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It’s the one part of us that we think of as being synonymous with life.
If it stops working, we die.
It’s also the only part of us that we say we can give to someone else.
And if they break it, it’s the worst feeling in the world.
You can’t talk about either love or life without talking about the heart, and the story
of the muscle that’s pumping in your chest right now is an epic saga that goes far back
in deep time.
In fact, it goes back at least 520 million years, when an underwater landslide in ancient
China buried a tiny arthropod called Fuxianhuia protensa.
Fuxianhuia was small, no bigger than one of the credit cards in your wallet, with a segmented
body, an impressive pair of mandibles, and eyes on the ends of two stalks.
It had the tools of a swift ocean predator, which means it probably needed a lot of oxygen.
And this may explain why Fuxianhuia is the earliest animal known to have had a blood
circulatory system, as well as an organ to keep its blood moving: the first known traces
of a very early heart.
Now, the path that connects this ancient predator to you isn’t a short, straight line.
It’s long, with lots of side-tracks that lead to other, more familiar organisms that
also have hearts.
It includes animals like slugs and snails.
And insects and crustaceans.
And the first vertebrates to walk on four legs.
It took hundreds of millions of years, and countless different iterations of the same
basic structure to lead to the heart that you have today.
So, yes: You’re right to think that your heart is important and worth protecting.
Because the story of its origin is an ancient one, as old as animal life itself.
In order to understand where hearts came from, we have to go back to the earliest common
ancestor of everything that has a heart.
Now, we don’t have a fossil of that ancestor, but we do have some proof of its existence:
its genes.
We, and everything else that has a heart, carry some of the same genes that this organism
had.
Those genes are responsible for developing blood vessels that can contract to push blood
around.
And over time, an incredibly diverse group of animals has used this basic set of instructions
to arrive at different solutions for circulating blood.
We can tease apart the history of the heart by studying the genomes of living organisms
that share these heart-making genes.
And using that method known as the molecular clock -- which combines what we know about
genetic mutation rates with insights from well-dated fossils -- it seems that the common
ancestor of all animals with a heart lived way back in the Neoproterozoic Era, between
600 million and 700 million years ago.
But oddly enough, even though it gave rise to all of the organisms that have hearts,
this ancestor probably didn’t have a heart of its own.
So the genes that are used to form hearts today are actually older than the heart itself!
Instead of a heart, researchers believe that this ancestor used blood vessels to push blood
through its body.
This type of system is called a peristaltic pump, and it works kind of like your intestines
do, with muscles gradually squeezing to move material along.
Annelid worms and the marine chordates known as lancelets have both held on to this simple
model of blood circulation.
So scientists think this is probably what our ancestor had -- the simplest version of
a system that’s shared by all of its descendants.
Now, over evolutionary time, practically every animal that’s descended from that ancestor
has added complexity to this basic system.
And they did so independently, as natural selection acted time and again on that peristaltic
pump and the genes that make it possible.
The circulatory systems in these animals often became more complex, because they tended toward
more active lifestyles.
I mean, if you’re a worm, a peristaltic pump works just fine.
But for active predators and animals that need to move quickly, that just wasn’t effective
enough.
To supply a more complex and more active body with more oxygen, the solution for most animals
was a heart.
Now, not all animals today have what we’d call a true heart.
But some version of an organ for circulating blood has evolved in at least three major
groups -- the arthropods, the mollusks, and us vertebrates.
The arthropod version of a “heart” is really a long, tube-like structure that runs
the length of the animal’s body called a dorsal vessel.
Its job is to collect blood and propel it in one direction: toward the head.
It doesn’t look much like a heart, and not all researchers are willing to call it one.
But it performs a comparable function.
And that dorsal vessel has been around for a long time.
We know that, because we’ve seen it in those fossils of Fuxianhuia.
In 2014, researchers studied a specimen that was so well preserved that they could actually
make out traces of its dorsal vessel.
And the scientists were able to reconstruct it, based on its similarities with the hearts
of modern arthropods.
So we know Fuxianhuia had this dorsal vessel, but it’s not clear when this structure first
evolved in arthropods - or if it evolved more than once.
There’s still a healthy debate among researchers about when exactly different lineages gained
or lost their blood-pumping organs.
Because, different kinds of arthropods have dorsal vessels that are really different from
each other.
And, some don't have any at all!
So this makes it hard to say if these vessels are a single, ancestral feature, or something
that showed up on its own, convergently, many times.
Still, we do know that the earliest arthropods go back not long before Fuxianhuia, to about
542 million years ago, at the verrry end of the Ediacaran Period.
So it’s safe to assume that their version of a not-quite-heart is at least that old.
And this same reasoning applies to mollusks.
This group includes gastropods, like snails and slugs; cephalopods, like squid and octopodes;
and bivalves, like clams and mussels, among many others.
All of these animals have hearts, so it’s likely that their common ancestor had a heart
too.
