Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 32.1 Animals Are Multicellular, Heterotrophic Eukaryotes with Tissues That Develop from Embryonic Layers
Animals are incredibly diverse but share key features that distinguish them from other life forms. They are multicellular, heterotrophic eukaryotes with specialized tissues and a unique developmental process that includes embryonic stages like the blastula and gastrula.
Defining Characteristics of Animals
- Heterotrophic Nutrition: Animals ingest and internally digest food using enzymes (unlike plants or fungi).
- Multicellular Eukaryotes:
- Lack cell walls; rely on collagen and extracellular matrix for structure.
- Have specialized tissues such as muscle and nervous tissue.
- Reproduction and Development:
- Primarily sexual reproduction with a dominant diploid stage.
- Zygote → cleavage → blastula → gastrula.
- Germ layers: ectoderm (outer), endoderm (inner).
- Many undergo a larval stage followed by metamorphosis.
Genetic Control and Development
- Development regulated by genes shared across animal species.
- Most animals have Hox genes:
- Contain homeoboxes, regulating embryonic development.
- Determine body plan and structure.
Origins of Animals and Their Closest Relatives
- Animals likely originated 710–770 million years ago.
- Choanoflagellates are the closest living relatives:
- Resemble sponge collar cells (choanocytes).
- Genetic similarities in adhesion and signaling genes.
- Common ancestor likely a flagellated, unicellular protist.
Multicellularity and Early Animal Evolution
- Multicellularity required:
- Evolution of cell adhesion and communication mechanisms.
- Fossils and molecular evidence suggest:
- Sponges appeared ~700 million years ago.
- Common ancestor of animals existed ~770 million years ago.
- Early embryos likely passed through cleavage, blastula, and gastrula stages.
In a Nutshell
Animals are defined by their multicellularity, internal digestion, tissue development, and unique embryonic processes. Their closest relatives are choanoflagellates, and they likely evolved from a simple, aquatic, flagellated ancestor. These shared traits, along with Hox genes, unify animals despite their vast diversity.