information to and from distant corners of our bodies.

Why do we need this extra second layer of communication?

Electrical signals are a great way to communicate quickly.

But often, distant cells and systems in our bodies need to stay in touch over time.

That gets tricky.

Consider this, controlling puberty with the nervous system alone, that would mean nerves firing around the clock for five or six years.

And more than just signaling long-term changes, hormones are a way for different parts of our body to stay in sync and maintain stability.

Imagine different parts of your body getting sleepy at different times, you'd be a mess.

But luckily, the release of melatonin is produced by a central gland so it affects the daily rhythms of every cell in your body simultaneously.

Electrical signals can't do that, but the nervous and endocrine systems aren't totally separated.

They work in feedback loops.

The endocrine system acts on neurons in the brain, which controls glands in the brain, which sends signals to other glands to tell them to make more or less hormone.

Stress hormones like cortisol can also alter brain function, even our ability to learn.

And increased levels of sex hormones can make animals act like complete idiots.

Communicating over large distances over time is a problem all multicellular life has had to solve, and it's an incredibly old one.

We find secreted chemical messengers in everything from single-celled organisms, to plants, to you, and your dog.

What's weird is that unrelated organisms separated by enormous distances on the tree of life share many of the same hormones.

They've just evolved to sometimes use the same messenger chemical in vastly different ways.

In human bodies, for example, the hormone, prolactin, is involved with breast growth and milk production, but elsewhere in the animal kingdom, it impacts everything from salt concentrations in fish to the way that bird bodies get ready to sit on their eggs.

Hormones from one species can even elicit responses in others.

Back in the 50s, the way we found out if women were pregnant was by injecting their pee into frogs or rabbits and then watching to see if human hormones kicked off ovulation.

All of this tells us that hormones and their chemical building blocks are an ancient and shared way that living things send and receive signals, maybe as old as complex life itself.

So if you're looking to blame someone for all those awkward puberty years, tell it to the slime molds.