The Miocene is the geological epoch that lasted from roughly 23 million to 5.3 million years ago, a stretch of almost 18 million years during which Earth’s climate swung from a warm, high-CO₂ world toward something recognizably cooler and drier. It was the epoch when grasslands overtook forests across entire continents, when the largest predatory shark ever known prowled the oceans, when apes spread from Africa into Eurasia, and when the very earliest members of the human lineage may have taken their first upright steps. Understanding the Miocene means understanding the period that set the stage for the modern world.
A Warm World That Slowly Cooled
The Miocene opened warm. Atmospheric CO₂ during the early Miocene fluctuated between roughly 420 and 700 parts per million, well above today’s pre-industrial baseline of about 280 ppm.1Paleoceanography and Paleoclimatology. Atmospheric CO₂ Estimates for the Late Oligocene and Early Miocene Using Multi‐Species Cross‐Calibrations of Boron Isotopes The warmest interval was the Miocene Climatic Optimum, spanning roughly 17 to 15 million years ago. During that window, CO₂ averaged around 500 ppm, and global temperatures were substantially higher than they are now.2Paleoceanography and Paleoclimatology. Evidence for Limited Atmospheric CO2 Rise at the Miocene Climatic Optimum Researchers have found, however, that the CO₂ rise at the onset of this warm spell was only about 100 to 150 ppm, which is surprisingly modest. That has led some scientists to argue the Miocene Climatic Optimum was less a sudden greenhouse spike and more a continuation of warmth inherited from the late Oligocene, sustained partly by shifts in ocean circulation rather than CO₂ alone.3Paleoceanography and Paleoclimatology. Evidence for Limited Atmospheric CO2 Rise at the Miocene Climatic Optimum
After the Climatic Optimum peaked, the planet began a long slide toward cooler conditions. The middle Miocene Climate Transition, centered around 14 million years ago, was a turning point. Southern Ocean bottom-water temperatures dropped by roughly 3 to 5 °C across this interval, and the Antarctic Ice Sheet expanded substantially.4Climate of the Past. Southern Ocean bottom water cooling and ice sheet expansion during the middle Miocene climate transition Interestingly, the deep-ocean cooling appears to have come first, before the main pulse of ice growth, suggesting the cooling helped trigger glaciation rather than simply following it. Atmospheric CO₂ also fell during this phase, with records showing values as low as around 300 ppm by the mid-Miocene, reinforcing the cooling trend through a positive feedback loop where carbon burial and ice expansion amplified each other.5Paleoceanography. CO2 drawdown following the middle Miocene expansion of the Antarctic Ice Sheet
Mountains, Monsoons, and Ocean Currents
Some of the most dramatic physical changes during the Miocene had nothing to do with greenhouse gases. The Himalaya and Tibetan Plateau were actively rising, and that uplift reshaped atmospheric circulation across Asia. Climate modeling experiments have linked the phased uplift of the Himalaya-Tibetan plateau to the stepwise evolution of the Asian monsoon system, which in turn influenced Northern Hemisphere glaciation patterns.6Nature. Evolution of Asian monsoons and phased uplift of the Himalaya–Tibetan plateau since Late Miocene times A prominent monsoon system appears to have developed around 8 million years ago following a major reorganization of atmospheric circulation over the continent, an event tied to a late Miocene uplift episode in the Himalayas and southern Tibet.7Global and Planetary Change. Late Miocene and mid-Pliocene enhancement of the East Asian monsoon as viewed from the land and sea
Down at the opposite end of the planet, the Antarctic Circumpolar Current was also transforming. For a long time, researchers assumed this massive current, which circles Antarctica and thermally isolates the continent, snapped into place once deep-water passages opened between landmasses tens of millions of years earlier. But recent work shows that a strong, deep-reaching current like the one we see today did not actually exist until the late Miocene. The expansion of the Antarctic Ice Sheet and sea ice after the middle Miocene Climate Transition intensified the density contrasts and westerly winds that drive the current, essentially turning what had been a weaker flow into the powerful circumpolar highway that amplified late Cenozoic global cooling.8Nature Geoscience. Late Miocene onset of the modern Antarctic Circumpolar Current The upshot is that tectonic “pre-conditioning” from the opening of seaways was necessary but not sufficient: ice sheet growth and atmospheric changes during the Miocene were what pushed the current into its modern configuration.9Climate of the Past. Links between CO2, glaciation and water flow: reconciling the Cenozoic history of the Antarctic Circumpolar Current
