Researchers have discovered a 380-million-year-old heart preserved inside a fossilized prehistoric fish.
They say the specimen records a key moment in the evolution of the blood-pumping organ found in all animals with bones, including humans.
The heart belonged to a fish known as Gogo, which is now extinct.
The “exciting” discovery, published in the journal Science, was made in Western Australia.
Lead scientist Professor Kate Trinajstic from Curtin University in Perth told BBC News of the moment she and her colleagues realized they had made the biggest discovery of their lives.
“We were huddled around the computer and we recognized that we had a heart and we almost couldn’t believe it! It was incredibly exciting,” he said.
Usually, bones rather than soft tissues turn into fossils – but at this site in the Kimberley, known as the Gogo Rock Formation – the fossils have preserved many of the fish’s internal organs, including the liver, stomach, intestine and heart.
“This is a critical moment for our own evolution,” said Professor Trinajstic.
“It shows the body plan that we evolved very early, and we’re seeing it for the first time in these fossils.”
Her collaborator, Professor John Long from Flinders University in Adelaide, described the find as “a shocking, shocking discovery”.
“We never knew anything about the soft organs of animals that long ago, until now,” he said.
The gogo fish is the first of a class of prehistoric fish called placoderms. These were the first fish to have jaws and teeth. Before them, fish were no longer than 30 centimeters, but placoderms could reach 29.5 feet (9 meters) in length.
Placoderms were the planet’s dominant life form for 60 million years, and were around more than 100 million years before the first dinosaurs walked the Earth.
Scans of the Gogo fish fossil showed that its heart was more complex than expected for these primitive fish. It had two chambers on top of each other, similar in structure to the human heart.
The researchers suggest that this made the animal’s heart more efficient and was the critical step that transformed it from a slow-moving fish to a fast-moving predator.
“This was how they could climb the ant and become a voracious predator,” Professor Long said.
The other important observation was that the heart was much further forward in the body than those of more primitive fish.
This position is thought to have been linked to the development of the Gogo fish’s neck and made room for the development of lungs further down the evolutionary line.
Dr Zerina Johanson of London’s Natural History Museum, who is a world leader in placoderms and is independent of Professor Trinajstic’s team, described the research as an “extremely important breakthrough” that helps explain why the human body is the way it is today .
“Many of the things you see we still have in our bodies; jaws and teeth, for example. We have the first appearance of front flippers and fins on the back, which eventually evolved into our arms and legs.
“There are many things that happen in these placoderms that we see developing in ourselves today, such as the neck, the shape and arrangement of the heart, and its location in the body.”
The discovery completes an important step in the evolution of life on Earth, according to Dr Martin Brazeau, a placoderm specialist at Imperial College London, who is also independent of the Australian research team.
“It’s really exciting to see this result,” he told BBC News.
“The fish that my colleagues and I study are part of our evolution. This is part of the evolution of humans and other animals that live on land and fish that live in the sea today.”
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