than 30,000 fossils of a little armored arthropod called Marrella splendens.

They're more than 500 million years old, and they're beautifully preserved, from

the tiny segments on their abdomens to the strange, curved appendages on their heads.

But one thing mars the beauty of these ancient animals: Many of their fossils are covered

in black smears, often found at either the head or the tail end of the body.

No other organism in the Burgess Shale has these weird black stains.

Those blotches turn out to be the earliest evidence in the fossil record of a substance

that almost all animals have in common with Marrella -- including us: blood.

And the story of blood is convoluted.

Because it's one of the most revolutionary features of our evolutionary history -- eventually

allowing nutrients and wastes to be carried around our bodies as we became more complex,

and more active.

But as time went on and conditions varied, the way in which blood did those jobs has

changed over and over again.

So, today, after the hundreds of millions of years that separate us from Marrella, we

animals have our familiar red blood.

But we also have blue blood.

And purple, and green, and even white.

The tale of how we got from black stains in rock to the blood in our veins is just one

example of how, in a world of constant change, the evolutionary response is always … fluid.

Blood does a lot of things.

It supplies oxygen to tissues.

It carries nutrients to cells and removes waste.

But not all animals actually need blood.

Some, like sponges, sea anemones, and jellies, have body tissues that are so thin that oxygen

can diffuse directly from the ocean water into their cells.

This means that the earliest animals on Earth probably didn't need blood, either, because

they were simple or slow-moving enough that they could use this diffusion to move materials

around.

But once animals became more complex and more active, another system was required: some

sort of system for circulating blood.

Now, the split between less complex animals --like sponges, jellies, and ctenophores -- and

every other kind of animal is one of the oldest evolutionary branching points in the entire

animal kingdom, taking place sometime in the late Proterozoic Eon.

So that means the common ancestor of all organisms with some kind of blood circulatory system

is thought to have lived more than 600 million years ago -- long before Marrella existed.

Unfortunately, there's no fossil of this ancestor that was the first to have a circulatory

system.

But we have some idea of what that organism might have looked like, because we know what

all living organisms with a blood circulatory system look like.

And they all share some important features.

Like, they all have bilateral symmetry, meaning they have two symmetrical sides, like you

and I do, rather than many sides, like a jelly or a sea-star.

And they pretty much all have an internal body cavity.

Most of them use it to support and cushion their internal organs, although one or two

animals have lost it over time.

Today, the simplest organisms that have these traits are the acoelomorphs: flat, worm-like

animals.

So our earliest blood-bearing ancestor might have looked a lot like them.

Now, we don't know exactly what the earliest blood was like, either.

But genetic researchers believe that some early forms of blood probably used the same

basic chemical model that many forms of blood use today.

Specifically, it probably worked with the help of special proteins.

These proteins probably served different purposes at first, like metabolizing nitric oxide or

trapping oxygen to keep it away from other tissues.

But in time, they were co-opted to perform another task -- to transport oxygen.

So, blood proteins are actually older than blood itself!

Molecular clock studies into the genes that code for them show that some blood proteins

may have evolved as much as 740 million years ago!

Today, for many animals, the blood protein of choice is a globin.

A globin molecule has a special prong on it that binds to an atom of iron, which in turn

is surrounded by a donut-shaped molecule called heme.

And on the opposite side of the donut, a molecule of oxygen can bind to the iron.

The basic protein structure that cradles this heme donut is called the globin fold.

And this fold is so distinct, and so good at holding onto and releasing oxygen, that

it's been used in many different forms, by many different organisms to do a variety

of jobs over the eons.

Today, in many animals, including you, blood carries oxygen around the body with the help

of a protein called hemoglobin.

Hemoglobin is what gives your blood its rich red color - that's the iron molecule inside.

But different kinds of hemoglobins have evolved in different kinds of animals: flatworms,

nematodes, arthropods, mollusks, and other animals have their own versions of oxygen-binding

proteins.

And they don't use them in quite the same way.

For example, we use hemoglobin to transport oxygen from our lungs to our various tissues.

But certain species of clams can use hemoglobin to store oxygen for their nerves to use when

oxygen is scarce.

And one type of nematode keeps a store of hemoglobin in the lining of its mouth to help

its mouthparts get enough oxygen to keep feeding in even low-oxygen conditions.

