The Cenozoic is the current geological era, stretching from the asteroid impact that ended the age of dinosaurs about 66 million years ago to the present day. It is sometimes called the “Age of Mammals,” though that shorthand undersells the breadth of change packed into this stretch of Earth history. The Cenozoic saw the planet swing from hothouse conditions warm enough for crocodilians near the poles to ice ages that buried much of the Northern Hemisphere under kilometers of glacial ice, and it produced everything from the first whales to the first grasslands to the first humans. Understanding it means tracing the interplay of climate, tectonics, and evolution across a period that shaped the modern world in almost every way.
Life After the Asteroid
The Cenozoic opened with a catastrophe. The Cretaceous-Paleogene (K-Pg) mass extinction wiped out roughly three-quarters of all species, including all non-avian dinosaurs. What happened next was one of the fastest ecological recoveries in the fossil record. A detailed fossil sequence from the Denver Basin in Colorado shows that within about 100,000 years of the impact, mammalian species richness had doubled and the maximum body size of mammals had already climbed back to near pre-extinction levels. By 300,000 years after the impact, maximum mammalian body mass had tripled, and dietary specialization was increasing alongside a rebound in plant diversity. The appearance of legumes (the bean family) around 700,000 years post-impact coincided with even larger mammals entering the scene.1Science. Exceptional continental record of biotic recovery after the Cretaceous–Paleogene mass extinction
Placental mammals, the group that includes everything from bats to elephants to humans, likely originated in the Late Cretaceous, but their major diversification unfolded in the earliest Paleocene, the first epoch of the Cenozoic. Rates of anatomical change in placental mammals during the five million years right after the extinction were roughly three times higher than background rates during the Cretaceous, consistent with an evolutionary radiation as mammals invaded ecological niches left vacant by the dinosaurs.2PubMed Central. Eutherians experienced elevated evolutionary rates in the immediate aftermath of the Cretaceous–Palaeogene mass extinction Genomic analyses reinforce the picture: cladogenic events, the branching points where new lineages split apart, accumulated heavily both just before and immediately after the K-Pg boundary, suggesting that both Cretaceous continental separation and the extinction itself played roles in the placental radiation.3Science. A genomic timescale for placental mammal evolution
The Hothouse Peak and the PETM
For the first twenty million years or so of the Cenozoic, the planet was warm and getting warmer. Atmospheric carbon dioxide was far higher than today, and ice sheets were essentially absent. The warmest sustained interval, the Early Eocene Climatic Optimum (roughly 53 to 50 million years ago), pushed prolonged ocean temperatures to the highest levels recorded anywhere in the Cenozoic. Continental interior temperatures in North America reached a mean annual temperature of about 23°C during this window, driving distinctive shifts in mammalian communities.4PubMed Central. Climate directly influences Eocene mammal faunal dynamics in North America
Superimposed on this general warmth were shorter, sharper spikes known as hyperthermals. The most dramatic was the Paleocene-Eocene Thermal Maximum (PETM), around 55.9 million years ago, when global temperatures jumped several degrees in a geologically brief pulse. The PETM was driven by a massive injection of carbon into the atmosphere, though the exact source and quantity have been debated for decades. One analysis of sedimentary records from Spitsbergen estimated that carbon was released at a peak rate in the range of 0.3 to 1.7 billion tonnes per year, much slower than today’s emissions from fossil fuels.5Nature Geoscience. Slow release of fossil carbon during the Palaeocene–Eocene Thermal Maximum Other researchers, using high-resolution isotope records from New Jersey shelf sediments, have argued for a more rapid release, on the order of 3,000 gigatons of carbon.6PubMed Central. Evidence for a rapid release of carbon at the Paleocene-Eocene thermal maximum A third study found evidence pointing to a much larger total carbon release, more than 10,000 petagrams, driven primarily by volcanism associated with the North Atlantic Igneous Province rather than from surface carbon reservoirs like methane hydrates.7PubMed Central. Very large release of mostly volcanic carbon during the Palaeocene-Eocene Thermal Maximum
The disagreement over the PETM’s carbon source and emission rate is not just academic bookkeeping. The PETM is the closest natural analogue we have to modern carbon emissions, so understanding how fast the carbon entered the atmosphere and what the consequences were matters for projecting our own climate future. Regardless of which estimate proves closest, all of them describe a world that responded to a carbon spike with ocean acidification, deep-sea extinction of bottom-dwelling organisms, and global reshuffling of ecosystems.
