• Geologic Period Neogene, 23 Ma
• Geologic Epoch Miocene, 23 Ma
• Geologic Epoch Pliocene, 5.3 Ma

The Neogene covers about 23 million years. During this period, mammals and birds continued to evolve into roughly modern forms, while other groups of life remained relatively unchanged. Early hominids, the ancestors of humans, appeared in Africa. Some continental movement took place, the most significant event being the connection of North and South America at the Isthmus of Panama, late in the Pliocene. This cut off ocean currents between the Atlantic and Pacific oceans, causing climate changes and creating the Gulf Stream. The global climate cooled considerably over the course of the Neogene, culminating in a series of continental glaciations in the Quaternary Period that follows.

In ICS terminology, from upper (later, more recent) to lower (earlier):

The Pliocene Epoch is subdivided into 2 ages:

• Piacenzian Age, preceded by
• Zanclean Age

The Miocene Epoch is subdivided into 6 ages:

• Messinian Age, preceded by
• Tortonian Age
• Serravallian Age
• Langhian Age
• Burdigalian Age
• Aquitanian Age

In different geophysical regions of the world, other regional names are also used for the same or overlapping ages and other timeline subdivisions.

The terms Neogene System (formal) and upper Tertiary System (informal) describe the rocks deposited during the Neogene Period.

The continents in the Neogene were very close to their current positions. The isthmus of Panama formed, connecting North and South America. India continued to collide with Asia, forming the Himalayas. Sea levels fell, exposing land bridges between Africa and Eurasia and between Eurasia and North America.

The global climate became seasonal and continued its overall drying and cooling trend which began in the beginning of the Paleogene. The ice caps on both poles began to grow and thicken, and by the end of the period the first of a series of glaciations of the current Ice Age began.

Marine and continental flora and fauna were fairly modern at this time. Mammals and birds continued to be the dominant terrestrial vertebrates, and took many forms as they adapted to various habitats. The first hominids, the ancestors of humans, appeared in Africa and spread into Eurasia.

In response to the cooler, seasonal climate, tropical plant species gave way to deciduous ones and grasslands replaced many forests. Grasses therefore greatly diversified, and herbivorous mammals evolved alongside it, creating the many grazing animals of today such as horses, antelope and bison.

The Neogene traditionally ended at the end of the Pliocene Epoch, just before the older definition of the beginning of the Quaternary Period; many time scales show this division.

However, there was a movement amongst geologists (particularly Neogene Marine Geologists) to also include ongoing geological time (Quaternary) in the Neogene, while others (particularly Quaternary Terrestrial Geologists) insist the Quaternary to be a separate period of distinctly different record. The somewhat confusing terminology and disagreement amongst geologists on where to draw what hierarchical boundaries, is due to the comparatively fine divisibility of time units as time approaches the present, and due to geological preservation that causes the youngest sedimentary geological record to be preserved over a much larger area and to reflect many more environments, than the older geological record. By dividing the Cenozoic Era into three (arguably two) periods (Paleogene, Neogene, Quaternary) instead of 7 epochs, the periods are more closely comparable to the duration of periods in the Mesozoic and Paleozoic eras.

The ICS once proposed that the Quaternary be considered a sub-era (sub-erathem) of the Neogene, with a beginning date of 2.588 Ma, namely the start of the Gelasian Stage. In the 2004 proposal of the International Commission on Stratigraphy (ICS), the Neogene would have consisted of the Miocene and Pliocene epochs. The International Union for Quaternary Research (INQUA) counter - proposed that the Neogene and the Pliocene end at 2.588 Ma, that the Gelasian be transferred to the Pleistocene, and the Quaternary be recognized as the third period in the Cenozoic, citing key changes in Earth's climate, oceans, and biota that occurred 2.588 Ma and its correspondence to the Gauss - Matuyama magnetostratigraphic boundary. In 2006 ICS and INQUA reached a compromise that made Quaternary a subera, subdividing Cenozoic into the old classical Tertiary and Quaternary, a compromise that was rejected by International Union of Geological Sciences because it split both Neogene and Pliocene in two.