And the ancestor of all mollusks goes even further back in the Ediacaran Period, to a
fossil of a squishy little seafloor dweller known as Kimberella.
This animal is sometimes interpreted as a mollusk.
And if it is one, then the common ancestor of mollusks, and its ancestral heart, has
to be at least as old as it and the other earliest fossil mollusks.
And that’s at least 550 million years old.
Now, unlike arthropods, mollusks have hearts with multiple chambers.
Generally, one or more chambers collect oxygenated blood from the gills.
These are called auricles.
Then they feed the oxygen-rich blood into another chamber, called a ventricle, which
pumps the blood out to the rest of the body.
And the more complex the mollusk, the more complex its heart is.
For example, in addition to a central chambered heart, most cephalopods like cuttlefish, octopodes,
and squid have two extra branchial hearts that pump blood to their gills.
Three hearts might seem like a lot, but they may have played a key role in helping some
cephalopods develop a more active lifestyle.
Now it’s our time!
We vertebrates have also evolved more complex hearts, but we followed a different path than
the mollusks.
Molecular clock studies suggest that some of the genes responsible for creating heart
muscle were evolving in vertebrates, and our closest relatives, from about 540 million
to 570 million years ago.
Meanwhile, the genes that create the specialized tissue that lines our blood vessels, known
as endothelium, date back between 510 million to 540 million years ago.
So it seems likely that, by the end of the Ediacaran Period, the vertebrate circulatory
system was also well on its way.
The earliest fossil we have of a vertebrate heart belongs to a fish called Rhacolepis,
and it has two chambers, as all fish hearts do.
But Rhacolepis lived only about 115 million years ago.
So while it’s the earliest physical evidence we have, it’s much too recent to tell us
much about the early evolutionary path our hearts have taken.
The hearts of our earliest vertebrate ancestors probably were kind of like that found in Rhacolepis,
in that it likely had two chambers: an atrium to collect blood, and a ventricle to push
it back out to the body.
And we think that because that’s how the hearts of vertebrates, including yours, start
out during embryonic development.
All vertebrate hearts develop according to a common pattern: In the embryo, a blood vessel
twists in on itself to create two chambers, and then develops depending on what kind of
organism it is and what other kinds of genetic instructions it has.
But the fact that every vertebrate develops a heart in this same way suggests that a two-chambered
plan is shared among us all.
And again, vertebrate hearts became more sophisticated, as the animals themselves became more complex
and more active.
Amphibians, for example, first appear in the fossil record around 360 million years ago,
and today, most of them have two atria, one for collecting oxygenated blood and one for
collecting deoxygenated blood, as well as a single ventricle.
But some of the more basal, or primitive, living amphibians have chambers that aren’t
totally separated, sort of like they have two and a half chambers.
So their common ancestor probably didn’t have three well-defined chambers, either,
and instead, that came later.
Mammals, meanwhile, have four chambers, and although our shared mammalian ancestor appeared
somewhere around 200 million years ago, it’s not totally clear when we acquired our four-chambered
heart.
In the mammalian heart, the right atrium and ventricle collect deoxygenated blood and send
it to the lungs, and the left atrium and ventricle collect the oxygenated blood from the lungs
and send it out to the body.
So, arthropods, mollusks and vertebrates all separately acquired some version of a heart
within the same general stretch of geologic time, in the Late Ediacaran Period and the
early Cambrian.
But even though all of these hearts look quite different, they’re all just variations on
a theme -- derived from the same basic set of genetic instructions that dates back to
that common ancestor that lived more than 600 million years ago.
And believe it or not, we can even track the evolution of those individual, ancient genes
that make our hearts possible.
In 1993, a researcher in Michigan found that the dorsal vessel in fruit flies develops
with the help of a gene known as tinman.
Y’know.
Like from the Wizard of Oz.
Because without a tinman gene, the flies don’t have a heart?
This gene instructs developing tissues to differentiate into various kinds of cardiac
cells.
And later studies found that we vertebrates have our own equivalent of the tinman gene.
And It turns out that our gene is similar enough to tinman, that researchers believe
they both evolved from a single, earlier gene that was present in that common ancestor that
lived more than 600 million years ago.
In humans, this gene is given the less-punny name of Nkx2-5, and just like in fruit flies,
it instructs the developing tissue that it’s expressed in, to generate the many different
kinds of cells that make a heart.
So, even though the dorsal vessel of a fly and the heart that’s beating in your chest
right now don’t seem to have a lot in common, we have our 600-million-year-old ancestor
to thank for the genes that make both of them.
Over vast stretches of geologic time, natural selection has acted on this genetic legacy
over and over, as animals became more complex.
The result is the variety of systems that we find today, in fruit flies, and cuttlefish,
and you -- as well as in Fuxianhuia, a humble arthropod from the distant past.
Like every other part of you, your heart is as complex as the story behind it.
So, the next time you give you heart away to someone, be sure that they know that your
gift is more than a half-billion years in the making.
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