One of Earth’s Earliest Fishes Probably Looked Like Floating Jell-O

Written by Joan W.
Updated: January 24, 2023
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To look back into ancient history and appreciate the evolution of life, the land, and the sea, you’ll want to imagine what the earth was like when life was just beginning. When one of the earth’s earliest fishes, the tiny Haikouichthys, were floating through oceans near what is now China, the earth was still very cold and only slowly warming.

During the Cambrian period (538.8 to 485.4 million years ago), before evidence of complex life appeared, our beautiful planet was cold and barren. Instead of several continents around the globe, there was one giant land mass that split into two during this period.

Almost all life on earth was relatively small and in the seas. Without many plants on land, dirt was lifeless and subject to erosion. Every once in a while, mudslides would pour sediment into the waters, burying marine organisms.

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The Ancient Origins of Modern Life

In this foreboding and primitive landscape, the variety of types of living creatures was accelerating faster than at any other time in history. As the world slowly warmed up, the precursors to life as we know it originated.

One of these critical links in the chain of life was a Jell-O-looking creature that offered more than meets the eye, the Haikouichthys. Why did the Haikouichthys stand out? It may have been the most ancient and primitive animal to have a backbone!

While many creatures that emerged during the Cambrian period had hard, protective outer shells, the Haikouichthys developed a bony structure on the inside. At only about one inch around, these tiny ancient early vertebrates may have been the biological forefather of fish, amphibians, reptiles, mammals, and humans.

Description and Size

Haikouichthys was a small, transparent fish that lived 520 million years ago, but there’s more to it than that. Although it was only about an inch in diameter and looked like jelly, it was one of the earliest organisms to have a skull. This tiny little fish had two eyes and a mouth on its head.

For a while, some scientists thought the infamous spine might not have actually been more than a primitive “notochord” running down its back. However, a 2003 discovery of 500 fossilized specimens confirmed that the Haikouichthys was a primitive vertebrate and did introduce some features that are commonplace today. 

For example, it was bilaterally balanced (the same on the left and right sides) and had a distinct head and tail. As a precursor to the dorsal fins on many modern fish, it had gill slits on its sides and a dorsal fin that ran from the back of its head and curved around its underside.

Diet: Primitive and Unique, Much Remains Unknown About Haikouichthys

Scientists believe the Haikouichthys was carnivorous, most likely a scavenger. They believe it was a scavenger rather than a hunter because the mouths of the Haikouichthys were not structured for biting or killing prey. 

With its developed musculature and flexible backbone, they would be able to scavenge the flesh of an injured sea creature and then quickly dart away unharmed. Their relative speed and tiny size created an advantage both in securing food and in protecting themselves from predators.

The Only Physical Evidence We Have Of Them Are Fossils

Fossils of these fish-ancestors are very rare and only found in the Yanshan region of China. Haikouichthys fossils were first presented to the world in 1999 by Chinese paleontologists Yu Liu and Hou Lianhai. 

Since these first two specimens were found near the city of Haikou, their name translates from Greek as “Haikou fish.”

While the first presentation only included two fossils, over 500 specimens have been discovered and examined since that time. It was very simple compared to the many fish we know today, with a small body and few internal organs laid out along its main axis like beads on a string. 

The skeleton was mostly made up of cartilage rather than bone. In fact, with its pale pinkish color, Haikouichthys probably looked like floating Jell-O.


Here, a fossil of

Haikouichthys ercaicunensis

 on display at the

Paleozoological Museum of China



Morosaurus millenii – Own work, CC BY-SA 4.0


Haikouichthys Lived in Shallow-Water Marine Environments Near China

Remains of Haikouichthys have only been found near China. They are believed to have lived in the warmer, shallow waters along the coastline.

While it is difficult for scientists to determine behaviors from the ancient, tiny fossils they are able to recover, they do surmise that Haikouichthys swam in shoals and were social animals. Collecting in groups would protect them from predators.

Extinction – When Did It Die Out?

If the theories are correct about the Haikouichthys’ contribution to the evolution of life on this planet, then they did not die out. Instead, they likely evolved into various creatures that, over millions of years, became recognizable animals alive today.

Despite Their Tiny Size, Haikouichthys Is an Evolutionary Giant

In the realm of science, there are few things more satisfying than finding a transitional form between two species. This is particularly true when it comes to the evolution of fish into tetrapods—the four-limbed animals we know today.

Haikouichthys is one such transitional form, having features that make it look like both a fish and a tetrapod. Its head is long and flat like that of a modern bony fish but possesses features that indicate that it had some sort of proto-jaw joint, similar to that found on land-dwelling vertebrates. These characteristics were previously unknown for this era in evolutionary history, making Haikouichthys an exciting discovery for paleontologists everywhere!

Haikouichthys is an evolutionary giant. While its body was only a few centimeters long and resembled something akin to Jell-O or a gummy bear, Haikouichthys was the first vertebrate to have a backbone—and it had one of the most complex brains for its size ever discovered among vertebrates.

Haikouichthys was the first of its kind. Despite how primitive it may seem in comparison, Haikouichthys is one of the most amazing fish that has ever existed. The fish was tiny, but despite its size, it may have been the ancestor of every single backbone animal on Earth.

Similar Animals to The Haikouichthys

Anomalocaris – A three-foot-long shrimp and jellyfish hybrid found in the oceans 500 million years ago.

Cephalaspis – A possible descendant of the Haikouichthys that vacuumed the prey it hunted.

Dunkleosteus – An ancient, extinct armored sea monster with a T-rex bite.

Tiktaalik – An ancient fishapod that could walk on land!

Elasmosaurus – An extinct giant reptile with an astonishingly long neck.

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The photo featured at the top of this post is ©

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About the Author

Joan Weisman is an experienced freelance marketing strategist and copywriter who helps small businesses expand their audience and stay top of mind with customers. She loves animals and dotes on her cat and dog.

FAQs (Frequently Asked Questions) 

Is Haikouichthys the first fish?

Haikouichthys was a prehistoric fish that lived 518 million years ago. It is important because it was the first fish in history and may also be the first animal with a backbone – the first vertebrate.

When did the Haikouichthys go extinct?

While the Haikouichthys does not exist on the planet anymore, scientists do not think they died out but instead evolved into other species, contributing to life on the planet today.

What did Haikouichthys evolve into?

This evolution has not been precisely mapped and confirmed, but some paleontologists suggest that Haikouichthys evolved into Cephalaspi (fish that vacuumed in its prey) and then into Hynerpeton (early four-limbed vertebrates).

What did the Haikouichthys eat?

Haikouichthys was a carnivore but lacked a jaw structure sufficient to hunt to harm prey. It was likely a scavenger that would bite injured sea creatures and then dart away to safety.

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