Animal Evolution and Classification

All true animals—members of the kingdom Animalia—are eukaryotic multicellular organisms that share a single common evolutionary ancestor (Fig. 3.4). This last common ancestor was likely a single-celled organism similar to modern day choanoflagellates. Choanoflagellates are single-celled eukaryotes that live in aquatic habitats. The term originates from the Greek root word choanos meaning collar. Each individual has a single whip-like flagellum and a collar-shaped structure used to capture food particles (Fig. 3.5 A). Multicellular organisms are living beings that consist of more than one cell. The phenomenon of mullticellularity evolved independently several times in different groups of living organisms such as algae, plants, fungi, and animals. Although choanoflagellates are individual single-celled organisms, some species group together to form colonies (Fig. 3.5 B).

These choanoflagellate colonies resemble some simple sponges. Sponges—in the phylum Porifera. This phylum is considered basal because it is closer to the root or ancestral end of a phylogenetic tree relative to other phyla (Fig. 3.4). Sponges are true multicellular animals. They have simple body plans lacking the tissues and organs found in other animal phyla. The phylum Porifera will be discussed in greater detail in the topic, Phylum Porifera.

Tissue Layers: Diploblasts and Triploblasts

Tissues are layers or groups of cells with similar structure and function. Humans (belonging within the phylum Chordata) have several different types of tissues. Examples include smooth muscle tissue in the stomach and nerve tissue in the eyes. Organs are groups of different tissues that perform a specific function. Examples of organs in a human include the stomach and the eye.

The first animals to evolve distinct, organized tissues gave rise to the phyla Cnidaria (jellyfish, anemones, and corals) and Ctenophora (comb jellies; Fig. 3.4). Cnidarians and comb jellies are diploblastic, meaning they develop from two layers of true tissues: an outer ectoderm and an inner endoderm. All other animal phyla are considered triploblastic, meaning they develop from three distinct tissue layers. These three tissue layers are the endoderm, mesoderm, and ectoderm.

Developmental Patterns: Protostomes versus Deuterostomes

Having three distinct tissue layers enabled triploblastic animals to evolve into a wide diversity of body forms and phyla. Most animal species are triploblasts. Triploblastic animal phyla can be separated into two large monophyletic groups: protostomes and deuterostomes.

During sexual reproduction, a single sperm cell fertilizes an egg cell. This fertilized egg cell, or zygote, soon begins to divide through the process of mitosis. After the zygote divides into two new cells, it is called an embryo. An embryo is a multicellular eukaryotic organism early in its development following fertilization and preceding birth or hatching. The cells of the embryo continue to divide and form a hollow sphere of cells called a blastula. The next stage of embryonic development is called gastrulation. Gastrulation is a developmental phase where cells in the ball-shaped blastula reorganize and fold inwards (Fig. 3.6). After gastrulation, the embryo is a hollow ball of cells with a deep dimple on one end.

In protostome animals, this deep dimple from gastrulation eventually develops into a mouth. Protostomes are a monophyletic group of animals whose embryos all develop in this “mouth first” pattern (proto– meaning first and –stome meaning mouth). The major phyla Mollusca, Annelida, and Arthropoda are all protostomes (Fig. 3.4).

The deep dimple from gastrulation develops into an anus in deuterostome animals. Deuterostome embryos produce mouths later in development (Fig. 3.6). Deuterostomes are a monophyletic group of animals whose embryos all develop in this “mouth second” pattern (deutero­– meaning second and ­–stome meaning mouth). Examples of deuterostome animals include the phyla Echinodermata and Chordata (Fig. 3.4). All vertebrate animals—including humans—are triploblastic deuterostomes.