Evolution of Animals

Evolution of Animals

Common Ancestor of Animals

• Similar to Choanoflagellates: sister to animilia
• Probably heterotrophic protist
• Characteristics
• Single flagellum surrounded by microvilli
• Similar cell morphology to Sponges (Porifera)
• Collar cells: extension of cell membrane
• Choanocytes: specialized cells that digest food

Evolution of Multicellularity

• Many independent origins of multicellularity Examples: Animals, fungi, plants etc.
• Probably a ball shaped choanoflagellate evolved an ecto- and endo- dermal layers > simple animal
• Choanoflagellates likely began to stick together (grow as a colony)
• Seeing how zygotes develop gives insight into the evolutionary origins
• When cells grouped together were more successful, multicellularity evolved
• Higher relative fitness

• Selective advantages
• Colonies were larger
• harder for predators to eat
• Easier for colonies to eat other things
• larger
• more coordination
• developed specialization of tasks: division of labor, more efficient
• Disadvantages
• Most cells in colony don't get to reproduce
• cells that do reproduce need support
• Somatic cells: handle growth and maintenance
• Requires cooperation
• Some cells may cheat Example: Cancer involves uncontrolled growth of selfish cells

Overcoming the Limits of Diffusion

• Diffusion: Random movement of molecules from high density to low density
• Only good over short distances
• Slow and ineffective
• Limits size of organism Examples: bacteria
• Solution
• Bulk Flow: moving molecules together in large groups
• Active transport of oxygen and nutrients in bulk
• Uses specialized tissue Examples: Circulatory system
• Not in sponges and jellies
• Metabolically active tissues are near the surface to maximize diffusion

Eras of Animal Evolution

• Paleozoic Era (542 - 421 MYA)
• Cambrian explosion: Rapid fossil evidence of many major groups of animals
• Evolution of complex life Examples: Vertebrates with skeletons
• Most major phyla appeared
• Mesozoic Era (251 - 65.5 MYA)
• Age of Dinosaurs
• First mammals appeared
• Diversification of flowering plants and insects
• Cenozoic Era (65.5 MYA - present)
• Mass extinction of terrestrial and marine animals
• Non-flying Dinosaurs and reptiles
• Mammals diversified and increased in size
• Filled vacant niches