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  • 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.

  • Theyre more than 500 million years old, and theyre 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 alwaysfluid.

  • 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 theyre 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 theyre 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!

  • Now, what do you want to learn about?

  • Leave us a comment, and don’t forget to go to youtube.com/eons and subscribe.

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

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血液如何进化(How Blood Evolved (Many Times))

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    joey joey 發佈於 2021 年 05 月 03 日
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