Mammals, a distinct class of vertebrate amniotes, have their roots in the Pennsylvanian geologic period, around 320 to 300 million years ago. Initially, amniotes quickly filled various survival and reproduction niches, leading to a natural selection that diversified these species. Shortly after their appearance, a crucial divergence occurred in the amniotes, separating the sauropsids (ancestors of reptiles and birds) and synapsids (other terrestrial vertebrates).
Within the synapsids, a group known as therapsids, specifically the large four-legged carnivores, emerged as the dominant land vertebrates, evidenced by fossils. A subgroup of therapsids, the cynodonts, distinguished by their “dog-teeth,” became particularly successful by the late Permian period.
However, the landscape of life drastically changed 252 million years ago with the End Permian mass extinction, also known as The Great Dying, which eradicated about 70 percent of all land-dwelling vertebrate species. This event ended the reign of large carnivorous therapsids but not the cynodonts. Around 220 million years ago, the first fossils of a new class of cynodonts emerged: mammals.
Mammals are characterized by features like hair, complex middle-ear bones, a neocortex, and mammary glands for nursing their young. Their evolution included a larger brain, enhanced sense of smell, generally smaller size, and a nocturnal lifestyle. These traits helped early mammals carve out successful survival niches during the Triassic period, even as it was dominated by large carnivores and herbivores, including dinosaurs and other predatory reptiles that survived The Great Dying.
Throughout the Triassic-to-Cretaceous periods (approximately 250 to 65 million years ago), Earth underwent several mass-extinction events. The most recent, marking the end of the Cretaceous, likely resulted from a large meteor impact. These events concluded the dominance of the ancient dinosaurs (though their lineage persists in modern birds) and opened new evolutionary niches. In this transformed world, mammals eventually evolved to become the dominant predators on Earth, a testament to their adaptability and resilience in the face of environmental upheavals.
