Fish as we know them are the result of over half a billion years of evolution. They were the first vertebrates on the planet, making them our very distant ancestors. That also means these animals provided the basic body plan for millions of years of evolution. Fish have adapted into tens of thousands of different living species. Along the way, they survived the biggest mass extinctions in Earth’s history.
At around 530 million years old, the Pikaia may have been the oldest known ancestor of modern vertebrates. It had a head distinct from the tail, bilateral symmetry, v-shaped muscles and a nerve cord running down the length of its body. The Haikouichthys, with eyes and a defined skull, may have been the earliest jawless fish. Along with the Myllokunmingia, these could be the first animals to have skulls. The Myllokunmingia was also the first known animal to have gill pouches.
Reconstructions of three Cambrian proto-fish
Left: Pikaia reconstruction taken by Citron at the Muséum national d'histoire naturelle
Middle: Haikouichthys reconstruction by Nobu Tamura of newdinosaurs.com
Right: Myllokunmingia reconstruction by Degan Shu of Northwest University, Xi'an, China
From 490 to 410 million years ago, the waters were dominated by jawless fish. Without lower jaws, they couldn’t consume larger prey. Many of these fish also had bony armour plates. The Astraspis and Arandaspis were big-headed finless fish that resembled giant tadpoles. With poor mobility and no lower jaw, they were bottom feeders who needed bulky armour to protect against large anthropods. Their descendants in the Silurian Period added forked tail fins for extra manoeuvrability.
Arandaspis reconstruction by Nobu Tamura of newdinosaurs.com
During the end of the Ordovician and beginning of the Silurian periods, the Earth went through its second biggest extinction events in history. These wiped out up to 70% of all species. The higher sea levels and warm shallow continental seas were hospitable for new marine life. There were many evolutionary milestones in this period. Placoderms were another class. They were armoured fish with movable jaws. This era also introduced spiny sharks and Guiyu oneiros, the earliest known bony fish.
Guiyu oneiros reconstruction by Arthur Weasley
From 416 to 358 million years ago, fish evolution diversified greatly. This is why the Devonian Period is called the ‘Age of Fishes’. Placoderms dominated this period, but didn’t have much life after it. Chondrichthyes, which includes sharks, rays and chimaeras, outclassed them, probably because of their agility.
Along with that were the Osteichthyans, fish with bony skeletons. They split off into ray-finned and lobed-finned species. Today, ray-finned are the most diverse and numerous vertebrates on Earth. The lobe-finned fish transitioned into tetrapods thanks to their fin structure. They became the ancestors of all land-living vertebrates, including humans. This is the stage of evolution where anglers (from lobe-finned fish) split off from their catches (ray-finned fish).
Three types of Devonian fish: a Placoderm, a lobe-finned bony fish and a shark
Left: Dunkleosteus fossil taken by James St. John at the Cleveland Museum of Natural History
Middle: Panderichthys reconstruction by Tyler Rhodes
Right: Cladoselache reconstruction by Nobu Tamura of newdinosaurs.com
In the Carboniferous Period, the decline of placoderms was a likely cause of evolutionary radiation for sharks. They found new environmental niches and adopted a variety of bizarre shapes. The Stethacanthus had a flat, serrated, brush-like dorsal fin. The Permian Period ended with the Permian-Triassic event. This was the largest mass extinction in Earth’s history. It wiped out around 96% of all marine species.
Stethacanthus reconstruction by DiBgd
After the mass extinction, evolution was effectively reset. For reasons we still don’t know, fish with cartilage skeletons suffered the most while bony fish were able to recover, and even diversify. One example is the Leedsichthys, the largest known bony fish. This is also when sturgeons appear in the fossil record. Having not evolved much since the Triassic, they’re informally called living fossils. Along with fish, the seas were eventually populated by marine reptiles like pliosaurs, crocodiles and turtles.
Leedsichthys reconstruction with a human for size comparison by Dmitry Bogdanov
From about 28 to 1.5 million years ago, the Megalodon came and went. It resembled a bigger version of the great white shark, with fossil lengths reaching 67 ft. Fossil evidence shows it fed on large whales, biting their fins first so they couldn’t escape. 3D digital models of its x-ray also suggest the Megalodon had more biting power than a T-rex!
Megalodon size comparison with a human by Matt Martyniuk
Despite that, the aquatic ecosystem has outlived its fiercest predator and even thrived. Over half of all living vertebrate species (about 32,000 species) are fishes. Today, they inhabit bodies of water from depths of 36,000 ft. in the Challenger Deep to elevations of 15,100 ft. in the Himalayan lakes. To find out more about the aquatic ecosystem today, check out our blog on Blue Planet II.