1. Evolutionary History of Vertebrates
Vertebrates originated about 525 million years ago during the Cambrian Explosion. The first
vertebrates were the jawless fish. 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 out of their gills. The jawless fish that lived during the Ordovician period all
went extinct by the end of the Devonian period. There are species of jawless fish like lampreys and
hagfish today, but they are only distant cousins of the first jawless species of fish thought to be the first
vertebrates. After the jawless fish came the armored fish, which developed paired fins. Next, the
cartilaginous fish, also known as sharks, skates, and rays, have skeletons made of cartilage rather than
bone and do not possess lungs. The last of the fish classes, the bony fish, includes many modern fish
species that we see today.
Amphibians were the first vertebrates to venture out onto land. Early amphibians retained many
fish-like characteristics but during the Carboniferous period amphibians diversified. They retained close
ties to water though, producing eggs that lacked a hard protective coating and requiring moist
environments to keep their skin damp. Additionally, amphibians underwent larval phases that were
entirely aquatic and only the adult animals were able to live in land habitats.
The next class of vertebrates is class reptilia. Reptiles arose during the Carboniferous period
shortly after amphibians and quickly took over as the dominant vertebrate on land. Reptiles did not
need any kind aquatic habitat, separating them from the amphibians. Reptiles developed hard-shelled
eggs that could be laid on dry land. They had dry skin made 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 enabled them greater mobility. All of these
characteristics eliminated the need to live close to water like the amphibians.
After reptiles came the class aves, or birds. Sometime during the early Jurassic, a group of
reptiles gained the ability to fly and these reptiles gave rise to the birds. Birds developed a range of
adaptations that enabled flight such as feathers, hollow bones, and warm-bloodedness. The last class of
vertebrates, Mammalia, also developed from reptiles. Mammals developed a four-chambered heart,
hair covering, and almost all do not lay eggs and instead give birth to live young. The exceptions that do
lay eggs are the monotremes, an order of mammals that include the platypus and the spiny anteater. All
monotremes are indigenous to either Australia or New Guinea.