Interrelationships of animal groups

All phyla began as invertebrates, but lineages of the phylum Chordata developed the internal skeletal armature, with spinal column, which was exploited in numerous fish groups and which eventually gave rise to terrestrial vertebrates. The number of phyla is uncertain partly because most of the branching patterns and the ancestral body plans from which putative phyla have arisen are not yet known. For example, arthropods (including crustaceans and insects) may have all diversified from a common ancestor that was a primitive arthropod, in which case they may be grouped into a single phylum; or several arthropod groups may have evolved independently from nonarthropod ancestors, in which case each such group must be considered a separate phylum. So far as known, all animal phyla began in the sea. See also: Arthropoda; Chordata; Invertebrate phylogeny; Vertebrata

Several lines of evidence bear significantly upon the ancestry of these animals and of their major subdivisions. Classically, animals were grouped according to the similarities of their adult body plans; however, although the body plans have provided a basis for the recognition of most of the phyla and other distinctive animal groups, they have not provided definitive evidence of the interrelations of the phyla. A second approach is to compare patterns of early development, which should be more conservative than adult features; such comparisons have permitted the grouping of some phyla into likely alliances. Hypotheses as to the body plans of phylum ancestors have been erected from these morphologic data. It has not proven possible to corroborate any hypotheses, however, because the earliest members of the phyla (and of invertebrate classes) appear abruptly in the fossil record, and their ancestors cannot be traced. Furthermore, numbers of extinct phyla or other major animal groups also appear in the fossil record, adding branches to the tree of life that must be reckoned with, but with no definitive indication of either their origins or branching patterns. Finally, genetic changes that have accumulated after branching events separated the major groups should have left a pattern of variation in the structure of genes that may be reconstructed to yield a phylogeny. If genetic changes have occurred in a reasonably regular manner, then the amount of divergence in the structure of genes and gene products, such as protein and ribonucleic acid (RNA) molecules, should be proportional to the time since branching occurred. Some genes change so fast that their pattern of divergence is useful only for recent branching events, whereas others are so conservative that they are used to study divergences that occurred billions of years ago. These methods are promising when used in conjunction with other evidence. See also: Fossil; Gene; Taphonomy

History of major animal groups

The simplest living phyla may have no direct relationships to other animals (Fig. 2), but they indicate the versatility of multicellular design. For example, members of the Porifera (sponges) contain several differentiated cell types that form body walls, but their architecture is limited to folding these walls into chambers that are sometimes highly convoluted. Members of the Cnidaria (also termed Coelenterata, including sea anemones, corals, and jellyfish) have two cellular body layers, separated by a gelatinous layer; the inner tissue forms a digestive cavity. See also: Cnidaria; Porifera

Fig. 2 Phylogenetic tree depicting the possible relations among the animal phyla. The branching pattern is constrained by models of development and of body-plan evolution and by molecular data. Branch length is not drawn to scale. (Copyright © McGraw-Hill Education)

Flatworms (Platyhelminthes) and their allies have solid bodies, with both external (ectoderm) and internal (endoderm) tissue layers separated by a third cellular layer (mesoderm); there is a gut lined with endoderm. All phylogenetic evidence suggests that a flatwormlike form gave rise to the remaining phyla, which possess body cavities (other than a gut) in which reproductive and other organs can be sequestered. In about 15 of these phyla, this cavity (the coelom) is lined by mesoderm and usually functions as a hydrostatic skeleton. See also: Platyhelminthes

The coelomates constitute all of the familiar, larger-bodied animals and are commonly grouped into two main divisions (Fig. 2). In one group, the body was originally segmented, as in annelid worms or arthropods, although the segmentation may be reduced or lost in some branches. In the other group, the coelom is divided into two or three regions; these include the Enchinodermata (starfish and sea urchins), Tunicata (sea squirts), and Chordata (including humans). Additionally, there are coelomate phyla that cannot yet be assigned to either of these groups; some, such as the Brachiopoda, Phoronida, and Bryozoa, display an enigmatic mixture of traits that otherwise characterize one or the other of the main groups. In about 10 phyla (not shown in Fig. 2), the body cavity is not a true coelom, but lies between endoderm and mesoderm and is termed a pseudocoel. These pseudocoelomates are soft-bodied, small to minute, and commonly parasitic; some of them have never been found as fossils. See also: Annelida; Brachiopoda; Bryozoa; Echinodermata; Phoronida; Tunicata (Urochordata)

Some features of the cells of primitive animals resemble those of the single-celled Protozoa, especially the flagellates, which have long been believed to be animal ancestors. Molecular phylogenies have supported this idea and also suggest that the phylum Cnidaria arose separately from all other phyla that have been studied by this technique. Thus, animals may have evolved at least twice from organisms that are not themselves animals, thereby representing a grade of evolution and not a single branch (clade) of the tree of life. Sponges also have been suspected of an independent origin, and it is possible that some of the extinct fossil phyla arose independently or branched from sponges or cnidarians. See also: Protozoa

