Unit Five. Evolution of Animal Life
20. History of the Vertebrates
Animals with backbones are among the most visible organisms in the living world. With over 54,000 species, they come in an astonishing range of sizes, from pygmy moles the size of your thumb and hummingbirds that are even smaller, to elephants like those above and whales that are even larger. Vertebrates first evolved in the sea, and today over half of all vertebrate species are fishes. But the great success of vertebrates has come with their successful invasion of land about 350 million years ago. Vertebrates, along with arthropods, now dominate animal life on land. Much of this success is due to the increased complexity of their internal organ systems, and particularly to the internal skeleton that is characteristic of vertebrates, which allows them to grow to great size. Humans, like the elephants above, are mammals, vertebrates with hair that nurse their offspring with milk. The first mammals appeared at the same time as the dinosaurs, but for over 150 million years were mostly a minor group. When dinosaurs became extinct 65 million years ago, mammals survived and have since taken over many of the ecological roles once occupied by dinosaurs.
When scientists first began to study and date fossils, they had to find some way to organize the different time periods from which the fossils came. They divided the earth’s past into large blocks of time called eras (the upper bars in figure 20.1). Eras are further subdivided into smaller blocks of time called periods (the darker blue bars below the era bars), and some periods, in turn, are subdivided into epochs, which can be divided into ages (not shown in the figure). It might be helpful to refer back to this figure throughout the chapter when different eras and periods are discussed.
Figure 20.1. An evolutionary timeline.
Vertebrates evolved in the seas about 500 million years ago (M.Y.A.) and invaded land about 150 million years later. Dinosaurs and mammals evolved in the Triassic period about 220 million years ago. Dinosaurs dominated life on land for over 150 million years, until their sudden extinction 65 million years ago left mammals free to flourish.
Virtually all of the major groups of animals that survive at the present time originated in the sea at the beginning of the Paleozoic era (the upper lavender bar), during or soon after the Cambrian period (545-490 M.Y.A.). Thus, the major diversification of animal life on earth occurred largely in the sea, and fossils from the early Paleozoic era are found in the marine fossil record.
Many of the animal phyla that appeared in the Cambrian period, like the bizarre trilobites you see above in figure 20.2, have no surviving close relatives. Ammonites, shelled, cephalopod mollusks that originated in the Paleozoic era, were among the most abundant creatures on earth 100 million years ago (figure 20.3).
Figure 20.2. Life in the Cambrian.
(a) Trilobites are shown in this reconstruction of a community of marine organisms in the Cambrian period, 545 to 490 million years ago. (b) A fossil trilobite.
Figure 20.3. Ammonite fossil from the Jurassic.
The first vertebrates evolved about 500 million years ago in the oceans—fishes without jaws. They didn’t have paired fins either—many of them looked something like a flat hotdog with a hole at one end and a fin at the other. For over 100 million years, a parade of different kinds of fishes were the only vertebrates on earth. They became the dominant creatures in the sea, some as large as 10 meters, larger than most cars.
Invasion of the Land
Only a few of the animal phyla that evolved in the Cambrian seas have invaded the land successfully; most others have remained exclusively marine. The first organisms to colonize the land were fungi and plants, over 500 million years ago. It seems probable that plants first occupied the land in symbiotic association with fungi, as discussed in chapter 18.
The first invasion of the land by animals, and perhaps the most successful invasion of the land, was accomplished by the arthropods, a phylum of hard-shelled animals with jointed legs and a segmented body. This invasion occurred about 410 million years ago.
Vertebrates invaded the land during the Carboniferous period (360-280 M.Y.A.). The first vertebrates to live on land were the amphibians, represented today by frogs, toads, salamanders, and caecilians (legless amphibians). The earliest amphibians known are from the Devonian period. Then about 300 million years ago, the first reptiles appeared. Within 50 million years, the reptiles, better suited than amphibians to living out of water, replaced them as the dominant land animal on earth. The pelycosaur, the fan-backed animal pictured in figure 20.4, was an early reptile. By the Permian period, the major evolutionary lines on land had been established and were expanding.
Figure 20.4. An early reptile: the pelycosaur.
The history of life on earth has been marked by periodic episodes of extinction, where the loss of species outpaces the formation of new species. Particularly sharp declines in species diversity are called mass extinctions. Five mass extinctions have occurred, the first of them near the end of the Ordovician period about 438 million years ago. At that time, most of the existing families of trilobites (see figure 20.2), a very common type of marine arthropod, became extinct. Another mass extinction occurred about 360 million years ago at the end of the Devonian period.
The third and most drastic mass extinction in the history of life on earth happened during the last 10 million years of the Permian period, marking the end of the Paleozoic era. It is estimated that 96% of all species of marine animals that were living at that time became extinct! All of the trilobites disappeared forever. Brachiopods, marine animals resembling mollusks but with a different filter-feeding system, were extremely diverse and widespread during the Permian; only a few species survived.
Mass extinctions left vacant many ecological opportunities, and for this reason they were followed by rapid evolution among the relatively few plants, animals, and other organisms that survived the extinction. Little is known about the causes of major extinctions. In the case of the Permian mass extinction, some scientists argue that the extinction was brought on by a gradual accumulation of carbon dioxide in ocean waters, the result of large-scale volcanism due to the collision of the earth’s landmasses during formation of the single large “super-continent” of Pangaea. Such an increase in carbon dioxide would have severely disrupted the ability of animals to carry out metabolism and form their shells.
The most famous and well-studied extinction, though not as drastic, occurred at the end of the Cretaceous period (65 million years ago), at which time the dinosaurs and a variety of other organisms went extinct. Recent findings have supported the hypothesis that this fifth mass extinction event was triggered when a large asteroid slammed into the earth, perhaps causing global forest fires and obscuring the sun for months by throwing particles into the air.
We are living during a new sixth mass extinction event. The number of species in the world is greater today than it has ever been. Unfortunately, that number is decreasing at an alarming rate due to human activity. Some estimate that as many as one-fourth of all species will become extinct in the near future, a rate of extinction not seen on earth since the Cretaceous mass extinction.
Key Learning Outcome 20.1. The diversification of the animal phyla occurred in the sea. The two animal phyla that have invaded the land most successfully are the arthropods and chordates (the vertebrates).