The Neogene Period spanned approximately 23 million years (23.04 to 2.58 million years ago), encompassing the Miocene and Pliocene Epochs. A gradual global cooling trend led to a drier, more seasonal climate, fundamentally altering ecosystems worldwide. The most significant shift was the widespread expansion of open habitats, specifically vast savannas and grasslands. This environmental change drove the rapid evolution and diversification of numerous animal groups that adapted to this new, grass-dominated world.
Adapting to Grasslands: Herbivorous Mammals
The proliferation of grasslands created a new niche for primary consumers, resulting in an explosive diversification among hoofed mammals, or ungulates. Species developed specialized body plans for running across open terrain and novel dental adaptations to process tough, abrasive grasses. The evolution of equids, the horse lineage, exemplifies this shift, as early three-toed forest browsers evolved into successful one-toed grazers. For instance, Pliohippus possessed high-crowned teeth designed for grinding silica-rich vegetation.
Proboscideans, the order that includes elephants, also underwent significant evolution. Early forms like gomphotheres, characterized by four tusks and shovel-like lower incisors, were common across North America and Eurasia. These were gradually replaced by more modern mastodons and the ancestors of modern elephants. Rhinoceroses also diversified, with some lineages developing longer legs for faster locomotion across the plains.
Artiodactyls, the even-toed ungulates, saw an immense radiation, leading to the appearance of many familiar groups. Early bovids, including modern cows and antelopes, expanded their variety, exhibiting teeth suited for grazing. Camels, which originated in North America, adapted to the drier, open habitats by developing long legs and efficient water retention. The success of these herbivores was linked to the dominance of C4 grasses, which became ecologically dominant late in the period.
Apex Predators and Specialized Carnivores
The abundance of fast-running, grazing mammals drove a corresponding evolution among terrestrial carnivores. Predators became more specialized and efficient hunters, leading to the rise of modern families like the Canidae (dogs and wolves) and the Felidae (cats).
Early large carnivores included the Amphicyonids, or “bear dogs,” which occupied a niche between modern bears and dogs. As the Neogene progressed, these forms declined while the Felidae became highly specialized. Saber-toothed cats, such as Machairodus, emerged, developing elongated canine teeth used for precision killing of large prey.
The development of modern carnivoran traits, including adaptations for pursuit hunting and specialized dentition, was a direct consequence of herbivore diversification. This period also witnessed the explosive radiation of bears (Ursidae) near the Miocene-Pliocene boundary. The predator-prey relationship became a dynamic evolutionary race across the developing savannas.
Marine and Avian Fauna
The oceans and skies also hosted significant evolutionary changes alongside terrestrial adaptations. In the marine environment, the Neogene saw the final stages of modern whale evolution. Both toothed whales (Odontocetes) and baleen whales (Mysticetes) diversified extensively, becoming fully aquatic and occupying niches from filter-feeding giants to agile pursuit predators.
Pinnipeds, including seals, sea lions, and walruses, evolved and spread widely, adapting to coastal and open-ocean environments. Dominating the marine food chain was the gigantic Megalodon shark, which reached lengths of up to 60 feet. This apex predator preyed on the newly abundant marine mammals.
In the skies, most modern avian orders were established, but the period saw a great diversification of passerines, or songbirds. Large, flightless predatory birds called “terror birds” (Phorusrhacids) remained major carnivores in South America before their decline. The success of modern bird groups was tied to the diversification of seed-producing plants in the grasslands.
The Emergence of Hominins
The later half of the Neogene is defined by the emergence of the hominin lineage. As the climate cooled and dried, African forests gave way to open woodlands and savannas, challenging the arboreal lifestyle of early primates. This environmental pressure encouraged a crucial adaptation in the ancestors of humans.
The first hominins appeared in Africa around 6 to 7 million years ago, separating from the lineage leading to modern chimpanzees. This divergence was marked by the gradual adoption of bipedalism, or walking upright on two legs. Bipedalism offered advantages in the open landscape, such as increased visibility and greater energy efficiency for travel.
The genus Australopithecus evolved around 4 million years ago, representing a diverse group of early hominins. Fossils like Australopithecus afarensis (“Lucy”) show habitual bipedalism combined with climbing adaptations, suggesting they exploited both ground and tree resources. Near the end of the period, around 2.4 million years ago, the first members of the genus Homo, such as Homo habilis, appeared, distinguished by larger brains and the first use of simple stone tools.