Animal monophyly is supported by gene sequences and morphology

The most convincing evidence that all the organisms considered to be animals share a common ancestor comes from phylogenetic analyses of their gene sequences. Relatively few complete animal genomes are available, but more are being sequenced each year. Analyses of these genomes, as well as of many individual gene sequences, have shown that the animals are indeed monophyletic. The best-supported phylogenetic tree for the major animal groups is shown in Figure 30.1. Table 30.1 summarizes the living members of those groups.

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Figure 30.1 A Phylogenetic Tree of the Animals This tree presents the best-supported current hypotheses of the evolutionary relationships among major groups of animals. The traits highlighted by red circles will be explained in this and the following two chapters.

Question

Q: Based on this tree, which of the depicted traits evolved multiple times among animals, and in which lineages?

Nervous systems are shown evolving three times: nerve nets in ctenophores and in cnidarians, and central nervous systems in bilaterians.

table 30.1 Summary of Living Members of the Major Animal Groups
Group Approximate number of living species described Major subgroups, other names, and notes
Ctenophores 250 Comb jellies
Sponges 8,500 Demosponges, glass sponges, calcareous sponges
Placozoans 2 Additional species have been discovered but not yet formally named
Cnidarians 12,500

Anthozoans: Corals, sea anemones

Hydrozoans: Hydras and hydroids

Scyphozoans: Jellyfish

Myxozoans: Parasitic mucous animals; sometimes placed in group distinct from cnidarians

Orthonectids 45 Microscopic wormlike parasites of marine invertebrates; relationships uncertain
Rhombozoans 125 Tiny (0.5–7 mm) parasites of cephalopods; relationships uncertain
PROTOSTOMES
Arrow worms 180 Glass worms
Lophotrochozoans
Bryozoans 5,500 Moss animals
Entoprocts 170 Sessile aquatic animals, 0.1–7 mm long, superficially similar to bryozoans
Flatworms 30,000 Free-living flatworms; flukes and tapeworms (all parasitic); monogeneans (ectoparasites of fish)
Gastrotrichs 800 “Hairy backs”
Rotifers and relatives 3,000 Rotifers, spiny-headed worms, and jaw worms
Ribbon worms 1,200 Proboscis worms
Phoronids 10 Sessile marine filter feeders
Brachiopods 450 Lampshells
Annelids 19,000

Polychaetes (generally marine; may not be monophyletic)

Clitellates: earthworms, freshwater worms, leeches

Mollusks 117,000

Monoplacophorans

Chitons

Bivalves: Clams, oysters, mussels

Gastropods: Snails, slugs, limpets

Cephalopods: Squid, octopuses, nautiloids

Ecdysozoans
Kinorhynchs 180 Mud dragons
Loriciferans 30 Brush heads
Priapulids 20 Penis worms
Nematodes 25,000 Roundworms
Horsehair worms 350 Gordian worms
Onychophorans 180 Velvet worms
Tardigrades 1,200 Water bears
Arthropods
Chelicerates 114,000 Horseshoe crabs, pycnogonids, and arachnids (scorpions, harvestmen, spiders, mites, ticks)
Myriapods 12,000 Millipedes, centipedes
Crustaceans 67,000 Crabs, shrimps, lobsters and crayfish, barnacles, copepods
Hexapods 1,020,000 Insects and their wingless relatives
DEUTEROSTOMES
Xenoturbellids 5 Secondarily simple marine worms; relationships uncertain
Acoels 400 Very small (mostly <2 mm) flattened marine worms; relationships uncertain
Echinoderms 7,500 Crinoids (sea lilies and feather stars), brittle stars, sea stars, sea daisies, sea urchins, sea cucumbers
Hemichordates 120 Acorn worms and pterobranchs
Tunicates 2,800 Sea squirts (ascidians), salps, and larvaceans
Lancelets 35 Cephalochordates
Vertebrates 65,000 Hagfish, lampreys, cartilaginous fish, ray-finned fish, coelacanths, lungfish, amphibians, reptiles (including birds), and mammals

Although animals were considered to belong to a single clade long before gene sequencing became possible, surprisingly few morphological features are shared across all species of animals. Two morphological synapomorphies have been identified that distinguish the animals:

  1. A common set of extracellular matrix molecules, including collagen and proteoglycans (see Figure 5.22)

  2. Unique types of junctions between cells (tight junctions, desmosomes, and gap junctions; see Figure 6.7)

Although some animals in a few groups lack one or the other of these traits, it is believed that these traits were possessed by the ancestor of all animals and subsequently lost in those groups. Similarities among animals in the organization and function of Hox and other developmental genes (see Chapter 19) provide additional evidence of developmental mechanisms shared by a common animal ancestor.

The common ancestor of animals was likely a colonial flagellated protist similar to existing colonial choanoflagellates. Choanoflagellate colonies have clearly retained similarities to the multicellular sponges (Figure 30.2). Why did early animals begin to form multicellular colonies? One hypothesis is that multicellular colonies are more efficient than single cells are at capturing their prey. Experiments with living species of choanoflagellates show that they spontaneously form multicellular colonies in response to signaling compounds that are found on certain species of planktonic bacteria they eat (Figure 30.3).

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Figure 30.2 Choanocytes in Sponges Resemble Choanoflagellate Protists (A) The similarity of choanoflagellate protist colonies to sponge choanocytes supports an evolutionary link between this protist lineage and the animals. (B) A sponge moves food-containing water through its body by beating the flagella of its choanocytes. Water enters the sponge through small pores and passes into water canals or an open atrium, where the choanocytes capture food particles from the water. The spicules are supportive, skeletal structures.

One hypothesis of animal origins postulates a choanoflagellate-like lineage in which certain cells in the colony became specialized—some for movement, others for nutrition, others for reproduction, and so on. Once this functional specialization had begun, cells could have continued to differentiate. Coordination among groups of cells could have improved by means of specific regulatory and signaling molecules that guided differentiation and migration of cells in developing embryos. Such coordinated groups of cells eventually could have evolved into the larger and more complex organisms that we call animals.

638

Nearly 80 percent of the 1.8 million named species of living organisms are animals, and millions of additional animal species await discovery (see Chapter 31 opening story). Evidence for the evolutionary relationships among animal groups can be found in fossils, in patterns of embryonic development, in the morphology and physiology of living animals, in the structure of animal proteins, and in gene sequences. Increasingly, studies of the phylogenetic relationships among major animal groups have come to depend on genomic sequence comparisons.