That sounds exciting.

Today we'll basically be learning the Dewey Decimel System

of evolution!

It's like filing!

You must be on the edge of your seat.

OK, shut up.

When it comes down to it, this science doesn't just

categorize organisms,

when you look a little deeper, you realize it's telling the

story of all life on earth.

And it's a pretty good story.

Every living thing on this planet is related to every other

living thing.

If you go far enough back, we all have a common ancestor.

An organism that both you and I are descended from.

Or something that a star fish and a blue whale are descended from.

Or, even weirder, that an oak tree and a salmon are

both descended from.

That organism lived.

It lived very long ago.

But it was here.

And I dig that.

The trick of taxonomy, is basically figuring out

where all those branches of the evolutionary tree are,

and finding some convenient labels to help us understand all of these

remarkable interrelationships.

Let's be clear though, taxonomy isn't about describing

life in all of it's ridiculous detail,

it's mostly about helping humans understand it,

because it's way too complicated without structure.

To get that structure biologists use the taxonomic system to

classify all the organisms on the Earth.

It's sometimes called the Phylogenetic Tree,

or the Tree of Life,

and it illustrates the evolutionary relationships

between all living species.

There are about 2 million known species, but there could be

anywhere from 5 million to 100 million species

scientists really have no freaking idea.

New species keep getting discovered all the time,

and the more organisms we have to keep track of,

the more complex the Phylogenetic Tree becomes.

So, there's not always a consensus about how to classify this stuff.

There's a lot of gray area in the Natural World.

Actually, let me rephrase that:

the Natural World is one giant Gray Area.

Sometimes it's just hard to know where to put

a certain group of organisms,

and eventually the group gets so big,

the classification system has to be messed with to make room for it.

So, the system isn't perfect,

but it's good enough that we've been using it for around 250 years.

[Sniffing] What's that?

Do you smell a Bio-lography coming on?

Carl Linnaeus was a Swede born in 1707.

And early in his career as a botanist he realized that the

botanical nomenclature of 18th century Europe was..

well,

just crap.

For instance, in his day, the "formal" name of a tomato plant

was Solanum caule inerme herbaceo, foliis pinnatis incisis,

racemis simplicibus.

Linnaeus actually said once, "I shudder at the sight

of most botanical names given by modern authorities."

Not only did this sloppiness bother him, he saw a whole

sugarstorm blowing in:

New plants were still being discovered in Europe,

but that was nothing compared to the crazy stuff that was

coming from the New World.

Linnaeus saw that pretty soon, naming conventions were

just going to collapse under all these new things to name.

And THEN what?

Linnaeus famously started off by naming himself.

He came from a peasant family, and at the time,

surnames were just for rich people, so when Carl

went to college, they asked him for his surname and he

just made one up: Linneaus, after the Linden trees that grew

on his family's homestead.

Linnaeus got a medical degree and became a professor at

Uppsala University where he devoted himself to the

study of nomenclature.

He had his students go places and bring back specimens for him to

study and categorize.

The method he eventually adopted was based on morphology,

or physical form and structure.

This wasn't necessarily a new idea.

Back then, people grouped organisms by analogous or homoplasic traits,

structures that appear similar but actually come from

completely independent origins.

By this definition, birds would be more closely related to

butterflies than to reptiles because birds and butterflies can both fly.

But Linnaeus had a good mind for this stuff and turned out to have

a real knack for choosing actual homologous traits for

his classification system

traits that stem from a common evolutionary ancestor.

Linnaeus didn't know jack about evolution

Darwin wouldn't come around for another 100 plus years

but he just intuited that some traits were more important than others.

For instance, he was struck by the fact that reproductive apparatus

seemed to be a good way of classifying plants.

He also caused a scandal by classifying the Class Mammalia

based on the female's ability to produce milk from their nipples.

Because apparently that was pretty racy stuff back then.

In his lifetime Linnaeus catalogued roughly 7,700 plants

and 4,400 animals, and he published his

classifications in a catalog called Systema Naturae,

which by the time he wrote the 12th edition, was 2,300 pages long.

In the meantime, Linneaus actually adopted a personal motto:

"God created, Linnaeus organized."

Although taxonomy has come a long way since Linnaeus, we still use a

bunch of the conventions that he invented.

For instance, we still arrange things into taxa, or groups of

organisms, and we still us the same Taxa as Linnaeus:

kingdom, phylum, class, order, family, genus and species.

We also still use Linnaeus' convention of binomial nomenclature

using a unique, two-part name for every species

the genus and species name, in Latin or Latin-ish.

This practice actually started back in the Middle Ages when educated

people were expected to know Latin.

We know a lot less latin now,

but we know a lot more about evolution which Linnaeus didn't.

And we have technologies like genetic testing to classify

relationships between organisms.

