10 Steps of Animal Evolution

From Fish to Primates

Plesiosaur in water
Plesiosaur, a marine reptile.

Mark Garlick / Getty Images

Vertebrate animals have come a long way since their tiny, translucent ancestors swam the world's seas over 500 million years ago. The following is a roughly chronological survey of the major vertebrate animal groups, ranging from fish to amphibians to mammals, with some notable extinct reptile lineages (including archosaurs, dinosaurs, and pterosaurs) in between.

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Fish and Sharks

Diplomystus fossil

Paul Kay / Getty Images

Between 500 and 400 million years ago, vertebrate life on earth was dominated by prehistoric fish. With their bilaterally symmetric body plans, V-shaped muscles, and notochords (protected nerve chords) running down the lengths of their bodies, ocean dwellers like Pikaia and Myllokunmingia established the template for later vertebrate evolution It also didn't hurt that the heads of these fish were distinct from their tails, another surprisingly basic innovation that arose during the Cambrian period. The first prehistoric sharks evolved from their fish forebears about 420 million years ago and quickly swam to the apex of the undersea food chain.

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Model reconstruction of Acanthostega at State Museum of Natural History in Stuttgart (Germany)

Dr. Günter Bechly / Wikimedia Commons / CC BY-SA 3.0

The proverbial "fish out of water," tetrapods were the first vertebrate animals to climb out of the sea and colonize dry (or at least swampy) land, a key evolutionary transition that occurred somewhere between 400 and 350 million years ago, during the Devonian period. Crucially, the first tetrapods descended from lobe-finned, rather than ray-finned fish, which possessed the characteristic skeletal structure that morphed into the fingers, claws, and paws of later vertebrates. Oddly enough, some of the first tetrapods had seven or eight toes on their hands and feet instead of the usual five, and thus wound up as evolutionary "dead ends."

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Solenodonsaurus janenschi

Dmitry Bogdanov / Wikimedia Commons / CC BY 3.0

During the Carboniferous period, dating from about 360 to 300 million years ago, terrestrial vertebrate life on earth was dominated by prehistoric amphibians. Unfairly considered a mere evolutionary way-station between earlier tetrapods and later reptiles, amphibians were crucially important in their own right, since they were the first vertebrates to figure out a way to colonize dry land. However, these animals still needed to lay their eggs in water, which severely limited their ability to penetrate to the interior of the world's continents. Today, amphibians are represented by frogs, toads, and salamanders, and their populations are rapidly dwindling under environmental stress.

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Terrestrial Reptiles

Hylonomus lyelli

​Matteo De Stefano/MUSE / Wikimedia Commons / CC BY-SA 3.0

About 320 million years ago, give or take a few million years, the first true reptiles evolved from amphibians. With their scaly skin and semi-permeable eggs, these ancestral reptiles were free to leave rivers, lakes, and oceans behind and venture deep into dry land. The earth's landmasses were quickly populated by pelycosaurs, archosaurs (including prehistoric crocodiles), anapsids (including prehistoric turtles), prehistoric snakes, and therapsids (the "mammal-like reptiles" that later evolved into the first mammals). During the late Triassic period, two-legged archosaurs spawned the first dinosaurs, the descendants of which ruled the planet until the end of the Mesozoic Era 175 million years later.

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Marine Reptiles

Plesiosaurus dolichodeirus
Dmitry Bogdanov / Wikimedia Commons / CC BY 3.0

At least some of the ancestral reptiles of the Carboniferous period led partly (or mostly) aquatic lifestyles, but the true age of marine reptiles didn't begin until the appearance of the ichthyosaurs ("fish lizards") during the early to middle Triassic period. These ichthyosaurs, which evolved from land-dwelling ancestors, overlapped with, and were then succeeded by long-necked plesiosaurs and pliosaurs, which themselves overlapped with, and were then succeeded by the exceptionally sleek, vicious mosasaurs of the late Cretaceous period. All of these marine reptiles went extinct 65 million years ago, along with their terrestrial dinosaur and pterosaur cousins, in the wake of the K/T meteor impact.

