Evolution of Vertebrates

Instructor: Emily Lockhart

Emily has taught science and has a master's degree in education.

Vertebrates, or animals with backbones, have undergone a long and rich evolutionary history. In this lesson we'll explore our vertebrate heritage, from the earliest examples to modern day.

The Earliest Vertebrates

Look down at your hand. With its five fingers, your hand owes its configuration to an ancient lineage of vertebrate diversity. The intricacies of your hand began with evolution of vertebrates half a billion years ago. This lesson will look at the long evolutionary line of vertebrates. All of today's vertebrates are defined by a vertebrae that surrounds a central nervous system controlled by a brain.

These early vertebrates passively let water flow through and had a sedentary life. Early vertebrates developed structures that lead to gills, which then lead to lungs. These developments provided a new way to 'breathe'. First, the pharynx develops muscles that can swallow. This actively pumps water. Second, cartilage appears along the slits creating a springy support that keeps the mouth open. This leads to the ability of marine animals to actively move water into the body.

From the first 'breath' to the development of limbs, the journey to becoming a vertebrate is filled with many extraordinary feats. This lesson outlines the steps that early vertebrates went through on their path to becoming what we see on Earth today.

Image shows the diversity of vertebrates
manytypesofvertebrates

Vertebrate Diversification In The Oceans

The protovertebrate, described above, paves the way for the diversity seen in modern vertebrate species. The first vertebrates to evolve from this early predecessor are called agnathans. Early agnathans are found in the fossil record about 540 million years ago (MYA). These eel-like 'jawless' fish are similar to modern day lamprey and hagfish. The next stage in vertebrate development, characteristic to agnathans, is the development of a grasping mouth. Agnathans were able to push into sediment, which allowed these early vertebrates to draw in food. The mouth could pick at algae or food deposits on surfaces. These organisms were no longer suspension feeding; they were active feeding.

Cartilage, in the jaws, provided elastic recoil, allowing for a strong bite and to quickly close around prey. For the first time in vertebrate history there are jaws. These new vertebrates also gained a set of pectoral fins and pelvic fins. These fins have bony and cartilaginous supports, allowing for increased maneuvering and stealth. The 'jawed fish' make way for two major evolutionary lines- the cartilaginous fish and the bony fish.

Fish

'Jawed fish' give rise to the Age of Fishes. This era, called the Devonion era, leads to two lines of fish that we see in the planet's water today. The first line is the cartilaginous fish. They are alive and well today as rays and sharks. They are marked by defined vertebrae and cartilaginous support. The other fish lineage comprises all bony fish. From bony fish come the tetrapods.

Vertebrate Diversification On Land

Tetrapods are the first vertebrate line to appear out of water. These lobe-finned fish use their fins to drag themselves onto land. These early pioneers will diversify into amphibians, reptiles, birds, and mammals. Tetrapods will pass on to the terrestrial line of vertebrates lungs, paired appendages, jaws, and a backbone. Those remarkable hands of yours can be traced to the early tetrapod pectoral fin 395 MYA. The limbs of all terrestrial vertebrates are similar in structure, all due to having the tetrapod as a common ancestor.

Image shows the similar limb structure of all modern day descendants of first tetrapods
homologouslimbs

Amphibians

Three groups of amphibians remain from the lineage that first appears around 300 MYA. These are the frogs, salamanders, and caecilians. These slimy, water loving critters once ruled the lands, some getting the size of an average alligator! Early amphibians were the stop-gap between fish and late tetrapods. By the age of dinosaurs, these early vertebrates looked very similar to modern amphibian skeletons.

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