The Cenozoic Era began approximately 66 million years ago and extends to the present day. It is popularly known as “The Age of Mammals” because warm-blooded vertebrates diversified to dominate nearly every terrestrial and marine ecosystem. The era began with the Cretaceous–Paleogene (K–Pg) extinction event, a global catastrophe that eliminated the non-avian dinosaurs and many other life forms. This mass die-off cleared the ecological stage, setting the conditions for major evolutionary and geological transformations that followed.
The Starting Line and Early Mammalian Diversification
With the disappearance of large reptiles, small, generalized mammals were presented with vast ecological niches to fill. The Paleogene Period, which encompasses the Paleocene, Eocene, and Oligocene epochs, saw an immediate and rapid evolutionary rebound. Early Paleocene mammals were typically small, but they quickly diversified.
The Eocene Epoch was characterized by a warm, humid “hothouse” climate, which accelerated mammalian diversification. This widespread warmth, with tropical forests extending almost to the poles, led to the first appearance of many modern mammalian groups, including the ancestors of whales, bats, rodents, and early primates.
By the Oligocene Epoch, the climate began a long-term cooling and drying trend, causing tropical forests to recede. This shift favored the evolution of ungulates, or hoofed mammals, which rapidly diversified. These groups include modern deer, pigs, and rhinoceroses.
Continental Shifts and Global Cooling
The Neogene Period was defined by immense geological forces that reshaped the planet and drove global climate change. Continents continued drifting toward their current positions, resulting in massive tectonic collisions. The ongoing impact of the Indian subcontinent into Asia thrust up the immense peaks of the Himalayas, and similar forces created the Alps in Europe.
These mountain-building events exposed fresh rock to weathering, which is thought to have drawn carbon dioxide out of the atmosphere over millions of years. The formation of geographic barriers, such as the Isthmus of Panama, also altered global ocean currents and circulation patterns.
This combination resulted in a long-term global cooling trend that set the stage for the later Ice Ages. As the climate cooled and dried, forests were replaced by expansive grasslands and savannas. This new environment spurred evolutionary changes among grazing mammals whose teeth adapted to grind tougher grasses.
The Age of Ice and Megafauna
The cooling trend climaxed during the Quaternary Period, beginning 2.6 million years ago, characterized by cyclical climate fluctuations known as the Ice Ages. These cycles involved long glacial periods, lasting approximately 90,000 years, where continental ice sheets advanced across much of North America and Eurasia, separated by shorter, warmer interglacial periods. These repeated shifts led to the evolution of specialized, cold-adapted animals known as megafauna.
These large mammals included species like the woolly mammoth, woolly rhinoceros, and giant ground sloth. Megafauna were distinguished by massive body sizes and thick coats of fur or blubber, adaptations that helped them conserve heat in frigid conditions.
The end of the last major glacial period, around 11,700 years ago, coincided with a widespread mass extinction of these species. This event disproportionately affected large species weighing over 45 kilograms. The timing of these extinctions aligns strongly with the arrival of early human populations, suggesting human predation was the main cause of their demise.
The Emergence of Hominids
The evolution of the human lineage, beginning with early primates in Africa, was a defining development of the late Cenozoic. One of the earliest and most profound changes was the development of habitual bipedalism, the ability to walk upright on two legs. This adaptation was present in early hominins long before significant increases in brain size.
Bipedalism freed the hands, paving the way for the development of tool use, with the earliest stone artifacts dating back to 3.3 million years ago. This technological innovation preceded the emergence of the genus Homo. Homo habilis, appearing around 2.8 million years ago, is credited with the first widespread use of simple stone tools known as Oldowan technology.
Subsequent species, such as Homo erectus, developed more sophisticated tools and were the first hominins to migrate out of Africa and colonize parts of Eurasia nearly two million years ago. The evolution of Homo sapiens occurred in Africa approximately 300,000 years ago. These anatomically modern humans began their global migration around 60,000 to 100,000 years ago, eventually reaching Australia and the Americas.