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Animal Origins and the Evolution of Body Plans
,Animal Origins and the Evolution of Body Plans Animals: Descendants of a Common Ancestor
Body Plans: Basic Structural Designs Sponges: Loosely Organized Animals Cnidarians: Two Cell Layers and Blind Guts Ctenophores: Complete Guts and Tentacles The Evolution of Bilaterally Symmetrical Animals Simple Lophotrochozoans Lophophorates: An Ancient Body Plan Spiralians: Spiral Cleavage and Wormlike Body Plans ,Animals: Descendants of a Common Ancestor Evidence indicates that all animals are descendants of a single ancestral lineage.
All animals share a set of derived traits: Similarities in their small-subunit ribosomal RNAs
Similarities in their Hox genes Special types of cell–cell junctions: tight junctions, desmosomes, and gap junctions A common set of extracellular matrix molecules, including collagen ,Animals: Descendants of a Common Ancestor Animals evolved from ancestral colonial flagellated protists.
Within these ancestral colonies, a division of labor arose. Cells became specialized for different functions, such as movement, nutrition, and reproduction. The specialized units continued to differentiate while improving their coordination with other working groups of cells. These coordinated groups of cells evolved into animals. ,Animals: Descendants of a Common Ancestor Generalized traits characterize animals:
They are multicellular organisms that must take in pre-formed organic molecules.
They acquire these organic molecules by ingesting other organisms, living or dead, and digesting them within their bodies. Animals must expend energy to acquire these organic molecules. Most have circulatory systems that carry O2, CO2, and nutrients. ,Animals: Descendants of a Common Ancestor Much of the diversity in the animal kingdom evolved as animals acquired the ability to capture and eat many different types of food and to avoid becoming food for other animals.
The need to move in search of food has favored sensory structures that provide animals with detailed information about their environment. Animals expend a considerable amount of energy to maintain relatively constant internal conditions while taking in foods that vary chemically. ,Animals: Descendants of a Common Ancestor Clues to the evolutionary relationships among animals are found in the fossil record, patterns of embryonic development, comparative physiology and morphology, and the structure of molecules such as the small-subunit RNAs and mitochondrial genes.
The sponges, cnidarians, and ctenophores separated from the other animal lineages early in evolutionary history. The remaining animals have been divided into two major lineages: the protostomes and the deuterostomes. ,Figure 32.1 A Current Phylogeny of the Animals
,Animals: Descendants of a Common Ancestor Animals form layers of cells during their development from a single-celled zygote to a multicellular adult.
The embryos of diploblastic animals have two cell layers: an outer ectoderm and an inner endoderm. The embryos of triploblastic animals have a third layer, the mesoderm. The existence of three cell layers distinguishes the protostomes and deuterostomes from simple animals that diverged earlier. ,Animals: Descendants of a Common Ancestor Protostomes and deuterostomes differ in the fate of the blastopore , the opening of the cavity that forms in the spherical embryo.
In the protostomes, the mouth arises from the blastopore. In the deuterostomes, the blastopore gives rise to the anus. ,Animals: Descendants of a Common Ancestor Most protostomes and the deuterostomes exhibit a pattern of early cell division in the fertilized egg called radial cleavage.
In radial cleavage, cells divide along a plane either parallel to or at right angles to the long axis of the fertilized egg. One major protostome lineage evolved a pattern of early cell division called spiral cleavage. ,Body Plans: Basic Structural Designs The entire structure of an animal, its organ sys

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