The Rise of Grasslands and Fire
If there is one landscape transformation that defines the Miocene’s legacy, it is the explosive spread of grasslands. For most of Earth’s history, grasses were minor players in plant communities. During the Miocene, that changed. A particular kind of grass, the C₄ type, which uses a more efficient form of photosynthesis suited to warm, dry, and low-CO₂ conditions, expanded dramatically. In northwest Argentina, isotopic evidence from soil carbonates shows C₄ plants appearing between about 7.3 and 6.7 million years ago, and similar signals from fossil teeth across both North and South America confirm the expansion was a global phenomenon, though it unfolded at different rates in different regions.10Earth and Planetary Science Letters. The expansion of C4 grasses and global change in the late Miocene: Stable isotope evidence from the Americas
Fire played a crucial role in establishing these grasslands. Between about 8 and 6 million years ago, fire frequency, seasonal precipitation, and C₄ grass dominance all surged in tandem, suggesting that fire and grasslands fed off each other in a self-reinforcing loop. Grasses provided fuel for fires, fires cleared woody vegetation and opened the landscape for more grasses, and the cycle accelerated.11PubMed Central. Grassland fire ecology has roots in the late Miocene This fire-grassland feedback is not just ancient history. It still governs the ecology of modern savannas and prairies, meaning the dynamic that shapes those landscapes today was born in the late Miocene.
The spread of grasses also drove evolutionary change in the animals that ate them. Grasses are abrasive, and herbivores that relied on them needed tougher teeth. Horses are the textbook example. Their evolution of high-crowned molars, an adaptation for grinding abrasive plant material, correlates strongly with dietary abrasion levels. Some early Miocene horse populations showed especially high abrasion, suggesting intervals of strong selection pressure for harder-wearing teeth, right before the first appearance of the horse subfamily in which high-crowned dentitions evolved.12PubMed. Dietary change and evolution of horses in North America
Megatooth Sharks and the Miocene Ocean
The Miocene ocean was a dramatically different place. Its most famous resident was Otodus megalodon, the megatooth shark, which grew to estimated lengths of 15 meters or more. Nitrogen isotope analysis of megalodon tooth enamel has revealed that it occupied a higher trophic position than any marine species known, living or extinct.13PubMed Central. Cenozoic megatooth sharks occupied extremely high trophic positions Zinc isotope data further suggest that megalodon and its relatives were flexible in what they ate, foraging across multiple trophic levels rather than specializing, which makes them better described as opportunistic supercarnivores than picky apex predators.14Earth and Planetary Science Letters. Miocene marine vertebrate trophic ecology reveals megatooth sharks as opportunistic supercarnivores Three-dimensional body modeling suggests megalodon’s preference for large prey, potentially including whales, let it minimize competition and provided enough stored energy for transoceanic migrations without frequent feeding stops. When it went extinct, the consequences for marine ecosystems were probably enormous, disrupting nutrient transfer and food webs across ocean basins.15PubMed Central. The extinct shark Otodus megalodon was a transoceanic superpredator: Inferences from 3D modeling
Whales were diversifying throughout the Miocene as well. Their evolutionary trajectory was closely tied to changes in ocean productivity. A comprehensive diversity analysis showed that much of the variation in whale species richness over geological time can be explained by the diversity of diatoms, the tiny single-celled algae that form the base of many marine food webs, combined with shifts in ocean temperature.16PubMed. Climate, critters, and cetaceans: Cenozoic drivers of the evolution of modern whales As oceans cooled and nutrient upwelling patterns changed through the Miocene, diatom blooms shifted, and whale communities responded.