Fruits, Seeds, and the Eocene Ecological Web
The warm Eocene was also when plant-animal interactions reached a level of complexity that still defines modern ecosystems. Fleshy fruits and a wide range of seed sizes had begun diversifying in the Late Cretaceous, but their peak variety arrived in the Eocene around 55 to 50 million years ago. During this initial phase, animals evolving fruit-eating habits and plants evolving new fruit types were expanding into ecological space that had not existed before. Early on, the most important seed dispersers were probably multituberculates, an extinct group of mammals, with primates and rodents becoming increasingly important later in the Eocene.8PubMed. Evolution of angiosperm seed disperser mutualisms: the timing of origins and their consequences for coevolutionary interactions between angiosperms and frugivores From the Eocene onward, the broad framework of fruit traits and frugivore niches was already in place, and subsequent evolution occurred mostly within that existing niche space rather than creating entirely new categories.
Interestingly, the earlier Mesozoic picture was quite different. Gymnosperm-dominated ecosystems before the Cenozoic were mostly open and had low proportions of specialized fruit-eaters, suggesting that the tight coevolution between plants and their seed dispersers was largely a Cenozoic phenomenon.9PubMed. Patterns of variation in fleshy diaspore size and abundance from Late Triassic-Oligocene
From Land to Sea in the Eocene
Among the most spectacular evolutionary transitions of the Cenozoic was the return of mammals to the ocean. Early whale ancestors were four-legged terrestrial animals, and the Eocene preserves a remarkably detailed fossil sequence of their shift to fully aquatic life. Bone microstructure tells the story at the cellular level. Early whale relatives like remingtonocetids and protocetids had dense, heavy bones that functioned as ballast, keeping them stable while wading and swimming in shallow water. Their forelimbs were still compact and powerful, used for propulsion.10PubMed Central. Transition of Eocene whales from land to sea: evidence from bone microstructure
As later cetaceans like basilosaurids moved into open water, the pattern reversed. Their forelimb bones became spongy, reflecting the shift from propulsion to steering. Their hind limbs shrank to vestigial nubs but paradoxically became extremely dense. The broad pattern was a transition from normal terrestrial bone to abnormally dense bone (useful as ballast in shallow water) and then to lighter bone suited for deep-water dynamic buoyancy control. A striking finding from histological studies is that these microstructural changes actually preceded the gross anatomical changes visible in the skeleton: the bones were adapting to water before the body shape fully did.11PubMed. Sink or swim? Bone density as a mechanism for buoyancy control in early cetaceans At the genetic level, cetacean genomes show accelerated evolution in genes involved in bone remodeling and the signaling pathways that control bone development, confirming that natural selection was actively reshaping the skeleton for aquatic life.12PubMed Central. Accelerated evolution and diversifying selection drove the adaptation of cetacean bone microstructure
The Big Chill at the Eocene-Oligocene Boundary
Around 34 million years ago, the Cenozoic’s long warm phase gave way to a dramatic cooling. Antarctica, which had been essentially ice-free, developed an ice sheet for the first time. Drill core records from the western Ross Sea show that as atmospheric CO₂ fell below roughly 750 parts per million, orbitally paced glacial cycles began on Antarctica. Initially, at CO₂ levels of 600 ppm or above, the ice sheet was small, restricted to the interior of the continent, and highly sensitive to wobbles in Earth’s orbit. A more stable, continent-scale ice sheet that reached the coastline did not form until around 32.8 million years ago, when CO₂ dropped below about 600 ppm.13PubMed. Antarctic Ice Sheet variability across the Eocene-Oligocene boundary climate transition
