Cladogram of major animal phyla based on development. The colored bars at top mark clades based on similar developmental structures. The labels on the branches show when a particular trait evolved. Fifteen of the most significant animal phyla (of approximately 35 total) are shown. Choanoflagellates are protists that live in colonies and are thought to be the living organisms most closely related to animals.
TissuesDinosaurs are prehistoric and extinct, but they are not the most primitive animals. That honor belongs to sponges. We are not talking about the type of sponge at the kitchen sink. We're talking about the SpongeBob SquarePants type of sponge. They're bright pink, purple, and orange sponges that live in the ocean and make up the phylum Porifera. Sponges are joined by another phylum, Placozoa, in a clade called the Parazoa. The animals in both phyla are asymmetrical. They have specialized cells that perform different functions within the organism but their cells do not join together to form tissues. Tissues are groups of cells, like skin cells, working together on the same job.
At some point early in the evolutionary history of animals (about 940 million years ago), scientists believe that some of the Parazoa started to form tissues during development. This gave those animals such an evolutionary advantage that almost all animals today have tissues. We call all animals with tissues the Eumetazoa. We know—Sneezy would make more sense. They begin development as a single cell (zygote) that multiplies and becomes a ball of cells (embryo), which then forms into tissues called germ layers. Simple animals like jellyfish only form two germ layers: an outer layer, or ectoderm, and an inner layer, or endoderm.
Another evolutionary step occurred when some animals developed a third, middle germ layer, called the mesoderm. The bodies of all animals in the clade Bilateria are formed from three germ layers. Each germ layer always becomes the same type of tissue in adult animals. For example, the outer germ layer (or ectoderm) always becomes the nervous system and skin (or its associated scales, feathers, or hair) in all animals in the Bilateria clade.
SymmetryWithin the Eumetazoa, there are two clades representing another accomplishment in EvoDevo: symmetry. If you draw an imaginary line through the middle of the organism, does it look the same on both sides of the line? Simple animals like jellyfish exhibit radial symmetry. Their body structures are arranged like the spokes of a wheel around a central axis. They also receive information about their environment equally from every "side" of their bodies. That's like a teacher having eyes in the back of her head, and on both sides—bad news bears for anyone texting in class. Animals that show radial symmetry are within the clade known as Radiata and they include the phyla Cnidaria and Ctenophora.
Aurelia aurita jellyfish exhibit radial symmetry. Image from here.
During at least one part of development, most animals exhibit bilateral symmetry, which means that there is only one line that can be drawn through them to create symmetry. A corn dog has bilateral symmetry. An animal that is bilaterally symmetrical has some specialized structures at one end or the other, indicating that it interacts with its environment in a directional fashion—usually "head on." Conveniently, this is also the best way to attack with a corn dog. Animals exhibiting bilateral symmetry are part of the clade called Bilateria.
Within the Bilateria, we can see three clades of animals based on the presence and type of body cavity. A body cavity, or coelom (pronounced "see-lum"), is an inner, fluid-filled space between the digestive tube and the outer wall of an organism. In humans, this space is where our inner organs (such as the kidneys) "float."
The most primitive bilateral animals developed in a simple fashion from their 3 germ layers. They had an outer "skin" made from the ectoderm, a muscle layer and inner gel-like layer made from the mesoderm, and an inner tube for digestion, made from the endoderm. These animals include the flatworm and ribbon worm phyla, Platyhelminthes and Nemertea, and are called Acoelomates, because they have no coelom.
The next clade is called the Pseudocoelomates because they have a "quasi-coelom." They have a space between their digestive tract and their outer wall but it isn't completely set apart as a body cavity because it is only lined with a mesodermal tissue layer on the outer edge closest to the outer wall. Starting from the center of their bodies, we find a digestive tract (from endoderm), a fluid-filled space, a muscle layer (from mesoderm), and an outer "skin" (from ectoderm). Included in this clade are the phyla Nematoda and Rotifera.
The last clade within Bilateria is called the Coelomates because the animals within this group have true coeloms. Their body cavities are surrounded entirely by a layer of tissue that develops from the mesoderm. Thus, again starting from the center of their bodies, we find a digestive tract (from endoderm), a muscle layer (from mesoderm), a fluid-filled space, another muscle layer (from mesoderm), and an outer "skin" (from ectoderm).
Patterns of Cleavage and GastrulationBilateral animals can also be grouped into two other clades (Protostomes and Deuterostomes) based on two other adaptive patterns of EvoDevo.
Animals begin as a single cell, which divides into two cells, which then divide into four cells all in a single plane (they're flat). During the next cell division, the four new cells will be added on top of the four original ones, creating a third dimension. If these new cells appear directly above the four original ones, the pattern is called radial cleavage, which is characteristic of deuterostomes. If the new cells are added diagonally above the four original ones, the pattern is called spiral cleavage, which is characteristic of protostomes. This difference isn't the key difference between protostomes and deuterostomes, though.
Later in development, the embryo, called a blastula at that point, undergoes a process called gastrulation. The embryo consists of a ball of cells, one layer on the outside (ectoderm) and one mass of cells inside (mesoderm). Some of the outer, ectodermal cells push themselves in toward the center of the ball (imagine poking your finger into a balloon), creating a third layer of cells (endoderm), which form an inner canal with an opening to the outside world. This opening is called the blastopore. In most protostomes, the blastopore becomes the mouth of the organism. In deuterostomes, it becomes the anus and the mouth forms later from a second opening. This difference is seen in the names of the two clades, which come from Greek. "Stoma" means "mouth," "proto" means "first," and "deutero" means "second."
All acoelomates and pseudocoelomates and some coelomates are protostomes. That suggests that the developmental patterns of deuterostomes developed in a group of coelomates later in evolutionary history.