Scientists 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. Many of their fossils are covered in black smears. These are 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 human: blood. The story of blood is convoluted. It is because it’s one of the most revolutionary features of evolutionary history. As time went on and conditions varied, how blood did those jobs has changed over and over again.
How did blood evolve?
After hundreds of millions of years, the blood separates us from Marrella. Humans and animals have familiar with red blood. But we also have blue, purple, green, and even white blood! The tale of how we got from black stains in rock to the blood in veins is just one example of how in the world of constant change. And the evolutionary response is always fluid.
Blood does a lot of things. It supplies oxygen to tissues and carries nutrients to cells, and removes waste. But not all animals 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. It means that the earliest animals on Earth probably didn’t need blood, either, because they were slow-moving or straightforward enough that they could use this diffusion to move materials around.
Once animals became more complex and more active, another system was required: some sort of system for circulating blood. Now, the split between less difficult animals like sponges, jellies, 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 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 researchers have some idea of what that organism might have looked like because they 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. 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 acoelomorphas: flat, worm-like animals. So our earliest blood-bearing ancestor might have looked a lot like them. 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 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 globin. A globin molecule has a particular 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. This fold is so distinct and so good at holding onto and releasing the oxygen that it’s been used in many different forms by many other organisms to do various jobs over the eons.
Today, in many animals, including humans, blood carries oxygen around the body with the help of a protein called hemoglobin. Hemoglobin is what gives 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 versions of oxygen-binding proteins.
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 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. 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. 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. 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 the proof can be found in our old friend Marrella. In 2014, scientists analyzed those weird stains on the Marrella fossils. They found that these fossils 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 human blood is.
Instead, it contained copper. Marella is the earliest organism to use copper rather than iron. And rather than hemoglobin, Marrella probably used a different protein called hemocyanin. Hemocyanins seem to have evolved independently of hemoglobin, using a different kind of metal to carry oxygen and developing a different protein structure. These proteins probably didn’t evolve from the globin fold but instead were adapted from some sort of enzyme.
It turns out that the genetic sequence of the hemocyanins found in mollusks is different from that found in arthropods. 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 oxygen transport proteins?
It could be because Hemocyanin works a little better in colder temperatures, even though hemoglobin is more efficient. And some organisms have retained both kinds 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 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. 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. It is produced when hemoglobin is broken down, and having a lot of this stuff might make their blood more resistant to disease. 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 blood to clot too quickly 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 with 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, wave after wave of convergent evolution gave rise to the blood of many kinds and many colors.
Blood types evolution
There’s a lot of mystery around blood types. Eight main blood groups make up most of the world’s population: A, B, AB, and O group, and a negative and positive for each. Scientists still research why blood evolved different types. But for now, science can at least tell a little bit about blood.
The positive or negative symbol that comes after ABO blood type is based on the Rhesus Blood Group System. Rhesus blood grouping is similar to the ABO system because it also has to do with the presence or absence of an antigen outside the red blood cell. In this case, the protein-based Rh antigen. This becomes important again when it comes to blood transfusions. A body with Rh-negative blood will reject Rh-positive blood.
Many experts hypothesize that they developed to help fend off disease, but it remains a hypothesis since this all happened millions of years ago. It all comes down to our genes. The genetic information parents pass down helps determine what color hair is, how susceptible to a disease, or how tall or short. It is the same for blood type, and it all comes down to the ABO gene, which has three different versions or alleles: A, B, and O.
Each parent has two of these alleles because they got one from their parents and passed one down to their child. This all comes together and gets encoded in DNA to create the blueprint. It is the process that the body will use to make blood. DNA will instruct the enzymes to build A antigens, B antigens, or both, none depending on the inherited allele combination when new blood is made.
Scientists recently discovered that enzymes found in the gut, when added to blood, could strip away the sugar-based antigens on the cell’s surface. That would effectively change type A or B to type O, known as the universal donor type, since it can be given to A, B, AB, or O patients. They have succeeded at finding an enzyme in some human gut bacteria that can convert blood. But of course, it’s more complicated than that.
For thousands of years, nobody understood blood. A Greek doctor from 200 CE believed that it was created from food and liver. And this school of thought lived on for nearly 1500 years. It wasn’t until the early 17th century that a British doctor named William Harvey discovered that blood circulated through the body. It spawned a new age of experimentation with blood.
In 1665, an English physician successfully kept one dog alive by transfusing it with the blood of another dog. But then things got kind of weird. Just two years later, doctors began experimenting with xenotransfusions. That is, transfusing humans with animals’ blood, such as sheep. And those human patients died. It wasn’t until 1900 that we finally realized people and animals have different types of blood that determine whose blood can mix with whose. That’s where those other letters come into play.
- Type A blood, immune system will perceive.
- Type B blood as an intruder, and trigger an auto-immune response. That can cause kidney failure, extensive blood clotting, and even shock.
- O blood has no antigens but has A and B antibodies. Also known as the universal donor type.
- AB blood, however, can accept both A and B blood without triggering that auto-immune response.
Things start to get a little more complicated when scientists introduce the rhesus factor or the negative and positive part of your blood type. Positives can accept negatives, but the opposite is extremely dangerous. And to further complicate things, scientists have discovered dozens of more blood types, such as the Duffy blood group, which can determine susceptibility to malaria. Or the Hh blood type, which 1 in 10,000 people in India has. But the vast majority of humans fall into this A, B, O system.
As for why humans evolved this complicated system of blood types and compatibility, scientists still research. The original mutations are thought to date back nearly 20 million years. But look, whatever the biology is behind blood typing, it’s a real, practical thing that matters. And in many parts of the world, knowing blood type is fairly common knowledge. In Japan, it’s linked to personality.
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Drosophila as a Genetic Model for Hematopoiesis.
The Evolving Erythrocyte: Red Blood Cells as Modulators of Innate Immunity.
Electron-microscopic observations of normal coelomocytes from the earthworm, Lumbricus terrestris.