Neogene

• 1. Introduction

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  The Neogene is a geologic period that stretched across 20.45 million years, from the close of the Paleogene to the dawn of the Quaternary. It is not ancient history in the way the age of dinosaurs is ancient. It is the era that shaped the world you recognise today: the Himalayas still rising, the Amazon cut off from the Pacific, the first members of our own genus picking their way across Africa. What happened during those millions of years to transform the planet into something so familiar? And why do geologists still argue about when, exactly, it ended?

• 2. Naming and Classification

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  Moritz Hörnes, an Austrian palaeontologist who lived from 1815 to 1868, coined the term "Neogene" in 1853. Before his proposal, the time span now split between the Paleogene and the Neogene was called the Tertiary Period. That older label never fully disappeared. Even after it lost its status as a formal stratigraphic term, "Tertiary" survived in informal use among geologists who grew up with it. The Neogene sits as the second period of the Cenozoic Era and the eleventh period of the longer Phanerozoic. It is divided into two epochs: the earlier Miocene and the later Pliocene. The Miocene alone breaks into six named ages, running from the Aquitanian at the base up through the Burdigalian, Langhian, Serravallian, Tortonian, and Messinian. The Pliocene is shorter, containing just the Zanclean and the Piacenzian. Different parts of the world have developed their own regional names for overlapping stretches of this same time.

• 3. A Changing Map

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  The continents during the Neogene already sat close to the positions they occupy today, but key connections were still being forged. The Indian subcontinent kept pressing into Asia, pushing up the Himalayas as it went. Sea levels fell enough to open land bridges, one linking Africa to Eurasia and another joining Eurasia to North America. The most consequential event on the map, however, was the formation of the Isthmus of Panama late in the Pliocene. That narrow strip of land stitched North and South America together for the first time. It also sealed off the warm ocean currents that had previously flowed between the Pacific and the Atlantic. With that corridor closed, only the Gulf Stream remained to carry heat northward toward the Arctic Ocean. The downstream effects of that single geological accident reached far beyond the water: it altered climate patterns across the entire Northern Hemisphere and set the stage for the glaciations that would define the period that followed.

• 4. Climate Swings

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  The Early Miocene opened in a relatively cool state, with mid-latitude sea temperatures and continental thermal gradients already resembling those of the present. Then, during the Middle Miocene, Earth warmed into what geologists call the Middle Miocene Climatic Optimum, a warm phase driven by the emplacement of the Columbia River Basalt Group. That warmth did not last. Around 11 million years ago the climate lurched cooler, and ice caps at both poles began to grow, reinforced by positive feedbacks as sea ice expanded. Between 7 and 5.3 million years ago, a episode known as the Late Miocene Cooling set in, driven by declining concentrations of carbon dioxide in the atmosphere. The Pliocene brought a temporary reprieve. From about 5.3 to 2.7 million years ago, the Pliocene Warm Interval interrupted the longer cooling trend. The Pliocene Thermal Maximum peaked between 3.3 and 3.0 million years ago. Carbon dioxide levels during the Pliocene Warm Interval were similar to those of the present day, which is why researchers treat it as an analogue for the projected climate of the near future under anthropogenic warming. The Sahara during the Pliocene was not always a desert. Wet "Green Sahara" phases struck North Africa roughly every 21,000 years, with the most intense episodes coinciding with periods when Earth's orbital eccentricity was high. Toward the close of the period, a weakening of the Indonesian Throughflow reduced heat transport toward Antarctica, triggering a further cooling that reinforced itself as sea levels dropped and the Leeuwin Current carried progressively less warmth southward. The first glaciation of the current Ice Age had begun.

• 5. Life Transformed

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  Nineteen million years ago, roughly 70 percent of all pelagic shark species vanished in a single pulse. The oceans that emerged from that loss were dominated by a different cast of large predators, including Megalodon and Livyatan. On land, the story ran in a different direction. Mammals and birds kept diversifying. Ungulates in North America became measurably more cursorial, lengthening their strides across the Oligocene-Miocene boundary as grasslands opened up and favoured animals that could run. An explosive radiation of bears took place at the boundary between the Miocene and the Pliocene. Ice-age giants such as mammoths and woolly rhinoceros were present in Pliocene landscapes. Meanwhile, some lineages bowed out. The reptile group Choristodera went extinct early in the period. The amphibians known as Allocaudata disappeared at its end. The reptilian genera Langstonia and Barinasuchus, terrestrial predators and the last members of the crocodilian-related group Sebecosuchia, also vanished during the Neogene. The first hominins, the ancestors of humans, appeared near the period's close. Homo habilis, the first member of the genus Homo, emerged in Africa.

• 6. Grasslands and Green

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  About 20 million years ago, gymnosperms in the form of certain conifers and cycad groups began diversifying rapidly in response to the shifting climate. As conditions grew cooler and more seasonal, tropical plant species gave way to deciduous ones. Grasses diversified enormously, and herbivorous mammals evolved in step with them, producing the lineages that led to horses, antelope, and bison. Lower atmospheric carbon dioxide over the last 10 million years of the period allowed plants to expand and gain ecological dominance across those grasslands. Daisy-family plants, known formally as Asteraceae, underwent a significant adaptive radiation of their own. Eucalyptus leaves preserved in Miocene deposits from New Zealand offer one of the period's stranger footnotes: the genus grew there during the Miocene even though it is not native to New Zealand today, having arrived on the islands from Australia only through later human introduction.

• 7. The Boundary Dispute

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  Geologists have argued for decades about where the Neogene ends. The traditional boundary placed the period's close at the end of the Pliocene Epoch, just before the Quaternary began. But marine geologists pushed to absorb the Quaternary into the Neogene entirely, while terrestrial geologists insisted the Quaternary's record was distinct enough to deserve its own period. The International Commission on Stratigraphy, known as the ICS, proposed at one point that the Quaternary become a sub-era within the Neogene, starting at 2.58 million years ago with the base of the Gelasian Stage. The International Union for Quaternary Research, INQUA, countered that both the Neogene and the Pliocene should end at 2.58 million years ago and that the Gelasian should move into the Pleistocene. A 2006 compromise made the Quaternary a sub-era, but the International Union of Geological Sciences rejected it because it split both the Neogene and the Pliocene in two. Following discussions at the 2008 International Geological Congress in Oslo, Norway, the ICS decided in May 2009 to resolve the standoff by making the Quaternary the youngest period of the Cenozoic, with its base fixed at 2.58 million years ago and including the Gelasian Age. That decision formally set the Neogene's end at 2.58 million years ago, the boundary it carries in current usage.