Final answer:
The Phanerozoic Eon is marked by the expansion of multicellular organisms, beginning with the Cambrian explosion. It saw significant evolutionary changes, including the colonization of land by plants and animals, and various mass extinctions that shaped today's biodiversity.
Step-by-step explanation:
The Phanerozoic Eon is characterized by the expansion of multicellular organisms. This eon began with the Cambrian Period over 541 million years ago. During the Phanerozoic, there was a significant diversity in life forms, beginning with the Cambrian explosion where numerous multicellular organisms including sponges and trilobites became abundant.
Throughout the Phanerozoic, especially in the Paleozoic Era, there was significant evolutionary advancement with the emergence of various classes, families, and species. This era saw the colonization of land by plants and the subsequent adaptation of marine animals and semi-aquatic arthropods to terrestrial habitats.
Several major extinction events occurred during this time, yet some species survived, leading to the persistence and further evolution of most animal phyla into the complex life forms present today. Importantly, the Phanerozoic Eon saw the rise of vertebrates moving onto land and the diversification of these land-dwelling animals into distinct lineages such as synapsids and sauropsids, leading to mammals, reptiles, dinosaurs, and birds.