Mammal evolution began around 200 million years ago during the late Triassic period. Early mammals were small, shrew-like creatures that emerged from a group of reptiles known as synapsids. These early mammals adapted to various environments, leading to the development of distinct features such as fur and mammary glands. Over millions of years, mammals diversified into various groups, including monotremes, marsupials, and placental mammals. The extinction of the dinosaurs around 66 million years ago allowed mammals to thrive and occupy ecological niches, resulting in the wide variety of species we see today, from whales to elephants.