Vertebrate Evolution

What is a vertebrate?

The animal kingdom is typically divided into two main groups: the invertebrates and the vertebrates. The vertebrates are distinguished by the presence of a backbone, and are the organisms that come to mind when one thinks of an animal: fish, amphibians, reptiles, birds, and mammals.

The vertebrate branch of the "tree of life"

The invertebrates, however, represent a vast number of species, which are classified into phyla that include:

• Sponges (phylum Porifera)
• Jellyfish and sea anemones (phylum Cnidaria),
• Flatworms (phylum Platyhelminthes)
• Roundworms (pphylum Nematoda)
• Segmented worms (phylum Annelida)
• Insects, spiders and crustaceans (phylum Arthropoda)
• Snails, clams and squid (phylum Mollusca)
• Starfish and sea urchins (phylum Echinodermata)

The invertebrates are worth mentioning because they represent the evolutionary history of the vertebrates. The vertebrates are a relatively recent “branch” on the tree of life and retain features of almost every invertebrate group. Some people find it intriguing that, of the invertebrates, our closest relatives are starfish and sea urchins.

The vertebrates are actually a subphylum within the phylum Chordata. Chordates are characterized by having a nerve cord running along the length of the back. While there are a few invertebrate chordates, the vertebrates are noted for having an endoskeleton (endo = internal), which includes the backbone. Interestingly, not all vertebrates have backbones — or skeletons — made of bone. More primitive forms, like sharks and rays, have skeletons made entirely of cartilage.

How have vertebrates evolved?

When scientists describe vertebrate evolution, they most often frame it as a transition from water to land. Once on land, the vertebrates are described as evolving to occupy diverse habitats and live very active lifestyles. What are some of the adaptations that made these transitions possible?

Jaws: The earliest vertebrates in evolutionary history are the fish. The earliest fish had no jaws — they sucked and rasped flesh of their prey rather than biting it. These fish include hagfish and lampreys. Fish that arose later, including the sharks and the bony fish, have jaws. Jaws represent a much more efficient and effective mode of capturing, feeding on, and swallowing prey.

Lungs and limbs: In order for vertebrates to succeed on land, they had to be able to breathe and move around. These adaptations are first seen in a primitive group of fish, of which a living example exists — the lungfish. Although they take in oxygen primarily through gills, they also have lungs. Their fleshy fins are supported by bone, and they can walk around in their habitats. The amphibians are thought to have evolved from fish like this. As their name implies – “amphibian” comes from the Greek word “amphibious,” meaning “double life” – the adaptations of amphibians truly reflect mixed habitats.

Watertight skin and eggs: To live exclusively on land requires the ability to avoid water loss. The next adaptations in vertebrate evolution included skin that acts as a watertight barrier. Evolving from amphibians, the reptiles are the first vertebrate group to show this adaptation. Reptiles also have what is called an amniote egg. Amniote eggs contain their own water supply and are surrounded by a leathery or hard shell. Birds, which are known to have evolved from reptiles, also have amniote eggs. Their feathers are actually modified scales.

Endothermy: Birds and mammals possess an adaptation known as endothermy (endo = internal; therm = temperature). This is what we typically call “warm-bloodedness..” This occurs as body temperature is regulated internally using heat supplied by the burning of food for fuel. Endothermy permits a degree of independence from environmental conditions. With this adaptation, birds and mammals have further evolved to possess diverse ways of feeding, avoiding predators, finding suitable habitats, and reproducing.

In the video, Dr. Douglas Zook noted an important idea. Often, people think of vertebrate evolution as being “ladder-like,” where earlier forms are replaced and improved upon by more modern forms. This isn’t how vertebrate evolution occurred. From common ancestors, each group branched into their own successful lineages. The most primitive living fish is just as successful in an evolutionary sense as the most recently evolved mammal — the human being.

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