The heart is a complicated electrical system that generates its electrical signal and runs the pumps. It causes them to contract and relax in a specific timing sequence. Either it is speeding up or slowing down the electrical impulses responsible for each heartbeat. If the heart beats too slowly, it will not pump enough blood to meet the body’s demands. If the heart beats too quickly, it will not have enough time between contractions to fill and will fail to pump enough blood to the body. As an essential part of the body, what is the origin of the heart?
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. 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.
The path that connects this ancient predator is long, with many side-tracks leading to other, more familiar organisms with hearts. It includes animals like slugs, snails, 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.
How did the heart evolve?
The history of the heart’s origin is an ancient one, as old as animal life itself. To understand where hearts came from, we have to go back to the earliest common ancestor of everything with a heart. Scientists don’t have a fossil of that ancestor but have some proof of its existence: its genes. It is because the heart carries some of the same genes that this organism had. Those genes are responsible for developing blood vessels that can contract to push blood around.
How was the heart shape created? Over time, an incredibly diverse group of animals has used this basic set of instructions to arrive at different solutions for circulating blood. Researchers 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. It combines 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. Even though it gave rise to all organisms with hearts, this ancestor probably didn’t have a heart of its own.
So the genes that are used to form hearts today are 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 like intestines do, with muscles gradually squeezing to move material along. Annelid worms and the marine chordates are known as lancelets, have both held on to this simple model of blood circulation.
So scientists think this is what ancestor had the simplest version of a system shared by all of its descendants. Over evolutionary time, practically every animal descended from that ancestor has added complexity to this basic system. They did so independently, as natural selection repeatedly acted 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.
For 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. Not all animals today have a genuine heart. But some version of an organ for circulating blood has evolved in at least three major groups:
The arthropod version of a “heart” is 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.
In 2014, researchers studied a specimen that was so well preserved that they could make out traces of its dorsal vessel. The scientists were able to reconstruct it based on its similarities with the hearts of modern arthropods. So Fuxianhuia had this dorsal vessel, but it’s not clear when this structure first evolved in arthropods. 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 different from each other. 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.
This group includes gastropods (snails and slugs), cephalopods (squid and octopodes), and bivalves (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 tiny seafloor dweller known as Kimberella. This animal is sometimes interpreted as a mollusk. And if it is one, then the common ancestor of mollusks. Its ancestral heart has to be at least as old as it and the other earliest fossil mollusks. That’s at least 550 million years old.
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. It 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.
Vertebrates have also evolved more complex hearts. Molecular clock studies suggest that some of the genes responsible for creating heart muscle evolved vertebrates from about 540 million to 570 million years ago. Meanwhile, the genes that make the specialized tissue that lines blood vessels, known as the endothelium, date back between 510 million to 540 million years ago. By the end of the Ediacaran Period, the vertebrate circulatory system was also well on its way. The earliest fossil 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.
The hearts of the 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. That’s how the hearts of vertebrates start 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. Then it develops depending on what kind of organism it is and what other types of genetic instructions it has.
Every vertebrate develops a heart that suggests that a two-chambered plan is shared among us all. 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.
Some of the more basal or primitive living amphibians have chambers that aren’t 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 have four chambers, and although shared, mammalian ancestors appeared somewhere around 200 million years ago.
- In the mammalian heart, the right atrium and ventricle collect deoxygenated blood and send it to the lungs. 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.
In 1993, a researcher in Michigan found that the dorsal vessel in fruit flies develops with the help of a gene known as tinman. 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 vertebrates have the equivalent of the tinman gene. And It turns out that our gene is similar enough to tinman. Researchers believe that they both evolved from a single, earlier gene 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. It instructs the developing tissue. To generate the many different kinds of cells that make a heart. However, the dorsal vessel of a fly and the present heart doesn’t seem to have a lot in common with 600-million-year-old ancestors.
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 fruit flies, cuttlefish, and Fuxianhuia, a humble arthropod from the distant past.
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