For decades, the standard explanation for Antarctic glaciation centered on the opening of ocean gateways around the continent, particularly the Drake Passage between South America and Antarctica. The idea was that once ocean currents could circle Antarctica freely, they thermally isolated the continent and allowed it to freeze. Coupled climate-ice sheet modeling has complicated that story. Simulations show that declining atmospheric CO₂ was the dominant control, with small ice caps first forming on the highest plateaus and then merging into a continental-scale sheet as a CO₂ threshold between roughly two and three times pre-industrial levels was crossed. In the models, neither ocean gateway opening nor mountain uplift significantly changed the timing of the major ice sheet transition, given a background of gradually falling CO₂.14Palaeogeography, Palaeoclimatology, Palaeoecology. A coupled climate–ice sheet modeling approach to the Early Cenozoic history of the Antarctic ice sheet
Grasslands, Grazers, and the Miocene Transformation
The Miocene epoch, roughly 23 to 5 million years ago, reshaped Earth’s surface in ways you can still see from a car window. Before the Miocene, most terrestrial landscapes were forested. During the mid to late Miocene, open grasslands expanded across every inhabited continent, creating biomes that had no real precedent. A key innovation was C₄ photosynthesis in grasses, a biochemical pathway that works more efficiently under dry, warm, low-CO₂ conditions. C₄ grasses evolved repeatedly within a single grass lineage and were consistently associated with higher diversification rates. The mid to late Miocene was the peak period for both C₃ and C₄ grass diversification, coinciding with the global spread of open habitats.15PubMed Central. C4 photosynthesis promoted species diversification during the Miocene grassland expansion
What drove the expansion is still debated. Falling atmospheric CO₂ is one candidate, since it favors C₄ plants over C₃ competitors. But isotope evidence from the Americas suggests that changes in rainfall seasonality, particularly a shift toward summer-dominated precipitation regimes, could also explain much of the expansion.16Earth and Planetary Science Letters. The expansion of C4 grasses and global change in the late Miocene: Stable isotope evidence from the Americas The Miocene also saw continued global cooling, punctuated by the Middle Miocene Climate Transition around 14 million years ago, when permanent ice sheets returned to Antarctica. Enhanced biological productivity in the eastern equatorial Pacific may have contributed to CO₂ drawdown during this step, with massive increases in siliceous plankton accumulation recorded just before the most prominent cooling pulse.17Geology. Middle Miocene climate cooling linked to intensification of eastern equatorial Pacific upwelling
Grasslands brought with them a whole new set of evolutionary pressures. Mammals that had been adapted to browsing on soft leaves in forests now faced abrasive, silica-rich grasses. Horses evolved high-crowned teeth. Antelopes and their relatives diversified. Flightless predatory birds known as terror birds (Phorusrhacidae) thrived as apex predators in South America from the Middle Eocene through the Early Pleistocene, with species ranging from modestly sized runners of 30 to 70 kilograms to true giants.18PubMed Central. The evolution and ecology of gigantism in terror birds (Aves, Phorusrhacidae)
The Himalayan Question
One of the Cenozoic’s longest-running scientific debates concerns the role of the Himalayas in driving global cooling. The idea, proposed in the 1990s, is elegant: as the Indian plate collided with Asia and pushed up the Tibetan Plateau, the enormous amount of fresh rock exposed to the atmosphere accelerated silicate weathering, a chemical process that consumes CO₂. More weathering, less CO₂, cooler planet. It became one of the most widely cited explanations for the Cenozoic’s long-term cooling trend.
Recent work has thrown cold water on the hypothesis. An analysis of erosion and weathering fluxes from the Himalayan-Tibetan system found that the rate of CO₂ consumption from chemical weathering actually decreased by about half between 16 and 5.3 million years ago, particularly in the Indus system, as erosion slowed and the source rocks shifted away from the most reactive types. Falling weathering fluxes during a period of global cooling directly contradicts the idea that Himalayan uplift drove the cooling.19Geophysical Research Letters. Himalayan‐Tibetan Erosion Is Not the Cause of Neogene Global Cooling A separate analysis of the timing of CO₂ consumption rates also concluded that Asian mountain building did not act as the dominant control over late Cenozoic atmospheric CO₂.20Chemical Geology. The impact of Himalayan-Tibetan erosion on silicate weathering and organic carbon burial The cooling clearly happened, but the tidy narrative linking it to a single tectonic cause has weakened considerably.
