Three million years ago, Earth was a warmer, wilder, and very different place. The planet sat in the middle of the Pliocene epoch, a period when global temperatures ran roughly 4°C above pre-industrial levels, sea levels stood about 17 meters higher than today, and several species of early human ancestors walked across an African landscape that was just beginning to dry out. At the same time, continents were shifting into modern positions, ocean currents were being rerouted, and a massive shark was entering its final decline. Here’s what was happening across the planet.
A Much Warmer World
The mid-Pliocene warm period is one of the most studied intervals in Earth’s climate history, largely because atmospheric carbon dioxide levels were similar to what we see today. CO2 concentrations hovered between roughly 330 and 394 parts per million, a range that brackets the levels we crossed in the late 20th century. But because the climate system had been in that state for a long time rather than rising rapidly, the planet had fully adjusted. Global average temperatures were around 4.1°C warmer than pre-industrial conditions, with some reconstructions placing the figure even higher.
That warmth reshaped the surface of the Earth. Sea levels were approximately 17.5 meters (about 57 feet) higher than they are now, meaning huge swaths of modern coastline were underwater. Florida, for instance, was largely submerged. The Arctic was far less icy, and ice sheets in Greenland and West Antarctica were substantially smaller or absent entirely during the warmest phases.
Forests Replaced Tundra and Desert
The warmer climate supported far more forest cover than exists today. In the Northern Hemisphere, high-latitude forests expanded into regions that are now barren tundra. Cool conifer forests and warm mixed forests pushed deeper into the interiors of Europe and Asia, occupying land that is currently grassland and shrubland. Temperate woodlands even extended into parts of North Africa.
Arid deserts were less widespread. Many areas that are dry and barren today were instead covered by tropical shrublands and savanna vegetation. The overall picture was a greener, more heavily forested planet, with tundra and desert biomes compressed into much smaller zones than they occupy in the modern world.
Our Ancestors Were Already Walking Upright
Three million years ago, Africa was home to several species of early human relatives. The most famous is Australopithecus afarensis, the species that includes “Lucy,” a partial skeleton discovered in Ethiopia. These hominins stood about 3 to 5 feet tall, walked on two legs, and had brains roughly a third the size of ours. They lived in a mix of wooded and open environments across eastern Africa.
They weren’t alone. Australopithecus africanus lived in southern Africa, while Kenyanthropus platyops inhabited what is now Kenya. Paranthropus aethiopicus, a more robust species with powerful jaws built for tough plant foods, was also emerging around this time. None of these species had yet developed the larger brains or more advanced tool use associated with our own genus, Homo, which wouldn’t appear for roughly another half million years.
But tool use was already underway. Stone tools discovered at a site called Lomekwi 3 in West Turkana, Kenya, date to 3.3 million years ago, making them the oldest known stone tools in the world. These crude implements predate the more familiar Oldowan tool tradition by 700,000 years, and they were found alongside Pliocene hominin fossils in what was then a wooded environment. Exactly which species made them remains uncertain, but the discovery pushed back the origins of technology by a staggering margin.
The Americas Were Joined by a Land Bridge
One of the most consequential geological events of this era was the final closure of the Isthmus of Panama, the narrow strip of land that connects North and South America. The land bridge had been forming for millions of years as volcanic activity and tectonic movement slowly pushed the seabed upward, but final closure occurred around 4 to 3 million years ago.
The consequences were enormous. For ocean circulation, the closure cut off the flow of water between the Atlantic and Pacific oceans, helping establish the Gulf Stream pattern that carries warm water northward along the eastern coast of North America today. This reorganization of ocean currents likely contributed to the gradual cooling and ice sheet growth that would eventually tip the planet into the Pleistocene ice ages.
For land animals, the bridge triggered what biologists call the Great American Biotic Interchange. North American carnivores and hoofed mammals migrated south, while South American species like ground sloths, glyptodonts (armored relatives of armadillos), and other unique mammals headed north. The exchange was lopsided: North American predators, equipped with more specialized teeth and larger brains, devastated many native South American species. Today, only about 10% of North American mammals trace their ancestry to South American migrants, including opossums, porcupines, and armadillos.
Megalodon Was Dying Out
The oceans three million years ago still contained one of the most formidable predators in Earth’s history: Megalodon. This enormous shark, which could reach lengths of 50 feet or more, had been a dominant marine predator since the middle Miocene, roughly 16 million years ago. It fed on marine mammals and was a true apex predator with a cosmopolitan range.
But Megalodon was in decline. Fossil analysis places its extinction at around 2.6 million years ago, meaning that three million years ago the species was in its final stretch. The causes likely involved a combination of cooling oceans, shifting prey availability, and competition. After Megalodon disappeared, baleen whales underwent a dramatic transformation, evolving to the gigantic sizes we see today. While Megalodon lived, most baleen whale species had been relatively small-bodied. With its top predator gone, the ocean’s food web reorganized, and filter-feeding whales filled new ecological space.
A Nearby Star Exploded
Around 2.6 to 2.8 million years ago, a massive star in a nearby star cluster (likely in the Scorpius-Centaurus association) exploded as a supernova. This wasn’t a theoretical guess. Scientists found the evidence embedded in Earth’s own geology: traces of a rare radioactive form of iron, iron-60, locked inside microfossils on the ocean floor. Magnetotactic bacteria had incorporated supernova debris into tiny chains of magnetite crystals, which then fossilized and preserved the signal for millions of years.
The iron-60 signal begins around 2.7 million years ago, peaks at roughly 2.2 million years ago, and fades out around 1.7 million years ago. The debris likely reached Earth as interstellar dust grains that penetrated the solar wind, entered the atmosphere, partially vaporized on entry, and then settled into marine sediments. The timing overlaps with a known marine extinction event that affected mollusks, marine snails, and bivalves, as well as a global cooling period. While a supernova could theoretically damage Earth’s ozone layer and trigger extinction through increased UV radiation, the star was probably too far away (more than 30 light-years) for that kind of catastrophic, direct effect. The connection between the supernova and the extinction remains an open question.
A Climate That Looks Familiar
The Pliocene world three million years ago is often described as the closest natural analog to the climate we’re heading toward. CO2 levels in the 330 to 394 ppm range are comparable to where we’ve been since the 1990s (and we’ve now passed 420 ppm). The difference is that Earth’s climate system had thousands of years to fully respond to those CO2 levels during the Pliocene. Ice sheets had time to melt, oceans had time to warm, and sea levels had time to rise to their equilibrium point of roughly 17 meters above today’s levels. That gap between where we are now and where the Pliocene settled represents what scientists sometimes call “committed warming,” the changes that are still catching up to the CO2 already in the atmosphere.