BISC 121Lg Lecture Notes - Lecture 13: Cambrian Explosion, Hox Gene, Symmetry In Biology
Document Summary
Animals inhabit almost all environments, though most are aquatic (marine) Animals have structure, nutrition, and life history: probably evolved from colonial flagellated protists. Traditional animal phylogeny is based on grades in body plans. Cambrian explosion (occurred 535~525 million years ago) marks the appearance of many major groups of living animals: could have been because of: Evolution of hox gene complex: few new phyla have evolved since the cambrian explosion recent evolution has generated new versions of old designs. What defines animals: multicellular heterotrophic eukaryotes, lack cell walls, have tissues, sexual reproduction and a diploid adult stage, hox genes regulate early development. Bilateral symmetry- dorsal and ventral side, right and left side, anterior and posterior ends and a head: cephalization- development of head. Animals are monophyletic: common ancestor is choanoflagellates. Protostomes: have spiral and determinate division at the 8 cell stage, schizocoelous- solid masses of mesoderm split to form coelom, mouth develops from blastopore.