Why Mammals Got Big, Then Bigger, Then Smaller
The trajectory of mammalian body size across the Cenozoic is not a simple story of constant growth. After the post-extinction recovery, the maximum body size of terrestrial mammals shows a pattern of peaks and troughs that tracks both ecology and climate. The primary driver was diversification to fill available niches, but environmental temperature and available land area appear to have set an upper ceiling on how large mammals could become.21PubMed. The evolution of maximum body size of terrestrial mammals
When multiple mammalian orders are examined together, the times at which they reach peak body size cluster in three windows: the Middle Eocene, the Oligocene, and the Plio-Pleistocene. The Eocene peak occurred when temperatures and mammalian diversity were both high, and is best explained by niche expansion in lush, productive ecosystems. The Plio-Pleistocene peak, the most statistically robust of the three, happened when both temperatures and diversity were low, and is best explained by global cooling itself. Cold climates favor large body size in mammals because larger animals retain heat more efficiently. Since the Eocene, there is a strong correlation between the frequency of maximum-size records and global temperature trends.22PubMed Central. Patterns of maximum body size evolution in Cenozoic land mammals: eco-evolutionary processes and abiotic forcing
Diatoms, Oceans, and a Preservation Puzzle
One of the Cenozoic’s most striking patterns in the marine realm is the apparent rise of diatoms, the tiny silica-shelled algae that today dominate ocean productivity. Their fossil abundance increases markedly in the late Cenozoic, and this trend has been linked to everything from the rise of grasslands (which supply dissolved silica to rivers) to the evolution of baleen whales (which depend on diatom-fueled food webs) to declining CO₂. Continental erosion of silicate rocks over the past 40 million years does appear to have been critical to diatom success, giving them a nutrient advantage over other siliceous plankton like radiolarians.23PubMed Central. Continental erosion and the Cenozoic rise of marine diatoms
But there is a catch. A diagenetic modeling study suggests that much of the observed increase in diatom fossils may be an artifact of changing preservation conditions rather than an actual biological boom. When you assume a constant flux of diatoms to the seafloor and let temperature and sedimentation rate evolve as observed, you produce a late Cenozoic increase in preserved diatom fossils that closely matches the real record. The implication is unsettling: the cooling climate improved silica preservation and simultaneously drove the rise of grasslands and whales, and what looks like a causal chain connecting these phenomena may instead be parallel responses to the same underlying driver.24PubMed Central. Revisiting the sedimentary record of the rise of diatoms
Ice Ages, Orbital Cycles, and the Role of CO₂
The Quaternary period, the most recent 2.6 million years of the Cenozoic, is defined by repeated glacial-interglacial cycles. Northern Hemisphere ice sheets advanced and retreated dozens of times, carving landscapes and reshuffling ecosystems with each swing. The astronomical theory of ice ages, proposed by Milutin Milankovitch, correctly identified orbital variations as the pacemaker, but a puzzle remains: orbital theory predicts a dominant cycle of about 41,000 years, while the strongest periodicity in the actual record is closer to 100,000 years. Ice core records from Antarctica resolved part of the mystery by showing unambiguously that atmospheric CO₂ was lower during glacial periods. During the last deglaciation, CO₂ began rising significantly, by about 50 ppm, several thousand years before any major reduction in ice volume, strongly suggesting that greenhouse gases played an active role in amplifying or even triggering deglaciations.25Quaternary Science Reviews. Quaternary glaciations: from observations to theories
The Megafaunal Extinctions
The end of the Pleistocene brought a wave of large-mammal extinctions across most continents: woolly mammoths, giant ground sloths, sabertooth cats, and dozens of other species disappeared within a relatively narrow window. The debate over whether climate change or human hunting was responsible has persisted for decades. A global quantitative analysis found that models combining both human arrival and climatic variables best explain the observed pattern of extinctions across five landmasses.26PubMed Central. Quantitative global analysis of the role of climate and people in explaining late Quaternary megafaunal extinctions A separate analysis sharpened the picture further: the timing of human arrival was the single best predictor, with climate acting as an additive rather than synergistic factor. Climate variation contributed, but human impact was the necessary condition that drove the extinctions.27Quaternary International. Bigger kill than chill: The uneven roles of humans and climate on late Quaternary megafaunal extinctions
