Four million years ago, Earth was in the midst of one of the most consequential periods in the history of life: early human ancestors were walking upright across the African landscape, global climates were cooling and drying, and entire ecosystems were reshaping themselves in ways that would set the stage for the modern world. The period around 4 million years ago, situated in the geological epoch known as the Pliocene, sits at a crossroads between a warmer, wetter past and the ice ages that would follow.
Our Ancestors Were Already Walking Upright
The most famous story from 4 million years ago is a human one. Several species of early hominins, the group that includes humans and our closest extinct relatives, were living in eastern and southern Africa. The best-known of these is Australopithecus anamensis, which lived between roughly 4.2 and 3.9 million years ago in what is now Kenya and Ethiopia. These were small-brained, ape-like creatures, but their leg bones and joints show clear signs of habitual bipedalism. They walked on two legs, at least some of the time, while still retaining features suited for climbing trees.
Shortly after, around 3.9 to 3 million years ago, Australopithecus afarensis appeared, the species that includes the famous “Lucy” skeleton. The transition between these two species likely happened right around the 4-million-year mark. These early hominins had brains roughly a third the size of a modern human’s, stood about 3.5 to 5 feet tall, and lived in mixed environments of woodland and grassland. They ate a varied diet of fruits, seeds, roots, and possibly insects or small animals.
Upright walking is the defining trait of the human lineage, and by 4 million years ago it was already well established. The evolutionary pressures behind it remain debated, but the shift toward more open landscapes likely played a role. Walking on two legs is more energy-efficient than knuckle-walking over long distances, which matters when food sources are spread across a drier, more open terrain.
Africa’s Forests Were Giving Way to Grasslands
The world 4 million years ago was warmer than today, but it was cooling. Global temperatures during the mid-Pliocene were about 2 to 3 degrees Celsius warmer than pre-industrial levels, and sea levels were roughly 15 to 25 meters higher. The Arctic had limited ice coverage, and forests extended much farther north than they do now. But a long-term cooling trend was underway, driven by shifts in ocean circulation, the uplift of mountain ranges, and changes in atmospheric carbon dioxide levels.
In Africa, this cooling and drying trend had a dramatic effect on the landscape. Dense tropical forests that had once blanketed much of eastern Africa were thinning and fragmenting, replaced by a patchwork of woodland, bushland, and expanding grasslands. The East African Rift System, a massive geological fracture still active today, was reshaping the topography. As the land on the eastern side of the rift rose, it created a rain shadow that dried out the interior, accelerating the spread of savanna environments.
This ecological transformation is directly linked to human evolution. As forests shrank, the ancestors of humans had to adapt to life in more open, seasonal environments where food and water were less predictable. The shift toward bipedalism, changes in diet, and eventually the development of tool use and larger brains all trace back, at least in part, to the environmental pressures that were intensifying around 4 million years ago.
The Panama Seaway Was Closing
One of the most significant geological events happening around this time was the formation of the Isthmus of Panama, the narrow land bridge connecting North and South America. Before roughly 4 to 3 million years ago, a deep-water seaway separated the two continents, allowing ocean currents to flow freely between the Atlantic and Pacific. As tectonic activity gradually pushed the land bridge upward, that connection closed off.
The consequences were enormous. With the seaway blocked, warm Caribbean water that had previously flowed into the Pacific was redirected northward, strengthening the Gulf Stream. This pumped more moisture and heat toward the North Atlantic, which paradoxically helped trigger the buildup of ice sheets in the Northern Hemisphere over the following million years. The closure of the Panama Seaway is considered one of the key factors in the onset of the Pleistocene ice ages.
The land bridge also triggered what paleontologists call the Great American Biotic Interchange. For the first time in tens of millions of years, animals could walk between North and South America. Over the next few million years, species migrated in both directions. Ancestors of armadillos, porcupines, and opossums moved north, while cats, dogs, bears, horses, and deer moved south. South America’s native predators, many of them marsupials, were largely displaced by the more competitive placental carnivores arriving from the north.
Global Wildlife Looked Very Different
The animal world 4 million years ago was recognizable but strange. Many modern mammal families already existed, but the individual species were often larger, more diverse, or found in unexpected places. Africa was home to a wider array of large herbivores than today, including several species of elephants, giant pigs, and diverse antelopes. Saber-toothed cats were apex predators across multiple continents. Hyenas were more varied and widespread than their modern descendants, with some species filling ecological roles now occupied by wolves.
In North America, horses were abundant and diverse, with multiple species grazing the open plains. Camels, which actually originated in North America, were still millions of years from crossing into Asia. Giant ground sloths roamed South America, some reaching the size of elephants. The oceans held species of whales and sharks, including close relatives of the great white, hunting in waters warmer than today’s.
Australia had already separated from the other southern continents long before this period and was evolving its own unique fauna. Giant marsupials were diversifying, including early relatives of the massive Diprotodon, the largest marsupial that ever lived. Giant flightless birds occupied predatory niches on several continents, though their dominance was beginning to wane as mammalian predators diversified.
The Climate Was a Preview of Our Future
Scientists study the mid-Pliocene intensely because it represents the last time Earth’s climate was as warm as projections suggest it will become by the end of this century. Carbon dioxide levels 4 million years ago hovered around 350 to 450 parts per million, a range we have already entered and are continuing to push upward. Understanding what the planet looked like under those conditions helps climate scientists model what changes to expect in sea level, ice coverage, and weather patterns.
The picture from the Pliocene is sobering. With temperatures only modestly warmer than today, the Greenland ice sheet was significantly smaller, the West Antarctic ice sheet may have been largely absent, and sea levels were high enough to submerge many of today’s coastal cities. Forests grew in areas that are now tundra. The subtropics were wetter, and deserts like the Sahara were smaller or periodically green. It was a habitable, thriving world, but one with a very different coastline and very different patterns of rainfall and vegetation than the world human civilization was built around.
Four million years ago sits at a pivot point. The warm, relatively stable conditions of the Pliocene were beginning to unravel, setting off a chain of environmental changes that would drive ice ages, reshape continents, and push a small lineage of African apes down the unlikely evolutionary path toward modern humans.