And yet we still use Linnaeus's morphology-based system because

genetic evidence generally agrees with classifications that are made

based on structure and form.

However, because there was a lot of life that Linnaeus had no idea

about, we had to stick a new taxa above Linnaeus' Kingdom.

We call it Domain.

And it's as broad as you can get.

The Domains are Bacteria, Archaea and Eukarya.

The bacteria and archae are prokaryotes, meaning their

genetic material goes commando with no nucleus to enclose it.

While the Eukarya make up all the life forms with a nucleus and

include pretty much all the life that you think of as life,

and quite a lot of the life that you don't think about at all.

It might seem like, since all macroscopic life only gets

one domain, it's kinda silly to give prokaryotes two

and for a long time, we didn't.

We didn't divide them up into different domains.

They hung out together in a single domain called Monera.

But it later became clear that Bacteria, which live pretty much

everywhere on earth, including inside of you and deep in the

Earth's crust, and Archaea which are even more hardy

than bacteria, have distinct evolutionary histories.

Archaea being more closely related to eukaryotes and,

yes, thus me and you.

They have totally different cell membranes and the enzymes they use

to make RNA, their RNA polymerase, is much more like ours.

Under the domain Eukarya, which is by far the most

interesting and even occasionally adorable domain, we have Kingdoms:

Protista, Fungi, Plantae and Animalia.

Now, scientists have settled on these four. For now.

But these are categories that are a human creation, but there

are good reasons for that human creation.

The unscientific truth is that we looked at life and divided it up

based on what we saw. So we were like,

"Well, protists are single-celled organisms, so, they're very

different from the rest of the domain.

Plants get their energy from the sun and fungi look and act very

different from plants and animals, and we already know what

animals are, so they have to get their own kingdom."

And though scientists are loathe to admit it, that system of just

looking and dividing things up actually worked pretty well for us.

Not perfectly, but pretty well.

But there's a reason why this worked so well.

Evolutionarily, there are actual categories.

Each of these kingdoms is a huge branch in the tree of life.

At each branch, an evolutionary change occurred that was

so massively helpful that it spawned a vast diversity

of descendents.

Plants or Plantae are the autotrophs of the Domain Eukarya.

Autotrophs meaning that they can feed themselves,

through photosynthesis of course.

Their cellulose-based cell walls and chloroplasts giving

them a distinct difference from all other multi-cellular life.

There are two other sorts of -trophs.

The heterotrophs, which get their energy by eating other organisms.

And Chemotrophs, which are weird and crazy and only show up

in bacteria and achaea, and they get their energy from chemicals.

Now the kingdom Protista is weird because it contains both

autotrophs and heterotrophs.

Some protists can photosynthesize, while others eat living things.

Protists are basically a bunch of weird, eukaryotic single-celled

organisms that may or may not be evolutionarily related

to each other

scientists are still trying to figure it out.

Some are plant-like, like algae,

some are more animal-like, like amoebas,

and some are fungus-like, like slime molds.

Protists are one of those gray areas I was telling you about.

So don't be surprised if, by the time you're teaching this

to your biology students, there are more than

four kingdoms in Eukarya.

Fungi, which are, you know—the funguses.

They include mushrooms, smuts, puffballs,

truffles, molds, and yeasts

and they're pretty cool because they have cell walls like plants,

but instead of being made of cellulose,

they're made of another carbohydrate called chitin,

which is also what the beak of a giant squid is made out of,

or the exoskeleton of a beetle.

Because fungi are heterotrophs like animals, they have these

sort of digestive enzymes that break down their food

and get reabsorbed. But they can't move,

they don't require a stomach for digestion

they just grow on top of whatever it is they're

digesting and digest it right where it is.

Which is super convenient!

And finally, we have Kingdom Animalia.

Which is the lovely kingdom that we find ourselves and 100% of

adorable organisms in.

Animals are multicellular, always.

We're heterotrophic, so we spend a lot of our time hunting down food

because we can't make it ourselves.

Almost all of us can move, at least during some stage

of our life cycle.

And most of us develop either two or three germ layers

during embryonic development,

wait for it...

...unless you're a sponge.

So like I said, we use this taxonomic system to describe

the common ancestry and evolutionary history

of an organism.

Looking at the phylogenetic tree, you can tell that humans are more

closely related to mice than we are to fish,

and more closely related to fish than we are to fruit flies.

So how about we pick an organism and follow it all the way through

the taxa, from kingdom to species, just to see how it works.

I know!

Let's pick this kitty.

Because I know she'd like it.

Right, cat?

So, kitties have cells that have nuclei and membrane

surrounded organelles.

And they're multicellular and heterotrophic and have

three germ layers of cells when they're embryos,

so they're in the kingdom Animalia.

And they have a spinal cord running down their backs,