Following formal discussions at the International Geological Congress, Oslo Norway, August 2008, the International Commission on Stratigraphy (ICS) decided in May 2009 to make the Quaternary the youngest period of the Cenozoic Era with its base at 2.588 Mya and including the Gelasian age, which was formerly considered part of the Neogene Period and Pliocene Epoch. Thus the Neogene Period ends bounding the succeeding Quaternary Peiod at 2.588 Mya.

The Miocene (symbol M_I) is a geological epoch of the Neogene Period and extends from about 23.03 to 5.332 million years ago (Ma). The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words μείων (meiōn, “less”) and καινός (kainos, “new”) and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene Epoch and is followed by the Pliocene Epoch. The Miocene is the first epoch of the Neogene Period.

The earth went from the Oligocene Epoch through the Miocene and into the Pliocene as it cooled into a series of Ice Ages. The Miocene boundaries are not marked by a single distinct global event but consist rather of regional boundaries between the warmer Oligocene and the cooler Pliocene.

The plants and animals of the Miocene were fairly modern. Mammals and birds were well established. Whales, seals and kelp spread. The Miocene Epoch is of particular interest to geologists and palaeoclimatologists as major phases of Himalayan Uplift had occurred during the Miocene Epoch affecting monsoonal patterns in Asia, which were interlinked with Northern Hemisphere glaciation.

The Miocene faunal stages from youngest to oldest are typically named according to the International Commission on Stratigraphy:

These subdivisions within the Miocene are defined by the relative abundance of different species of calcareous nanofossils (calcite platelets shed by brown single - celled algae) and foraminifera (single - celled protists with diagnostic shells). Two subdivisions each form the Early, Middle and Late Miocene. Regionally, other systems are used. These ages often extend across the ICS epoch boundary into the Pliocene and Oligocene:

Continents continued to drift toward their present positions. Of the modern geologic features, only the land bridge between South America and North America was absent, although South America was approaching the western subduction zone in the Pacific Ocean, causing both the rise of the Andes and a southward extension of the Meso - American peninsula.

Mountain building took place in Western North America, Europe, and east Asia. Both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines. Well studied continental exposures occur in the American Great Plains and in Argentina.

India continued to collide with Asia, creating dramatic new mountain ranges. The Tethys Seaway continued to shrink and then disappeared as Africa collided with Eurasia in the Turkish - Arabian region between 19 and 12 Ma. The subsequent uplift of mountains in the western Mediterranean region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea (known as the Messinian salinity crisis) near the end of the Miocene.

The global trend was towards increasing aridity caused primarily by global cooling reducing the ability of the atmosphere to absorb moisture. Uplift of East Africa in the Late Miocene was partly responsible for the shrinking of tropical rain forests in that region, and Australia got drier as it entered a zone of low rainfall in the Late Miocene.

Grasslands underwent a major expansion; forests fell victim to a generally cooler and drier climate overall. Grasses also diversified greatly, co-evolving with large herbivores and grazers, including ruminants. Between 7 and 6 million years ago, there occurred a sudden expansion of grasses which were able to assimilate carbon dioxide more efficiently but were also richer in silica, causing a worldwide extinction of large herbivores. The expansion of grasslands and radiations among terrestrial herbivores such as horses can be linked to fluctuations in CO₂. Cycads between 11.5 and 5 m.y.a. began to rediversify after previous declines in variety due to climatic changes, and thus modern cycads are not a good model for a "living fossil".

Both marine and continental fauna were fairly modern, although marine mammals were less numerous. Only in isolated South America and Australia did widely divergent fauna exist. In the Early Miocene, several Oligocene groups were still diverse, including nimravids, entelodonts, and three - toed horses. Like in the previous Oligocene epoch, oreodonts were still diverse, only to disappear in the earliest Pliocene. During the later Miocene mammals were more modern, with recognizable dogs, racoons, horses, beaver, deer, camels, and whales, along with now extinct groups like borophagine dogs, gomphotheres, three - toed horses, and semi - aquatic and hornless rhinos like Teleoceras and Aphelops. Islands began to form between South and North America in the Late Miocene, allowing ground sloths like Thinobadistes to island hop to North America.

