Historical Geology Understanding Our Planet\'s Past

most significant advancements.The vertebrate skeleton was light, strong, and

flexible, with efficient muscle attachments.The biggest advantage was that the

skeleton grew along with the animal. In contrast, invertebrates must shed their

external skeletons in order to grow, placing them in great danger to predators.

These new internal skeletons enabled the wide dispersal of free-swimming

species into a variety of environments.

The most primitive of chordates, which include the vertebrates, was a

small, fishlike oddity called amphioxus. Although the animal did not have an

actual backbone, it nonetheless is placed in direct line to the vertebrates. The

earliest vertebrates lacked jaws, paired fins on either side of the body, or true

vertebrae and shared many characteristics of modern lampreys. The origin of

the vertebrates led to the evolution of one of the most important novelties—

namely the head. It was packed with paired sensory organs, a complex three-

part brain, and many other features missing in invertebrates.

The traditional view held that eyes had evolved independently several

times in the distant past. However, the discovery of a gene shared by fruit flies,

squids, mice, and humans suggest that the eye probably evolved only once in

life’s evolutionary history. The complex eyes of modern animals probably

originally evolved from light-sensitive nerve cells that eventually developed

Figure 60Marine flora
and fauna of the late
Ordovician.
(Courtesy Field Museum of
Natural History)

Historical Geology