Final answer:
Mammals became the dominant type of life during the Cenozoic Era, which began around 65 million years ago, following the mass extinction at the end of the Cretaceous Period.
Step-by-step explanation:
The Cenozoic Era is commonly referred to as the age of mammals, and it is during this period that mammals became the dominant type of life on Earth. After the mass extinction at the end of the Cretaceous Period, which marked the closure of the Mesozoic Era also known as the 'Age of Reptiles', mammals diversified and filled ecological niches previously occupied by dinosaurs. The Cenozoic began approximately 65 million years ago and extends to the present day. This era saw the evolution and radiation of mammals into a wide array of forms and habitats, exploiting opportunities that arose due to the absence of the then-dominant dinosaurs. Over time, mammals flourished, becoming the dominant animals, especially evident after the last ice age that ended about 12,000 years ago leading to further mammalian diversification including the evolution of Homo sapiens.