Saber-Toothed Cats, Bear Dogs, and Terror Birds
On land, the Miocene hosted a roster of predators that looked nothing like today’s top carnivores. Saber-toothed cats were widespread and diverse, and they coexisted alongside bear dogs, massive predatory mammals that combined features of bears and canines. At the exceptional fossil site of Cerro de los Batallones in Spain, stable isotope analysis of three sympatric apex predators from the late Miocene, two saber-toothed cats and a bear dog, revealed how they divided up resources. The two cats, one leopard-sized and the other tiger-sized, hunted prey in more wooded parts of the habitat, probably targeting animals of different body sizes to reduce competition. The bear dog, by contrast, fed on prey from more open woodland, its carbon isotope values differing significantly from those of the cats.17PubMed Central. Resource partitioning among top predators in a Miocene food web This kind of niche partitioning, where predators carve out different feeding territories within overlapping ranges, is a recurring theme in Miocene ecosystems.
Biomechanical studies have shown that saber-toothed skulls were more varied in function than their dramatic canines might suggest. Barbourofelis fricki, a Miocene saber-tooth, had a stoutly built skull capable of withstanding stress in various killing scenarios, making it potentially more of a generalist than the later and more famous Smilodon. The researchers concluded that extreme morphological specialization in saber-tooths did not necessarily translate to ecological specialization.18PubMed Central. Comparing cranial biomechanics between Barbourofelis fricki and Smilodon fatalis: Is there a universal killing-bite among saber-toothed predators?
South America, still an island continent for most of the Miocene, had its own set of apex predators: terror birds. These large, flightless, cursorial birds filled the top-predator niche in the absence of big mammalian carnivores. Fossil evidence from La Venta in Colombia shows that gigantic terror birds inhabited middle Miocene tropical environments, not just the open habitats they are sometimes stereotyped as preferring.19Papers in Palaeontology. A gigantic new terror bird (Cariamiformes, Phorusrhacidae) from Middle Miocene tropical environments of La Venta in northern South America Recent work from the same fossil locality has even documented direct evidence of trophic interactions between terror birds and crocodylians, underscoring the complexity of these ancient food webs.20PubMed Central. Direct evidence of trophic interaction between a crocodyliform and a large terror bird in the Middle Miocene of La Venta, Colombia
Apes Spread Across the Old World
The Miocene was the golden age of apes. Hominoids, the group that includes modern apes and humans, had diverged from Old World monkeys back during the Oligocene in Africa.21PubMed. Systematics of Miocene apes: State of the art of a neverending controversy During the early Miocene, they radiated across Africa. The earliest known apes, genera like Proconsul and Rangwapithecus, were arboreal creatures associated with evergreen rainforest.22PubMed. Middle Miocene dispersals of apes Then, in the middle Miocene, they left. A wave of “out of Africa” dispersal carried apes into Europe and Asia, followed by a burst of species formation in Asia around the mid-Miocene and a subsequent “out of Asia” expansion back into Africa and beyond.23Journal of Biogeography. Miocene hominoid biogeography: pulses of dispersal and differentiation At their peak, Miocene apes were far more diverse than the handful of living species we have today; they occupied habitats from tropical forest to subtropical woodland across three continents.
