Unit Five. Evolution of Animal Life
The features described in the previous section evolved over millions of years. The great diversity of animals today is the result of this long evolutionary journey. The multicellular animals, or metazoans, are traditionally divided into 35 distinct and very different phyla. How these phyla are related to each other has been the source of much discussion among biologists.
The Traditional Viewpoint
Taxonomists have traditionally attempted to create animal phylogenies (family trees; see section 15.5) by comparing anatomical features and aspects of embryological development. A broad consensus emerged over the last century about the main branches of the animal family tree.
The First Branch: Tissues. The kingdom Animalia is traditionally divided by taxonomists into two main branches: (1) Parazoa (“beside animals”)—animals that for the most part lack a definite symmetry and possess neither tissues nor organs, mostly composed of the sponges, phylum Porifera; and (2) Eumetazoa (“true animals”)—animals that have a definite shape and symmetry and, in most cases, tissues organized into organs and organ systems. In figure 19.1, all animals shown to the right of Parazoa are eumetazoans.
All the branches in the animal family tree trace back to one ancestor at the base of the phylogeny. This shared ancestor was probably a choanoflagellate, a colonial flagellated protist that lived in the Precambrian era over 700 million years ago.
Figure 19.1. The animal family tree: The traditional viewpoint.
Biologists have traditionally divided the animals into 35 distinct phyla. The diagram above illustrates the relationships among some of the major animal phyla. The bilaterally symmetrical animals (those to the right of Radiata in the figure above) are sorted into three groups that differ with respect to their body cavity: acoelomates, pseudocoelomates, and coelomates.
The Second Branch: Symmetry. The eumetazoan branch of the animal family tree itself has two principal branches, differing in the nature of the embryonic layers that form during development and go on to differentiate into the tissues of the adult animal. Eumetazoans of the subgroup Radiata (having radial symmetry) have two layers, an outer ectoderm and an inner endoderm, and thus are called diploblastic. All other eumetazoans are the Bilateria (having bilateral symmetry) and are triploblastic, producing a third layer, the mesoderm, between the ectoderm and endoderm.
Further Branches. Further branches of the animal family tree were assigned by taxonomists by comparing traits that seemed profoundly important to the evolutionary history of phyla, key features of the body plan shared by all animals belonging to that branch. Thus, the bilaterally symmetrical animals were split into groups with a body cavity and those without (acoelomates); animals with a body cavity were split into those with a true coelom (body cavity enclosed by mesoderm) and those without (the pseudocoelomates); animals with a coelom were split into those whose coelom derived from the digestive tube and those that did not, and so on.
Because of the either-or nature of the categories set up by traditional taxonomists, this approach has produced a family tree like the one in figure 19.1, with a lot of paired branches. The arbitrary nature of the divisions has always been obvious to biologists, but in spite of that, most biologists feel that it faithfully represents the general nature of the evolutionary history of metazoans.
The traditional animal phylogeny, while accepted by a broad consensus of biologists for almost a century, is now being reevaluated. Its simple either-or organization has always presented certain problems—puzzling minor groups do not fit well into the standard scheme. Results hint strongly that the key body-form characteristics that biologists have traditionally used to construct animal phylogenies—segmentation, coeloms, jointed appendages, and the like—are not the always preserved characters we had supposed. These features appear to have been gained and lost again during the course of the evolution of some unusual animals. If this pattern of change in what had been considered basic characters should prove general, our view of how the various animal phyla relate to one another is in need of major revision.
The last decade has seen a wealth of new molecular RNA and DNA sequence data on the various animal groups. The new field of molecular systematics uses unique sequences within certain genes to identify clusters of related groups. Using these sorts of molecular data, a variety of molecular phylogenies have been produced in the last decade. While differing from one another in many important respects, the new molecular phylogenies have the same deep branch structure as the traditional animal family tree (compare the lower branches in the “new” family tree in figure 19.2 with the lower branches in figure 19.1). However, most agree on one revolutionary difference from the traditional phylogeny used in this text and presented in figure 19.1: The protostomes (which have a different pattern of development than the deuterostomes—a topic that will be discussed later in this chapter) are broken into two distinct clades. Figure 19.2 is a consensus molecular phylogeny developed from DNA, ribosomal RNA, and protein studies. In it, the traditional proto- stome group is broken up into Lophotrochozoa and Ecdysozoa.
Figure 19.2. The animal family tree: A new look.
New phylogenies suggest that the protostomes might be better grouped according to whether they grow by adding mass to an existing body (Lophotrochozoa) or by molting (Ecdysozoa).
Lophotrochozoans are animals that grow by adding mass to an existing body. They are named for a distinctive feeding apparatus called a lophophore found in some phyla of the molecularly defined group. These animals—which usually live in water, have ciliary locomotion, and trochophore larvae —include flatworms, mollusks, and annelids.
Ecdysozoans have exoskeletons that must be shed for the animal to grow. This sort of molting process is called ecdysis, which is why these animals are called ecdysozoans. They include the roundworms (nematodes) and arthropods. How an animal grows wasn’t a key characteristic when classifying animals in the traditional approach, but proves to be an important characteristic when comparing animals molecularly.
This new view of the metazoan Tree of Life is only a rough outline—at present, molecular phylogenetic analysis of the animal kingdom is in its infancy. Phylogenies developed from different molecules sometimes suggest quite different evolutionary relationships. For this reason, in this text you will explore animal diversity guided by the traditional animal family tree. However, the childhood of the new molecular approach is likely to be short. Over the next few years, a mountain of additional molecular data can be anticipated. As more data are brought to bear, the confusion can be expected to lessen and the relationships within groupings more confidently resolved.
Key Learning Outcome 19.2. Major groups are related in very different ways in molecular phylogenies than in the more traditional approach based on form and structure.