A HISTORY OF LIFE ON EARTH 97 Figure 5.7 Members of the Edi- acaran fauna. (A) Mawsonites spriggi, may be a cnidarian relative of sea anemones, although this is not certain. (B) The actual relation- ship of the wormlike Dickinsonia costata to later animals is unknown. (A O The Natural History Museum, London; B O Ken Lucas/Visuals Unlimited.) Paleozoic Life: The Cambrian Explosion The Paleozoic era began with the Cambrian period, starting about 542 Mya. For the first 10 Myr or so, animal diversity was low; then, during a period of just 10 to 25 million years, almost all the modern phyla and classes of skeletonized marine animals, as well as many extinct groups, appeared in the fossil record. Tlus interval marks the first appearance of brachiopods (Figure 5.8A), trilobites (Figure 5.8B) and other classes of arthropods, various classes of molluscs, echinoderms, and many others. The remarkably well-preserved Cambrian fauna of the Burgess Shale of British Columbia (Figure 5.9 and the chapter-opening photograph) includes animals rather different from any that succeeded them. The Cambrian diversifi- cation included the earliest jawless (agnathan) vertebrates: the recently discovered early Cambrian Haikouichthys had eyes, gill pouches, notochord, segmented musculature, and other features resembling those of larval lampreys (Figure 5.10A; Shu et al. 2003), and the late Cambrian conodonts had teeth made of cellular bone (Figure 5.10B). Most of the fun- damentally different body plans (often called Bauplane, German for "construction plans" or blueprints) known in animals apparently evolved during the Cambrian-perhaps the most dramatic adaptive radiation in the history of life. The reasons for this diversification, called the Cambrian explosion, have been the sub- ject of vigorous debate (Erwin 1991; Lipps and Signor 1992). How and why did so many great changes evolve in such a short time? By applying a molecular clock to the DNA se- quence divergence among the living animal phyla, Gregory Wray and colleagues (1996) showed that the phyla actually originated long before their first appearances in the fossil record, perhaps 1000 Mya. Before the "explosion," however, most animals either lacked skeletons or were so small that their Precambrian remains have not been found. The Cam- Figure 5.8 Two animal groups brian explosion thus consists of rapid diversification within clades that had evolved much ~~~~'p~~~e("A~~~,"~,"~i~mm brachiopod (Mucrospirifer, phylum98 CHAPTER 5 (A) earlier-diversification that included the evolution of shells and skeletons. A combination of genetic and eco- logical causes may account for this diversification (Knoll and Carroll 1999; Knoll 2003). Regulatory genes that govern the differentiation of body parts (such as the Hox genes; see Chapter 20) may have undergone major evolutionary changes at this time. Moreover, the extinction of the archaic Ediacaran animals-which may have been brought about by a decrease in oxygen levels-released the survivors from competition, al- lowing them to diversify just as mammals did after the demise of dinosaurs (see Chapter 7). Figure 5.9 Artist's reconstruc- Molecular phylogenetic studies show that animals are most closely related to the uni- of two of the peculiar cellular choanoflagellates. Sponges (phylum Porifera), which have inany choanoflagel- of the Cambrian Burgess Shale. (A) Opabitiia, probably an arthropod. late-like cells, are the sister group of the other animals, known collectively as Metazoa (8) Wi~~u~xia, perhaps related to (Figure 5.11). The radially symmetrical Cnidaria (jellyfishes, corals) and the Ctenophora annelid worms. Characteristics con- (comb jellies) are basal branches relative to the Bilateria-bilaterally symmetrical animals firmed in multiple fossils can be with a head, often equipped with mouth appendages, sensory organs, and a brain. The illustrated in a single drawing, Bilateria include three major branches: the deuterostomes, in which the blastopore formed which therefore often can convey structures more clearly than a pho- during gastrulation becomes the anus, and two groups of protostomes, in which the tograph of a single fossil specimen. blastopore becomes the mouth. The largest deuterostome phyla are the Echinodermata For that reason, taxa are often illus- (starfishes and relatives) and the Chordata (including the vertebrates, tunicates, and am- trated by drawings rather than by phioxus). Protostomes form two major clades: the Ecdysozoa (arthropods, nematodes, photographs in this chapter. (Draw- and some smaller phyla) and the Lophotrochozoa (molluscs, annelid worms, brachiopods, ings by Marianne Collins, from Gould 1989.) and a variety of other groups). The end of the Cambrian (500 Mya) was marked by mass extinction. The trilobites, of which there had been more than 90 Cambrian families, were greatly reduced, and several classes of echinoderms became extinct. As Stephen Jay Gould (1989) emphasized, if the early vertebrates had also succumbed, we would not be here today. The same may be said about every point in subsequent time: had our ancestral lineage been among the enor- mous number of lineages that became extinct, humans would not have evolved, and per- haps no other form of life like us would have, either. Figure 5.10 Early vertebrates of the Cam- brian. (A) Photograph and drawing of one of the earliest known vertebrates, Haikouichthys, of the early Cambrian. The drawing calls attention to features interpreted as eye, noto- chord, vertebral elements, dorsal fin, esopha- gus, gill pouches, ventral fin, and anus, indi- cating a postanal tail region characteristic of vertebrates. (B) Bony, toothlike structures of Cambrian conodonts. Conodonts were slen- der, finless chordates that are thought to be related to agnathans (jawless vertebrates such as lampreys). (A photo courtesy of D.- G. Shu, after Shu et al. 2003; B courtesy of James Davison, M.D.)A HISTORY OF LIFE ON EARTH 99 Embryonic Vertebrata (vertebrates) becomes Cephalochordata (amphioxus) the anus Urochordata (tunicates) 3 cell layers, bilateral Hemichordata (acorn worms) Echinodermata (starfishes, sea urchins, etc.) :!lv:etry> I 7 Bryozoa (moss animals) 1 locomotion re] Brachiopoda (brachiopods) Platyhelminthes (flatworms) 2 cell layers, radial symmetry Multi- cellularitv Pogonophora (tubeworms) Rotifera (rotifers) Annelida (segmented worms) J becomes Nematoda (nematode worms) the mouth