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All vertebrates evolved from a single common ancestor and share certain inherited characteristics, most notably a backbone. As the descendents of the first vertebrate evolved new features, more species branched off the tree, creating the diverse group of more than 50,000 vertebrates alive today.
Like most major evolutionary groups, vertebrates originated in the sea. All vertebrate species, including humans, descended from these early fishlike creatures. To this day, the majority of all vertebrate species still live in the ocean.
An evolutionary tree, or cladogram, shows which species and groups of species are most closely related. Species that branch from a common point, or node, share similar features because they descended from a common ancestor. Smaller groups connect to form larger groups at nodes lower on the tree. This series of nesting groups ultimately leads back to the common ancestor of them all.
Every node represents a point in evolution when a new species arose. Each new species passed on its characteristics to its descendents. As more new species evolved, more branches split off the tree.
The two evolutionary trees in this hall fit together. The entire vertebrate tree stems from a single branch in the larger tree—the Chordata, animals with a nerve chord in their back. The vertebrate tree evolved over the past 500 million years; the broader marine tree covers much more time, about 1.5 billion years. The first microbial life, including bacteria and archaea, arose about 2 billion years before then.
Just a few important events in the evolution of vertebrates are shown here. To explore a more detailed version of this tree, touch the computer screens.
Vertebrates: Origin of the backbone, which protects the nerve chord and anchors the bones and muscles. Every species on the vertebrate tree branches from this node.
Gnathostomes: All vertebrates, except for a small branch of some jawless fishes, inherited jaws from a single common ancestor, which is represented by this node.
Osteichthyes: In this group, a pocket in the gut evolved into the lungs and gas bladder of all bony fishes, lobefin fishes and four-limbed tetrapods, including humans.
The animals shown here are just a small sample of the more than 50,000 vertebrate species living today. To determine which species are most closely related, scientists look for similar characteristics shared by members of a group but not found outside the group. These shared characteristics indicate that group members share a common ancestor from which they inherited these traits. Shared characteristics may be found in an animal's physical structure, behavior and genetic code.
The following sections introduce two major groups on the vertebrate tree: rayfins, lobefins and tetrapods, and the jawless and cartilaginous fishes.
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