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Pteranodon dinosaurs flying - 3D render

Elenarts / Getty Images

Often mistakenly referred to as dinosaurs, pterosaurs ("winged lizards") were actually a distinct family of skin-winged reptiles that evolved from a population of archosaurs during the early to middle Triassic period. The pterosaurs of the early Mesozoic Era were fairly small, but some truly gigantic genera (such as the 200-pound Quetzalcoatlus) dominated the late Cretaceous skies. Like their dinosaur and marine reptile cousins, the pterosaurs went extinct 65 million years ago. Contrary to popular belief, they didn't evolve into birds, an honor that belonged to the small, feathered theropod dinosaurs of the Jurassic and Cretaceous periods.

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Hesperornis regalis skeleton at the Smithsonian museum of Natural History

Quadell / Wikimedia Commons / CC BY-SA 3.0

It's difficult to pin down the exact moment when the first true prehistoric birds evolved from their feathered dinosaur forebears. Most paleontologists point to the late Jurassic period, about 150 million years ago, on the evidence of distinctly bird-like dinosaurs like Archaeopteryx and Epidexipteryx. However, it's possible that birds evolved multiple times during the Mesozoic Era, most recently from the small, feathered theropods (sometimes called "dino-birds") of the middle to late Cretaceous period. By the way, following the evolutionary classification system known as "cladistics," it's perfectly legitimate to refer to modern birds as dinosaurs!

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Mesozoic Mammals

Reconstruction of a Megazostrodon species, in the Natural History Museum, London.

Theklan / Wikimedia Commons / CC BY-SA 4.0

As with most such evolutionary transitions, there wasn't a bright line separating the most advanced therapsids ("mammal-like reptiles") of the late Triassic period from the first true mammals that appeared around the same time. All we know for sure is that small, furry, warm-blooded, mammal-like creatures skittered across the high branches of trees about 230 million years ago, and coexisted on unequal terms with much bigger dinosaurs right up to the cusp of the K/T Extinction. Because they were so small and fragile, most Mesozoic mammals are represented in the fossil record only by their teeth, though some individuals left surprisingly complete skeletons.

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Cenozoic Mammals

hydracodon nebraskensis running rhino hooved Natural History Museum Los Angeles

Dawn Pedersen / Flickr / CC BY 2.0

After dinosaurs, pterosaurs and marine reptiles vanished off the face of the earth 65 million years ago, the big theme in vertebrate evolution was the rapid progression of mammals from small, timid, mouse-sized creatures to the giant megafauna of the middle to late Cenozoic Era, including oversized wombats, rhinoceroses, camels, and beavers. Among the mammals that ruled the planet in the absence of dinosaurs and mosasaurs were prehistoric cats, prehistoric dogs, prehistoric elephants, prehistoric horse, prehistoric marsupials and prehistoric whales, most species of which went extinct by the end of the Pleistocene epoch (often at the hands of early humans).

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Matteo De Stefano/MUSE / Wikimedia Commons / CC BY-SA 3.0

Technically speaking, there's no good reason to separate prehistoric primates from the other mammalian megafauna that succeeded the dinosaurs, but it's natural (if a bit egotistic) to want to distinguish our human ancestors from the mainstream of vertebrate evolution. The first primates appear in the fossil record as far back as the late Cretaceous period and diversified in the course of the Cenozoic Era into a bewildering array of lemurs, monkeys, apes, and anthropoids (the last the direct ancestors of modern humans). Paleontologists are still trying to sort out the evolutionary relationships of these fossil primates because new "missing link" species are constantly being discovered.

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Strauss, Bob. "10 Steps of Animal Evolution." ThoughtCo, Jul. 31, 2021, thoughtco.com/evolution-of-vertebrate-animals-4040937. Strauss, Bob. (2021, July 31). 10 Steps of Animal Evolution. Retrieved from https://www.thoughtco.com/evolution-of-vertebrate-animals-4040937 Strauss, Bob. "10 Steps of Animal Evolution." ThoughtCo. https://www.thoughtco.com/evolution-of-vertebrate-animals-4040937 (accessed February 7, 2023).