The Basics of Vertebrate Evolution

From Jawless Fish to Mammals

Coelacanth fossil
John Cancalosi / Getty Images

Vertebrates are a well-known group of animals that includes mammals, birds, reptiles, amphibians, and fish. The defining characteristic of vertebrates is their backbone, an anatomical feature that first appeared in the fossil record about 500 million years ago during the Ordovician period. Here are various groups of vertebrates in the order in which they evolved.

Jawless Fish (Agnatha)

The first vertebrates were the jawless fish. These fish-like animals had hard bony plates that covered their bodies, and as their name implies, they did not have jaws. Additionally, these early fish lacked paired fins. The jawless fish are thought to have relied on filter-feeding to capture their food, and most likely would have sucked water and debris from the seafloor into their mouth, releasing water and waste through their gills.

The jawless fish that lived during the Ordovician period all became extinct by the end of the Devonian period. While there are still some species of fish that lack jaws (such as lampreys, and hagfish), these modern-day jawless species are not direct survivors of the Class Agnatha, but are instead distant cousins of the cartilaginous fish.

Armored Fish (Placodermi)

The armored fish evolved during the Silurian period. Like their predecessors, they too lacked jawbones but did possess paired fins. The armored fish diversified during the Devonian period but declined and fell into extinction by the end of the Permian period.

Cartilaginous Fish (Chondrichthyes)

Cartilaginous fish, which include sharks, skates, and rays, evolved during the Silurian period. Cartilaginous fish have skeletons composed of cartilage rather than bone. They also differ from other fish in that they lack swim bladders and lungs.

Bony Fish (Osteichthyes)

Bony fish first arose during the late Silurian period. The majority of modern fish belong to this group. (Note that some classification schemes recognize the Class Actinopterygii rather than Osteichthyes.) Bony fish diverged into two groups: one that evolved into modern fish and one that evolved into lungfish, lobe-finned fish, and fleshy-finned fish. The fleshy-finned fish gave rise to the amphibians.

Amphibians (Amphibia)

Amphibians were the first vertebrates to venture onto land. Early amphibians retained many fish-like characteristics but diversified during the Carboniferous period. They retained close ties to water, however, requiring moist environments to keep their skin damp and producing fish-like eggs that lacked a hard protective coating. Additionally, amphibians underwent larval phases that were entirely aquatic; only the adult animals were able to survive terrestrial habitats.

Reptiles (Reptilia)

Reptiles arose during the Carboniferous period and quickly took over as the dominant form of land vertebrates. Reptiles freed themselves from aquatic habitats where amphibians had not. Reptiles developed hard-shelled eggs that could be laid on dry land. They had dry skin comprised of scales that served as protection and helped retain moisture.

Reptiles developed larger and more powerful legs than those of amphibians. The placement of the reptilian legs beneath the body (instead of at the side as with amphibians) enabled them greater mobility.

Birds (Aves)

Sometime during the early Jurassic period, two groups of reptiles gained the ability to fly; one of these groups later gave rise to the birds. Birds developed a range of adaptations, such as feathers, hollow bones, and warm-bloodedness that enabled flight.

Mammals (Mammalia)

Mammals, like birds, evolved from reptilian ancestors. Mammals developed a four-chambered heart, hair covering, and most (with the exception of monotremes such as the platypus and echidna) do not lay eggs, instead, giving birth to live young.

Progression of Vertebrate Evolution

The following table shows the progression of vertebrate evolution. The organisms listed at the top of the table evolved earlier than those further down.

Animal Group Key Features
Jawless Fish • no jaws
• no paired fins
• gave rise to placoderms, cartilaginous and bony fish
Placoderms • no jaws
• armored fish
Cartilaginous fish • cartilage skeletons
• no swim bladder
• no lungs
• internal fertilization
Bony fish • gills
• lungs
• swim bladder
• some developed fleshy fins (gave rise to amphibians)
Amphibians • first vertebrates to venture out onto land
• remained quite tied to aquatic habitats
• external fertilization
• eggs had no amnion or shell
• moist skin
Reptiles • scales
• hard-shelled eggs
• stronger legs positioned directly beneath the body
Birds • feathers
• hollow bones
Mammals • fur
• mammary glands
• warm-blooded