At the tail end of the Miocene, the earliest candidates for the human lineage appeared. Sahelanthropus tchadensis, found in Chad and dated to the late Miocene around 7 million years ago, has been the subject of fierce debate. Recent morphological work on limb bones associated with the species found features linked to bipedal hip and knee function, along with a femoral tubercle only known in bipedal hominins, leading the researchers to conclude that Sahelanthropus was an early biped that evolved from a chimpanzee-like Miocene ape ancestor.24PubMed Central. Earliest evidence of hominin bipedalism in Sahelanthropus tchadensis Not everyone agrees. A separate analysis of the same femoral shaft concluded the individual was not habitually bipedal and urged caution when considering the species’ relationships.25PubMed. Nature and relationships of Sahelanthropus tchadensis The disagreement is unresolved, and it hinges on fragmentary material from a single site. What is clear is that by the late Miocene, the ape lineage that would eventually produce humans was already beginning to experiment with upright locomotion in some form.
The Late Miocene Transition and the Messinian Salinity Crisis
The closing chapter of the Miocene was defined by continued drying and cooling. Proxy data and climate simulations have shown that declining CO₂ was the primary driver of temperature and precipitation decreases across most land areas during the middle to late Miocene. But vegetation changes acted as a powerful amplifier: as forests gave way to grasslands and scrub at northern high latitudes, the loss of forest cover accelerated cooling by as much as 10 °C in some regions and reduced rainfall at low and mid latitudes by up to 30 percent.26PubMed Central. Vegetation feedbacks accelerated the late Miocene climate transition In other words, the drying landscape and the cooling climate were feeding off each other, much like the fire-grassland feedback discussed earlier.
The shift in climate also reshaped European mammal communities. Across the Miocene-Pliocene boundary, around 5.3 million years ago, analysis of plant-eating mammals shows a sharp decrease in species with medium-height teeth and mixed diets, replaced by an increase in species with low-crowned teeth and more omnivorous habits.27Palaeogeography, Palaeoclimatology, Palaeoecology. Late Miocene and Pliocene large land mammals and climatic changes in Eurasia The interpretation is that as open, grassy habitats expanded in some areas and contracted in others, the herbivore community shuffled to match the available food.
The Miocene’s most spectacular closing act was the Messinian Salinity Crisis, which began around 5.96 million years ago when tectonic movements partially or fully cut off the Mediterranean Sea from the Atlantic Ocean. What followed was extraordinary: the precipitation of roughly a million cubic kilometers of salt as the Mediterranean evaporated. How much the sea level actually dropped has been a long-running argument. Some estimates, based on shallow-water fossils found in the deep basin, suggest near-complete desiccation, while evidence of freshwater deposits along former coastlines points to a much less extreme drawdown. Recent landscape evolution modeling offers a reconciliation: as the Mediterranean dropped, rivers eroding into surrounding continents carved deep gorges, and the retrogressive erosion along spillways from neighboring basins gradually added water back, producing a superimposed sea-level rise on top of climatic oscillations and eventually transitioning the basin to fresher-water conditions in the last stage of the crisis.28PubMed Central. Kilometric sea level changes during the Messinian salinity crisis caused by river erosion and climate When the Atlantic finally broke through again at the start of the Pliocene, the refilling of the Mediterranean was one of the most dramatic flooding events in geological history.
Freshwater Ecosystems and Hidden Radiations
While large mammals, sharks, and early hominins tend to dominate Miocene narratives, some of the epoch’s most consequential evolutionary events played out underwater in rivers and lakes. South America’s freshwater fish fauna, among the richest on Earth today, traces key bursts of diversification to the Miocene. The Neotropical armored catfish genus Hypostomus, for example, invaded the Paraná River basin in southeastern Brazil around 12.5 million years ago and subsequently experienced accelerated rates of species formation and morphological diversification. The basin’s geographical remoteness, its recent history of marine incursion, and its relatively low species diversity at the time all created the ecological opportunity for a classic adaptive radiation.29PubMed. Transcontinental dispersal, ecological opportunity and origins of an adaptive radiation in the Neotropical catfish genus Hypostomus (Siluriformes: Loricariidae) This pattern, an isolated or newly accessible habitat with few competitors allowing rapid diversification, repeated itself across Miocene freshwater systems worldwide and helps explain why tropical rivers today harbor such staggering fish diversity.