The major characteristic that defines the Cenozoic Era is the expansion and diversification of mammal species. The Cenozoic Era, also known as the Age of Mammals, began approximately 66 million years ago and continues to the present day. This era is marked by the dominance of mammals on Earth, as they rapidly evolved and diversified after the extinction of dinosaurs at the end of the Mesozoic Era. Various groups of mammals, including primates (which eventually led to the evolution of humans), appeared and thrived during this time, occupying diverse ecological niches and adapting to various environments. The Cenozoic Era also witnessed the development of grasslands, the formation of modern continents, and significant global climate changes.
