6.2 An outline of vertebrate evolution

Let's now place the early evolution of tetrapods in perspective by taking an overview of the whole of vertebrate evolution.

Figure 14 Geological ranges of vertebrates showing when various groups evolved from each other during the Phanerozoic. The width of each group indicates its approximate diversity (as number of families); note the scale bar. The wide horizontal separation of some newly evolved groups from their ancestors (e.g. mammals from reptiles) does not indicate huge, abrupt change but is simply a consequence of the way the diagram is drawn

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Given the ubiquity of our own primate species, and familiarity with domesticated animals such as cats, dogs, and rabbits, our view of the relative diversity and abundance of the main vertebrate groups tends to be biased towards mammals. However, a tally of living vertebrate species reminds us that mammals are not as dominant as some might imagine. In round terms, the total number of living vertebrate species is over 66 000 (which compares, for example, with some 110 000 species of molluscs and more than 80 000 species of roundworms (nematodes)). Living mammals number around 6000 species, and amphibians around 8000 species, whereas there are some 12 000 reptile species, 11 000 bird species and over 28 000 species of bony fish, and about 800 species of sharks and rays, the cartilaginous fish. (You may want to look up the latest figures online as you read this section, as these numbers are constantly refined.)

In discussions such as this, it is important to be aware which taxonomic category (e.g. order, family or genus) is being considered because the picture can change if other measures of diversity (e.g. species) are used.

As we have already seen, the vertebrates were initially slow to diversify. From their Cambrian start, it was not until late Ordovician times that the early jawed fish evolved into different groups, though some were subsequently to become extinct (the placoderms, see Atlas, pp. 76-77, and acanthodians, see Atlas, p. 83). From Figure 14 it becomes clear why the Devonian is often called the 'Age of Fishes': not only was there a greater diversity of fish groups than at any other time but most of the other vertebrate classes had yet to evolve.

Figure 14 reveals a variety of patterns in fish evolution, including some major setbacks. The placoderms became extinct at the end of the Devonian, coincident with a decrease in early tetrapod diversity. Sharks, rays and bony fishes suffered a major extinction at the end of the Permian, coincident with a similar setback for amphibians and reptiles, part of a mass extinction event. Today, the total number of fish families is at its highest ever, mainly due to the bony fishes.

Let's now look at the history of tetrapods. After emerging in late Devonian times, tetrapod communities were initially dominated by amphibians, especially in Carboniferous and Permian times, when a wide variety of amphibian groups evolved and died out. Amphibians remained large during the Palaeozoic Era, often 1-2 m long - huge by the standards of today's frogs, toads and newts, which have a relatively recent, Mesozoic origin. The first reptiles appeared in the early Carboniferous.

At first the earliest Carboniferous reptiles were quite small, superficially lizard-like predators. They lived mostly on arthropods, such as dragonflies and cockroaches, in the dense equatorial Carboniferous forests and swamps. The energy of sunlight trapped during photosynthesis became stored in vast amounts of plant litter that accumulated on the floor of the forests and in swamps. Eventually, this debris was buried, compressed and converted by heat and pressure into coal; 300 Ma later the energy from Carboniferous sunlight fuelled the Industrial Revolution.

Not until Permian times did reptiles begin to diversify widely and increase in size; some even returned to living in the sea. Others evolved certain mammal-like features, such as the beginnings of tooth differentiation. This allowed for separate functions such as large dagger-shaped teeth for defence and killing prey, and smaller cheek teeth for efficient processing of food before digestion. One of these groups that survived the end Permian mass extinction eventually evolved into the first mammals in the late Triassic.

After the end Permian extinction event, the reptiles recovered and gradually became the dominant group by the end of Triassic times, though both reptiles and amphibians were set back by another, late Triassic, extinction event.

Dinosaurs were a special group of reptiles - special not least because, after their origin in the late Triassic, they became the dominant land animals for over 150 Ma. They filled almost every niche possible for large land vertebrates. They included carnivores, herbivores and a few omnivores (mixed diet), and ranged in size from that of a chicken to vast plant eaters, such as the Brachiosaurus, which grew up to 25 m long and weighed up to 40 tonnes. Dinosaurs never, however, took to the air or to the oceans: other large reptiles - the pterosaurs - dominated the skies, and marine reptiles such as ichthyosaurs, plesiosaurs and mosasaurs flourished in the sea. The major surviving groups of reptiles, the crocodiles, turtles and tortoises, lizards and snakes also all appeared in Mesozoic times. Much evidence suggests that birds evolved from dinosaurs in late Jurassic times.

The great success of the Mesozoic reptiles came to a dramatic halt at the end of the Cretaceous. Reptile diversity was cut back from about 60 families to 30 and never fully recovered (Figure 14). The bony fishes rapidly increased in diversity in the early Tertiary.

For a long time after their late Triassic origin, mammals remained small, perhaps nocturnal, shrew-like creatures living in the nooks and crannies of the dinosaur world. But once the large Mesozoic reptiles had become extinct, mammals were able to diversify into the wide range of niches that became vacant. Plants, which also suffered some extinctions at the end of the Cretaceous, continued to have a role in the evolution of both vertebrates and invertebrates. The rise of flowering plants (angiosperms) and pollinating insects, together with climate change in the early Tertiary, promoted the evolution of grasses, which in turn supported the spectacular rise of grazing mammals. It was perhaps the climate-driven change in vegetation that promoted our own evolution from tree-dwelling primates in late Cenozoic times. Global cooling as part of the initial descent into the Quaternary Ice Age caused retreat and fragmentation of the extensive forests of central Africa and their replacement by more open grasslands. This gave an adaptive advantage to a group of more upright-walking primates - the hominids. Our own species, Homo sapiens, appeared only 100 000 to 150 000 years ago (depending on how the origin of the species is defined).