Unequivocally recognizable dabbling ducks, plovers, typical owls, cockatoos and crows appear during the Miocene. By the epoch's end, all or almost all modern bird families are believed to have been present; the few post - Miocene bird fossils which cannot be placed in the evolutionary tree with full confidence are simply too badly preserved instead of too equivocal in character. Marine birds reached their highest diversity ever in the course of this epoch.

Approximately 100 species of apes lived during this time. They ranged over much of the Old World and varied widely in size, diet and anatomy. Due to scanty fossil evidence it is unclear which ape or apes contributed to the modern hominid clade, but molecular evidence indicates this ape lived from between 15 to 12 million years ago.

In the oceans, Brown algae, called kelp, proliferated, supporting new species of sea life, including otters, fish and various invertebrates.

Cetaceans attained their greatest diversity during the Miocene, with over 20 recognized genera in comparison to only six living genera. This diversification correlates with emergence of gigantic macro - predators such as megatoothed sharks and raptorial sperm whales. Prominent examples are C. megalodon and L. melvillei. Other notable large sharks were C. chubutensis, Isurus hastalis and Hemipristis serra.

Crocodilians also showed signs of diversification during Miocene. The largest form among them was a gigantic caiman Purussaurus which inhabited South America. Another gigantic form was a false gharial Rhamphosuchus, which inhabited modern age India. A strange form Mourasuchus also thrived alongside Purussaurus. This species developed a specialized filter - feeding mechanism, and it likely preyed upon small fauna despite of its gigantic size.

The pinnipeds, which appeared near the end of the Oligocene, became more aquatic. Prominent genus was Allodesmus. A ferocious walrus, Pelagiarctos may have preyed upon other species of pinnipeds including Allodesmus.

Furthermore, South American waters witnessed the arrival of Megapiranha paranensis, which were considerably larger than modern age piranhas.

There is evidence from oxygen isotopes at Deep Sea Drilling Program sites that ice began to build up in Antarctica about 36 Ma during the Eocene. Further marked decreases in temperature during the Middle Miocene at 15 Ma probably reflect increased ice growth in Antarctica. It can therefore be assumed that East Antarctica had some glaciers during the early to mid Miocene (23 – 15 Ma). Oceans cooled partly due the formation of the Antarctic Circumpolar Current, and about 15 million years ago the ice cap in the southern hemisphere started to grow to its present form. The Greenland ice cap developed later, in the Middle Pliocene time, about 3 million years ago.

The "Middle Miocene disruption" refers to a wave of extinctions of terrestrial and aquatic life forms that occurred following the Miocene Climatic Optimum (18 to 16 Ma), around 14.8 to 14.5 million years ago, during the Langhian stage of the mid - Miocene. A major and permanent cooling step occurred between 14.8 and 14.1 Ma, associated with increased production of cold Antarctic deep waters and a major growth of the East Antarctic ice sheet. A Middle Miocene delta ¹⁸O increase, that is, a relative increase in the heavier isotope of oxygen, has been noted in the Pacific, the Southern Ocean and the South Atlantic.

The Pliocene (archaically Pleiocene) Epoch (symbol P_O) is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch. Prior to the 2009 revision of the geologic time scale, which placed the 4 most recent major glaciations entirely within the Pleistocene, the Pliocene also comprised the Gelasian stage, which lasted from 2.588 to 1.805 million years ago.

The Pliocene was named by Sir Charles Lyell. The name comes from the Greek words πλεῖον (pleion, "more") and καινός (kainos, "new") and means roughly "continuation of the recent", referring to the essentially modern marine mollusc faunas. H.W. Fowler called the term (along with other examples such as pleistocene and miocene) a "regrettable barbarism" and an indication that even "a good classical scholar" such as Lyell should have requested a philologist's help when properly coining words.