The earliest undoubted animal fossils (the Ediacaran fauna) are soft-bodied, and they first appear in marine sediments nearly 650 million years ago. This fauna lasted about 50 million years and consisted chiefly of cnidarians or cnidarian-grade forms, although it contains a few enigmatic fossils that may represent groups that gave rise to more advanced phyla. Then, nearly 570 million years ago, just before and during earliest Cambrian time, a diversification of body architecture began that produced most of the living phyla, as well as many extinct groups. The body plans of some of these groups involved mineralized skeletons that created, for the first time, an extensive fossil record because these skeletons are more easily preserved than soft tissues. The earliest mineralized skeletons are chiefly minute, including the small shelly fauna of earliest Cambrian age; moreover, although some of these fossils are primitive members of living phyla, many (perhaps 10 to 20 kinds of them) are distinctive and quite possibly represent phyla or at least major branches of phyla that soon became extinct, geologically speaking. The soft-bodied groups were also markedly diversified, although their record is so spotty that their history cannot be traced in detail. A single, exceptionally preserved soft-bodied fauna from the Burgess Shale of British Columbia, which is about 530 million years old, contains not only living soft-bodied worm phyla, but extinct groups (perhaps a dozen) that cannot be placed in living phyla and do not seem to be ancestral to them. Among these is Wiwaxia, a novel form that bears spinelike elements similar to some of the small shelly fossils of Early Cambrian age, thus providing evidence that the small shelly forms did indeed include novel types. Clearly, an early radiation of animals produced a vast array of novel body plans in an evolutionary episode that has never been surpassed. See also: Burgess Shale; Cambrian; Ediacaran biota

Following the early phase of rampant diversification and of some concurrent extinction of phyla and their major branches, the subsequent history of the durably skeletonized groups can be followed in a general way in the marine fossil record (Fig. 3). The composition of the fauna changed continually, but three major associations can be seen: one dominated by the arthropodlike trilobites during the early Paleozoic [panel (a) of Fig. 3], one dominated by articulate brachiopods and crinoids (Echinodermata) in the remaining Paleozoic [panel (b) of Fig. 3], and one dominated by gastropod (snail) and bivalve (clam) mollusks during the Mesozoic and Cenozoic [panel (c) of Fig. 3]. The mass extinction at the close of the Paleozoic that caused the contractions in so many groups (Fig. 3) may have extirpated more than 90% of marine species and led to a reorganization of marine community structure and composition into a modern mode. Resistance to this and other extinctions seems to have been a major factor in the rise of successive groups to dominance. On land, the fossil record is too spotty to trace in detail the invasions of the terrestrial environment, which must have been under way by the mid-Paleozoic. Annelids, arthropods, and mollusks are the more important invertebrate groups that made the transition to land. The outstanding feature of terrestrial fauna is the importance of the insects, which appeared in the late Paleozoic and later radiated to produce several million living species, surpassing all other life forms combined in this respect. See also: Cenozoic; Extinction; Extinction (paleontology); Insecta; Mesozoic; Mollusca; Paleozoic; Trilobita

Fig. 3 Marine fossil record of major animal groups during the last 680 million years. The width of the spindles is proportional to the number of families present. Note the shifting of dominance from (a) trilobites and other forms during the early Paleozoic to (b) articulate brachiopods, crinoids, and others in the late Paleozoic, and then to (c) gastropod and bivalve mollusks. (Credit: J. J. Sepkoski, Jr.)

Chordate origins

The phylum Chordata consists largely of animals with a backbone, that is, the Vertebrata, including humans (Fig. 4). The group, however, includes some primitive nonvertebrates, termed the protochordates: lancelets, tunicates, acorn worms, pterobranchs, and possibly the extinct graptolites and conodonts. The interrelationships of these forms are not well understood. With the exception of the colonial graptolites, they are soft-bodied and have only a very limited fossil record. They suggest possible links to the Echinodermata in developmental, biochemical, and morphological features. In addition, some early Paleozoic fossils, the carpoids, have been classified alternatively as chordates and as echinoderms by various investigators, again suggesting a link. In spite of these various leads, the origin of the chordates remains basically unclear. See also: Conodont; Graptolithina

Fig. 4 Vertebrate evolution in relation to geologic time. Classes are shown in uppercase letters; subclasses are shown in lowercase letters, except that “hagfishes” and “lampreys” are common names for orders. (Credit: M. Hildebrand)

Chordates are characterized by a hollow, dorsal, axial nerve chord, a ventral heart, a system of slits in the pharynx that serves variously the functions of feeding and respiration, a postanal swimming tail, and a notochord that is an elongate supporting structure lying immediately below the nerve chord. The protochordates were segmented, although sessile forms (such as the tunicates) show this only in the swimming, larval phase. Even the free-swimming forms were not truly active, and they used the gill apparatus primarily for feeding on small particles trapped on mucous streams generated in the pharynx. There are two hypotheses concerning the nature of the protochordates. One holds that all protochordates were originally sessile, although having active larvae, and that the vertebrates arose from the larval phase through increased development of the segmented muscular trunk and associated nervous system. The alternative hypothesis proposes that the sessile condition seen in many protochordates is secondary.

The first vertebrates were fishlike animals in which the pharyngeal slits formed a series of pouches that functioned as respiratory gills. An anterior specialized mouth permitted ingestion of food items large in comparison with those of the filter-feeding protochordates. Vertebrates are first known from bone fragments found in rocks of Cambrian age, but more complete remains have come from the Middle Ordovician. Innovations, related to greater musculoskeletal activity, included the origin of a supporting skeleton of cartilage and bone, a larger brain, and three pairs of cranial sense organs (nose, eyes, and ears). At first, the osseous skeleton served as protective scales in the skin, as a supplement to the notochord, and as a casing around the brain. In later vertebrates, the adult notochord is largely or wholly replaced by bone, which encloses the nerve chord to form a true backbone. All vertebrates have a heart that pumps blood through capillaries, where exchanges of gases with the external media take place. The blood contains hemoglobin in special cells that carry oxygen and carbon dioxide. During the exchanges, the oxygen content of the cells is increased and the content of carbon dioxide is decreased. In most fishes, the blood passes from the heart to the gills and thence to the brain and other parts of the body. In most tetrapods and in some fishes, blood passes to the lungs, is returned to the heart after oxygenation, and is then pumped to the various parts of the body.