East African Climate and Human Evolution
The Cenozoic’s climate story loops back to us in a very direct way. All major events in hominin evolution appear to have occurred in East Africa, a region whose landscape has been transformed over the past 10 million years by tectonic rifting. What was once a relatively flat area covered in mixed tropical forest became a heterogeneous patchwork of mountains over four kilometers high, lakes, deserts, and cloud forests. The progressive rifting created lake basins that were exquisitely sensitive to precipitation changes, and there is evidence that the presence of precession-driven, short-lived deep-water lakes coincided with key events in hominin evolution.28Quaternary Science Reviews. East African climate pulses and early human evolution
The long-term drying trend across Africa was punctuated by episodes of extreme climatic variability, and these pulses may have provided the selective pressure behind speciation and dispersal events. A significant speciation and brain expansion event around 1.8 million years ago appears to have been concurrent with the occurrence of highly variable, extensive deep-water lakes. Around 300,000 years ago, shifts in moisture balance across Africa changed the relative proportions of open versus wooded ecosystems, with eastern Africa increasing in grass cover and in the diversity of plant resources. This shift is broadly concurrent with the end of the long run of Homo erectus and the appearance of new Homo species along with Middle Stone Age technology.29Philosophical Transactions of the Royal Society B. The climate and vegetation backdrop to hominin evolution in Africa Periods of extreme climate variability, driven by both orbital forcing and local tectonic effects, seem to have acted as a catalyst for evolutionary change among mammals and hominins alike.30PubMed. High- and low-latitude forcing of Plio-Pleistocene East African climate and human evolution
When the Mediterranean Dried Up
One of the Cenozoic’s most dramatic geological episodes is also one of its strangest. Around 5.96 million years ago, tectonic movements partially or fully cut off the Mediterranean Sea from the Atlantic Ocean, triggering the Messinian Salinity Crisis. Over the course of several hundred thousand years, the Mediterranean repeatedly shrank and refilled, precipitating roughly a million cubic kilometers of salt. Landscape evolution modeling of the drawdown phase shows that as the sea level dropped, rivers carved deep canyons into the surrounding continents, and the retreat of water along spillways connecting to other basins gradually fed freshwater back into the Mediterranean, explaining the transition to less salty conditions in the crisis’s final stages.31Science Advances. Kilometric sea level changes during the Messinian salinity crisis caused by river erosion and climate The eventual reopening of the Strait of Gibraltar reflooded the basin in what is thought to have been a geologically rapid event. The Messinian Salinity Crisis left lasting marks on Mediterranean ecology, salinity, and sedimentation that are still studied today.
The Anthropocene Fingerprint
The question of whether the Cenozoic has entered a genuinely new epoch, the Anthropocene, has moved from informal shorthand to a formal scientific proposal. The case rests on a startling set of changes: manufactured materials like aluminum, plastics, and concrete now appear in sedimentary layers worldwide, alongside spikes in radionuclides from nuclear testing and particulates from fossil fuel burning. Carbon, nitrogen, and phosphorus cycles have all been substantially altered. Rates of sea-level rise and the extent of climate perturbation exceed anything recorded in the late Holocene. Species invasions are global, and extinction rates are accelerating.32PubMed. The Anthropocene is functionally and stratigraphically distinct from the Holocene
An analysis of geological archives worldwide found that diverse anthropogenic signals surged simultaneously starting around 1952, corresponding to the onset of what economists and Earth system scientists call the Great Acceleration. Between 1953 and 1958, the surge appeared nearly simultaneously across every region, from Antarctica to East Asia to Europe. This synchronous spike marks the moment when human activities began inscribing recognizable fingerprints in sedimentary strata on every continent, and when deviations from established natural climatic conditions became measurable.33PubMed Central. Toward defining the Anthropocene onset using a rapid increase in anthropogenic fingerprints in global geological archives Whether or not the Anthropocene gains formal ratification as a geological epoch, the stratigraphic evidence makes one thing clear: in the context of the Cenozoic’s 66-million-year arc, the past seventy years represent a discontinuity with no obvious precedent in rate or scope.