As with other older geologic periods, the geological strata that define the start and end are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The boundaries defining the onset of the Pliocene are not set at an easily identified worldwide event but rather at regional boundaries between the warmer Miocene and the relatively cooler Pleistocene. The upper boundary was set at the start of the Pleistocene glaciations.

In the official timescale of the ICS, the Pliocene is subdivided into two stages. From youngest to oldest they are:

• Piacenzian (3.600–2.588 Ma)
• Zanclean (5.332–3.600 Ma)

The Piacenzian is sometimes referred to as the Late Pliocene, whereas the Zanclean is referred to as the Early Pliocene.

In the system of North American Land Mammal Ages the Pliocene overlaps with two stages: the Blancan (4.75 – 1.806 Ma) and Hemphillian (9 – 4.75 Ma). In the system of South American Land Mammal Ages, the Pliocene overlaps with the Montehermosan (6.8 - 4.0 Ma), Chapadmalalan (4.0 - 3.0 Ma) and Uquian (3.0 - 1.2 Ma). In the Paratethys area (central Europe and parts of western Asia) the Pliocene contains the Dacian (roughly equal to the Zanclean) and Romanian (roughly equal to the Piacenzian and Gelasian together) stages. As usual in stratigraphy, there are many other regional and local subdivisions in use.

During the Pliocene epoch climate became cooler and drier, and seasonal, similar to modern climate.

The global average temperature in the mid - Pliocene (3.3 mya - 3 mya) was 2-3°C higher than today, global sea level 25 m higher and Northern hemisphere ice sheet ephemeral before the onset of extensive glaciation over Greenland that occurred in the late Pliocene around 3 Ma. The formation of an Arctic ice cap is signaled by an abrupt shift in oxygen isotope ratios and ice - rafted cobbles in the North Atlantic and North Pacific ocean beds. Mid - latitude glaciation was probably underway before the end of the epoch. The global cooling that occurred during the Pliocene may have spurred on the disappearance of forests and the spread of grasslands and savannas.

Continents continued to drift, moving from positions possibly as far as 250 km from their present locations to positions only 70 km from their current locations. South America became linked to North America through the Isthmus of Panama during the Pliocene, making possible the Great American Interchange and bringing a nearly complete end to South America's distinctive large marsupial predator and native ungulate faunas. The formation of the Isthmus had major consequences on global temperatures, since warm equatorial ocean currents were cut off and an Atlantic cooling cycle began, with cold Arctic and Antarctic waters dropping temperatures in the now - isolated Atlantic Ocean.

Africa's collision with Europe formed the Mediterranean Sea, cutting off the remnants of the Tethys Ocean. The border between the Miocene and the Pliocene is also the time of the Messinian salinity crisis.

Sea level changes exposed the land bridge between Alaska and Asia.

Pliocene marine rocks are well exposed in the Mediterranean, India and China. Elsewhere, they are exposed largely near shores.

The change to a cooler, dry, seasonal climate had considerable impacts on Pliocene vegetation, reducing tropical species worldwide. Deciduous forests proliferated, coniferous forests and tundra covered much of the north, and grasslands spread on all continents (except Antarctica). Tropical forests were limited to a tight band around the equator, and in addition to dry savannahs, deserts appeared in Asia and Africa.

Both marine and continental faunas were essentially modern, although continental faunas were a bit more primitive than today. The first recognizable hominins, the australopithecines, appeared in the Pliocene.

The land mass collisions meant great migration and mixing of previously isolated species, such as in the Great American Interchange. Herbivores got bigger, as did specialized predators.

In North America, rodents, large mastodons and gomphotheres, and opossums continued successfully, while hoofed animals (ungulates) declined, with camel, deer and horse all seeing populations recede. Rhinos, three toed horses (Nannippus), oreodonts, protoceratids and chalicotheres went extinct. Borophagine dogs went extinct, but other carnivores including the weasel family diversified, and dogs and fast running hunting bears did well. Ground sloths, huge glyptodonts and armadillos came north with the formation of the Isthmus of Panama.

In Eurasia rodents did well, while primate distribution declined. Elephants, gomphotheres and stegodonts were successful in Asia, and hyraxes migrated north from Africa. