Life is weird.

An octopus has a mini-brain  in each of its eight arms,   operating them semi-independently  from its central brain. Many plant species purposefully defend themselves   from being eaten by releasing toxins,  despite not having brains at all.

And wombats…take cube-shaped poos! When it comes to multicellular organisms—or living   things made of more than one cell—our  planet has a bevy of bizarre beasts. But what we call weird, be it the nervous system  of an octopus or the digestive tract of a wombat,   is actually the result of millions  of years of evolution selecting   for the traits that will lead to  an organism's continued survival.

Each of these internal systems fits the  needs of the organisms that evolved them. And while we have our differences,   all multicellular organisms use  the same basic operating system. We’re organized and regulated groups of  cells, collaborating to form systems that   operate in intricate harmony even  if it doesn’t always feel that way.

Hi! I’m Dr. Sammy, your friendly neighborhood  entomologist, and this is Crash Course Biology.

I’m just sayin’ Wombat poop, why is it cubed? Their butts and their rectums are round, round! I mean, one does not simply poop cubes!

I – I, I think I need a music  break to just mull this over. (THEME MUSIC) Wait wait! I got it! No... (Theme Music) Billions of years ago,   every living thing on Earth was  made up of just one cell each.

But eventually, some of these cells came together  to form the world’s first multicellular organisms. And while they might have looked  like blobs drawn by a toddler,   they set the stage for every kind of  fungus, plant, and animal on Earth. As you can imagine, multicellular organisms became  pretty different from single-celled ones in some   big ways – one of the biggest differences  between them is how they’re organized.

Unicellular forms, like bacteria and most  algae, can be organized pretty simply: They’ve   got molecules, maybe some organelles—the cell’s  organ-like structures—and then the cell itself. And that works for them, like how minimalists  will say, “I’m really good with just one mug.” But multicellular organisms  can have trillions of cells! And their insides, like my mug  collection, need to be organized.

Obviously, the bumble bee mugs go  on the shelf under the firefly mugs. I’m sure it’s the same at your house. So, multicellular organisms are arranged like  nesting dolls, one system inside of another.

On top of molecules, organelles, and  cells, these creatures also have tissues,   or groups of similar cells doing the same job. And they have organs, or groups  of tissues that work together. And animals, in particular, have organ systems,   where different organs  collaborate with each other.

Like, take this horse’s digestive system. It’s made up of organs, like an esophagus,   intestines, and a small-ish stomach, since,  like me, they tend to snack throughout the day. The lining of these organs  uses a variety of tissue   types to control what materials go in and out.

Then, one step down, each tissue type  is made up of different kinds of cells,   that in turn, contain organelles—all of  which are made of microscopic molecules. And all these different parts need to be  connected to, and communicating with, each other. Otherwise, we’re all just a  bunch of tissues and organs   flopping around and…see now that…that’s weird.

Thankfully our cells are good at   coordinating these complex systems, even  as they’re doing totally different things. You see, multicellular organisms have specialized  cells that take on their own individual functions. So, rather than one cell having to take on all of  the processes needed to keep an organism alive,   specialized cells share the load, and focus  on being really good at just one thing.

Like, a school fundraiser run by just one  person could potentially be a disaster. But when there’s a different volunteer  to take on each task—make flyers,   bake brownies, keep track of the  cash—things go a lot more smoothly. In a similar way, multicellular organisms  have cells that transport things,   cells that help muscles contract,  cells that communicate messages to   other parts of the body, even cells  that make other specialized cells.

And there are even specializations  within the specializations. Like, among the cells that transport useful  things in plant bodies, there are xylem cells   that transport water upwards from the roots and  phloem cells that transport sugar from the leaves. So, having multiple cells lets organisms   specialize to get a whole variety of work done.

But that’s not all: having multiple  cells also lets organisms get bigger. When you’re just one cell, you can grow only  so much, because of natural limitations set   by the balance between a cell’s volume and its  surface area – aka the surface-to-volume ratio. Like, picture a balloon.

The rubber outside of the balloon is its surface. And all the air that fits inside is its volume. Things like water and nutrients enter  a cell through the surface — a membrane   that decides what gets let  in or kept out of the cell.

Once that good stuff passes through the membrane,  it goes where it’s needed to do work in the cell. And in a tiny one-celled organism,  no matter where nutrients need to go,   it’s a pretty quick trip from the membrane. Kind of like a small town where you can zip from  one side to the other in only a few minutes.

But when you’re mushroom-, tulip-, or  whale-sized, that becomes a long commute   between the city limits—so long that things  wouldn’t get to where they needed to go in time. Plus, like a balloon, if the  volume inside gets too much   bigger than the surface, the membrane might burst. So, that’s one reason why we large organisms  have many tiny cells instead of one giant one.

It keeps us efficient enough to stay alive.

be easy to think that multicellular organisms are   somehow better than unicellular ones, or that  evolution is always working to add complexity. But really, there’s no direction to evolution. It’s a twisty, looping path  that sometimes makes living   things more complicated, and sometimes less so.

After all, when organisms get  more complicated, there are also   a lot more opportunities for things to go wrong. So, usually, complexity only sticks around  in a species if it helps them survive and   reproduce, like how fish ancestors  developed an organ called a fin. The fin helped them swim  faster, so it got passed on.

And multicellular life has  its own unique problems, too. When you’re a single-celled organism,   life is pretty simple: keep that  one cell alive, and you’re golden. Whereas complexity can mean our multi -celled bodies are under a lot of stress.

Let’s bring the house lights down and  settle in for the Theater of Life… In the 1930s, Argentinian scientist   Dr. Rebeca Gerschman had already made a name  for herself studying potassium in the blood. And when she moved to the US, she thought  that’s what she’d continue researching.

But then, other scientists made a  curious observation about fighter   pilots: They noticed that their  skin aged faster than normal. Gerschman thought she might know why. So, she started investigating the effects  of oxygen and other gases on mammals.

At high altitudes, fighter pilots  are breathing 100 percent oxygen,   as opposed to the 21 percent  in the air on the ground. They’re also exposed to a lot of radiation. Gerschman thought those things might be connected.

She did experiments testing the ways high oxygen  levels triggered the body’s stress response. But her biggest discovery was about free radicals. Free radicals are unstable molecules that bounce  around itching for something to react with.

Today, we know they’re formed when radiation  interacts with oxygen, but also by everyday   processes in our body, like when mitochondria  in our cells use oxygen to harness energy. And in 1954, Gerschman was the  first scientist to discover that   free radicals can damage and kill cells  in what’s now called oxidative stress. Something that could lead to  everything from disease to skin damage.

At first, the scientific community didn’t  accept this, likely thanks to prejudices   against women in science at the time,  and a misunderstanding of chemistry. But today, free radicals are  a huge topic in medicine. You know what they say,  more cells, more…elaborately specific problems.

But, on top of that, it also  just takes a lot of energy   to keep a multicellular organism functioning. We’re basically energy vacuums,  pulling in buckets of sunlight,   food, and nutrients to fuel and  regulate all the processes of life. Like, picture a bird in the wintertime.

At an organism’s base level, called equilibrium,   that bird’s internal temperature  would match the temperature around it. But most external temperatures are  way too cold to be safely mirrored   by the internal environment of a bird’s body. They need their insides to be nice  and steamy—around 41 degrees Celsius.

In order to maintain this, living  things rely on homeostasis,   or relatively steady, stable  conditions within the body. Homeostasis is a high-energy endeavor that lets  us regulate temperature, hydration, oxygen levels,   and more — the kind of things you’d put on a  dashboard to keep a character alive in The Sims. Without homeostasis, cells can’t do their jobs.

Like, say plant roots didn’t regulate  how much water they collected. Whether they sucked up too much or  too little, the plant wouldn’t be   able to photosynthesize—or,  harness energy from the Sun. So yeah, maintaining  homeostasis is pretty crucial.

And to find out how we do it, we need  to take a pit stop—in the bathroom. Go with me here. There’s an ideal water level for your toilet tank:   It’s where the water line  sits when you’re not using it.

When you flush, the water level drops below that,  as all the clean water rushes out of the tank. When this happens, a ball  inside the toilet tank drops,   and triggers a mechanism to fill the tank back up. Then, when the water gets to the right  height, that ball starts to float,   and that triggers the tank to stop  filling so it doesn’t overflow.

With this handy system, the toilet  keeps its water level just right. When something is less than ideal — either  too low or too high — it corrects itself. We call that negative feedback, and it’s one  way living organisms maintain homeostasis.

That’s right—you’ve got something  in common with a toilet! Bet you didn’t see that one comin’. There are also positive feedback  loops, though these are less common.

Rather than correcting something that’s off,   positive feedback loops happen when the result  of one reaction triggers more of that reaction. For example, when an apple ripens  on a tree, it produces ethylene. That causes its neighbors to ripen, too,   which in turn produces more ethylene  and causes more apple ripening.

Multicellular organisms from  animals to plants to fungi   have dozens of these systems  for maintaining homeostasis. There are systems for acquiring resources,  for waste disposal, infrastructure, transport. Plus things like defense,  repair, and communication.

You might say we’re, quite literally, busybodies. At the end of  the day, despite multicellular organisms being   vastly different—and sometimes, super weird—it’s  these complicated systems that make us work. Those octopus arm-brains help its tentacles  move faster and take in sensory information.

And wombat poop — well, I’ve had some time to  think now and what I think happens is that the   walls of the rectum must go “kuh-jhooo”  to flatten it on two sides and then the   top and bottom of the rectum must do the  same thing, sorta like a trash compactor. And then it flips over and does it one more time. You feel me?

And this somehow squeezes extra water  out of the food because Australia is dry… That’s my hypothesis and I’m sticking to it at  least until I can find a really small camera. Anyway, my obsession with Wombat poo aside,   at the most basic level, multicellular  organisms are all doing the same thing. We’re using a series of interacting  body systems to survive.

We’re organizing the molecules and cells within us  into intricate systems, and keeping ’em regulated. In the next several episodes, we’ll check out  how specific multicellular organisms function,   like vascular plants and vertebrates. I’ll see ya then.

Peace! This series was produced in  collaboration with HHMI BioInteractive. If you’re an educator, visit  BioInteractive.org/CrashCourse for   classroom resources and professional development  related to the topics covered in this course.

Thanks for watching this episode of Crash  Course Biology which was filmed at our studio   in Indianapolis, Indiana, and was made  with the help of all these nice people. If you want to help keep Crash  Course free for everyone,   forever, you can join our community on Patreon.