How Blood Evolved (Many Times)


In the famous formation in British Columbia
known as the Burgess Shale, we have found more 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
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