Even the bacterium E. coli has an especially strange version that seems to sense, rather

than transport, oxygen.

And as proteins go, hemoglobin is a molecule with an incredibly long history.

Some of the oldest confirmed hemoglobin in the fossil record is from exactly the organism

you might guess: a mosquito.

A 46 million-year-old mosquito was found fossilized in shale from Montana, and when scientists

probed its stomach in 2013, they didn't find the makings of Eocene Park.

Instead, they found chunks of hemes, presumably decomposed pieces of hemoglobin.

But hemoglobin is much older than this mosquito.

For example, the type that we use is specific to vertebrates, and according to molecular

clock studies, it's probably about as old as jawed vertebrates themselves, which date

back 450 million years.

Now, hemoglobin isn't the only blood protein that has evolved.

And proof can be found in our old friend Marrella.

In 2014, scientists analyzed those weird stains on the Marrella fossils, and found that they

were enriched with metal, compared to the rest of the rock.

But strangely, the metal that Marrella's blood was enriched with wasn't iron, like

our blood is.

Instead, it contained copper.

Marella is the earliest organism we know of to use copper rather than iron.

And rather than hemoglobin, Marrella probably used a different protein called hemocyanin.

Hemocyanins seem to have evolved totally independently of hemoglobin, not only using a different

kind of metal to carry oxygen, but also developing a different protein structure.

And these proteins probably didn't evolve from the globin fold, but instead were adapted

from some sort of enzyme.

And it turns out that the genetic sequence of the hemocyanins found in mollusks is totally

different from that found in arthropods.

And they're so different that scientists think mollusks and arthropods probably evolved

hemocyanin at totally different times -- the mollusk version around 740 million years ago,

and its arthropod counterpart 600 million years ago.

So hemocyanin is old, and the fact that both mollusks and arthropods have copper-bearing

blood proteins appears to be a feature of convergent evolution.

By the way, these hemocyanins are why horseshoe crabs have blue blood -- because copper turns

greenish blue when it's oxidized.

So Marrella's blood was probably blue, too.

But, if hemoglobin is good enough for us, why did mollusks and arthropods evolve their

own oxygen transport proteins?

This could be because Hemocyanin works a little better in colder temperatures, even though

hemoglobin is more efficient.

And some organisms have actually retained both kind of proteins, perhaps to provide

flexibility in case their environment changes radically.

So, Hemocyanin and Hemoglobin are the most common oxygen-carrying blood proteins found

in animals today, and they're the ones we know the most about.

But they aren't the only ones!

Many species of marine worms and brachiopods, for instance, use a totally different blood

protein hemerythrin.

It uses iron to transport oxygen, too, but it doesn't have that donut-shaped heme.

Because of this, the blood in those animals turns a bright violet when it's oxygenated.

And like hemocyanin, this protein is less efficient, but it's also simpler -- so simple,

in fact, that it's thought to have been used by the very earliest single-celled organisms.

Blood can also be green, too!

Some animals, like certain species of lizards, have a lime-green pigment in their blood called

biliverdin, which is produced when hemoglobin is broken down, and having a lot of this stuff

might actually make their blood more resistant to disease.

And other animals have even lost their blood proteins entirely, like the aptly-named Ice

Fish, which lives off the coast of Antarctica.

Its blood is a clearish white because, unlike other fish, it doesn't have any hemoglobin

or other proteins, at all.

That might be because having blood cells would cause its blood to clot too easily in such

cold temperatures.

Or maybe it was just a genetic accident.

But even without blood proteins, the Ice Fish gets along by having a low metabolism and

living in oxygen-rich waters.

So, the history of blood goes back hundreds of millions of years, connecting us to Marrella

and the even older ancestor of all organisms that have a circulatory system of some kind.

And the proteins that our blood use go back even further, practically to the dawn of complex

life itself.

Between that time in the deep past and today, there occurred wave after wave of convergent

evolution, giving rise to bloods of many kinds and many colors.

Thanks as always for joining me today, and extra big thanks to our current Eontologists,

Jake Hart, Jon Ivy, John Davison Ng and everybody's favorite hominin, STEVE!

If you want to join them and maybe have me mispronounce your name too

You can go to patreon.com/eons to make your pledge!

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