The Pliocene Epoch is a division of the geologic timescale that extends from 5.33 to 2.58 million years ago. It is the second and most recent epoch of the Neogene Period, following the Miocene and preceding the Pleistocene. This period is characterized as a time of global cooling after the warmer Miocene, yet it remained significantly warmer than the modern climate. The boundaries of the Pliocene are not marked by a single global event but by regional shifts from the preceding warmer period.
A Warmer World’s Geography and Climate
The Pliocene climate was, on average, warmer and wetter than the climate of today. During a phase known as the Mid-Piacenzian Warm Period, about 3.3 to 3.0 million years ago, global average temperatures were 2–3°C higher than they are now. Atmospheric carbon dioxide concentrations were comparable to modern levels, hovering around 400 parts per million, which makes the epoch a subject of scientific interest for understanding our own climate’s future.
During this epoch, the Earth’s continents continued to drift toward their present-day locations, moving from positions as far as 250 kilometers away to within 70 kilometers of where they are now. A significant geological event was the formation of the Isthmus of Panama around 3.5 million years ago. This connection redirected ocean currents, notably strengthening the Gulf Stream and contributing to warmer conditions in the North Atlantic. Sea levels during the Pliocene were higher than they are today, submerging coastal areas like Florida and parts of Europe.
The Rise of Modern Ecosystems
A gradual cooling and drying trend throughout the Pliocene spurred changes in terrestrial habitats. This climatic shift led to the expansion of grasslands, savannas, and deserts, often at the expense of tropical forests. These new environments drove the evolution of the animal life that inhabited them, giving rise to many species with modern characteristics.
This epoch is well known for its megafauna. In North America, landscapes were populated by giant ground sloths like Megalonyx and large, armored glyptodonts. Predators included the saber-toothed cat, Smilodon, while the large proboscidean Deinotherium roamed across Eurasia and Africa. In the oceans, the giant shark Carcharocles megalodon was a dominant predator.
The formation of the Isthmus of Panama triggered the Great American Biotic Interchange. This event allowed animals to migrate between North and South America for the first time in millions of years. North American animals like saber-toothed cats and gomphotheres moved south, while South American species, including ground sloths and porcupines, migrated north.
The Dawn of Humanity
The Pliocene marks an important chapter in human origins, witnessing the appearance of early human ancestors, or hominins. These were not modern humans but belonged to a group of species that were part of our evolutionary lineage. The most prominent of these was the genus Australopithecus, which first appeared in the fossil record during this epoch.
Australopithecines are defined by a combination of features, most notably their ability to walk on two legs, a method of locomotion known as bipedalism. This adaptation is clearly seen in their skeletal structure, yet their brain size remained relatively small compared to later hominins in the genus Homo. They retained some ape-like features, suggesting they were still adapted to climbing trees to some extent.
One of the most famous fossil discoveries from this period is “Lucy,” a complete skeleton of an Australopithecus afarensis. Her discovery provided strong evidence of bipedalism in early hominins. The environmental shifts of the Pliocene are thought to have played a direct role in this evolutionary development, as expanding savannas made walking upright advantageous.
The End of an Epoch
The Pliocene drew to a close as the global climate continued its cooling and drying trend, leading into the Pleistocene Epoch. This gradual transition culminated in the Quaternary glaciation, which marked the start of the ice ages. The stable and warm world of the Pliocene gave way to cyclical climate fluctuations involving the advance and retreat of massive continental ice sheets, reshaping the planet’s surface.
This climate change drove many Pliocene species to extinction, as animals that had thrived in the warmer conditions could not adapt to the increasingly cold world. However, this environmental pressure also spurred the evolution of new species. Animals adapted to the cold, such as the woolly mammoth, emerged in the new landscapes of the Pleistocene.