Slide 1 We are now going to talk through the major evolutionary events in animal evolution. Please be aware that this tree of animal evolution does not include all animal groups - but instead shows the groups that demonstrate these major events in animal evolution. As we talk through the tree - we will see that there are major bifurcations or branching points. The first bifurcation in the tree of animal evolution is the division between those animals that do not have true tissues - known as the parazoa - and those animals that do have true tissues - the eumetazoa. The group of organisms that demonstrates no true tissues are the sponges. All other animals on the tree fall within the eumetazoa and have true tissues.
Slide 2 Another aspect of animal morphology is symmetry. The parazoa - those animals that lack true tissues - also lack symmetry. All of the eumetazoa have either radial symmetry or bilateral symmetry.
Slide 3 Our second bifurcation in the tree of animal evolution is the division between those animals with radial symmetry and those with bilateral symmetry. The representative group of radially symmetrical organisms is the group that includes the jellyfish. All other animals are members of the Bilateria and have bilateral symmetry.
Slide 4 Other distinguishing characterstics of the animals include developmental events. Developmental biology is the study of irreversible events from zygote to death. This includes the control of cell growth, differentiation of cells into different types, and the processes that gives rise to tissues, organs and anatomy. Slide 5 For the purposes of this course, we will only be looking at the earliest stages of animal development. As we have already learned, fertilization of an egg by a sperm produces a diploid zygote. This zygote then divides via mitosis to produce the earliest cells in the embryo. These earliest mitotic cell divisions are known as cleavage. Cleavage results in different types of cells in different types of organisms. In some organisms, the cells that result from these early cleavages are determinate. In other organisms, including humans, the cells are indeterminate. This means they have a characteristic known as totipotency.
Slide 6 When a cell is determinate it means that the fate of the cells are determined when the cell is formed. When a cell is indeterminate it means that the developmental fate of the cells in the developing embryo are not determined by the identity of the parent cell - they can differentiate into any cell type. This means that these cells are totipotent - the ability of a cell to divide and produce all the cell types in an organism.
Slide 7 Totipotent cells are also known as stem cells. Stem cells are a controversial issue because embryonic stem cells require the destruction of an embryo. This diagram shows the basic steps in cultivating stem cells. First, a fertilized egg is produced using in vitro fertilization - this means artificially producing an embryo in a lab. The resulting zygote develops for about 6 days - at which time it has produced an inner stem cell mass of totipotent cells. These cells are then harvested and, because they are totipotent, can be coaxed to develop into different cell types that can potentially be used in human therapies.
Slide 8 There are some key stages in development that distinguish the different branches of animal evolution. The first distinct structure to form during development is an 8-cell mass of cells. This develops into a hollow mass of cells known as a blastula.
Slide 9 The blastula then develops into a structure known as a gastrula through a process known as gastrulation. Gastrulation is the movement of surface cells into the developing embryo and during this process cells begin to differentiate into different tissue types. This is the first appearance of tissues in the developing embryo.
Slide 10 During gastrulation, either two or three tissue layers can result. Organisms that only produce two embryonic tissue layers (endoderm and ectoderm) are known as being diploblastic. Organisms that produce three embryonic tissue layers (endoderm, ectoderm, and mesoderm) are known as being triplobastic. Endoderm ultimately will develop into digestive organs and the tissues that line internal organs. Mesoderm will develop into skeleton and muscles. Ectoderm will develop into skin and nervous tissue.
Slide 11 At this point we can look once again the second bifurcation in the tree of animal evolution. Not only does it divide the Radiata from the Bilateria - but it also divides the diplobasts from the triplobasts. All of the radially symmetrical organims are diplobastic and produce only two embryonic tissue layers. All of the bilaterally symmetrical organisms are triploblastic and produce three embryonic tissue layers during gastrulation.
Slide 12 Another developmental difference between the branches within the bilateria is whether or not there is an internal fluid filled cavity known as a coelom. Animals within the Bilateria can be either acoelomates - animals that lack a body cavity, or Psuedocoelomates which are animals that have a body cavity that forms between endoderm and mesoderm. The true coelomates are animals that have a coelom that forms within the mesoderm tissue. The coelom is a body cavity - it is not the same as the digestive tract.
Slide 13 This image from the tutorial shows the differences between these three types of animals. In this image - endoderm is yellow, mesoderm is red, ectoderm is blue and the coelom is white. The acoelomates do not have a body cavity - they have a solid body. The true coelomates have a coelom that is completely surrounded by mesoderm. The pseudocoelomates have a body cavity that is surrounded by mesoderm on one side and endoderm on the other.
Slide 14 This brings us to the 3rd and 4th bifurcations in the tree of animal evolution. The 3rd bifurcation is between the aceolomates and those animals with a body cavity. The 4th bifurcation is between the pseudocoelomate and the true coelomates. The acoelomates are represented by the flatworms while the pseudocoelomates are represented by the nematodes. All of the remaining animals are members of the true coelomates.
Slide 15 Within the true coelomates, there is a final division - the protostomes and the deuterostomes. This distinction results during gastrulation when the movement of cells forms a structure known as an archenteron which will become the gut. The protostomes are those animals in which the mouth forms first. These animals have spiral cleavage at the 8-cell stage and the cells are determinate at this stage. The deuterstomes are those animals in which the mouths forms secondarily. These animals have radial cleavage at the 8-cell stage and the cells are indeterminate.
Slide 16 The 5th bifurcation in the tree of animal evolution is the division between protostomes and deuterostomes. The protostomes are represented by the molluscs, annelids, and arthropods. The deuterostomes are represented by the echinoderms and the chordates. The remaining tutorials will look at all of these different groups of animals in more detail.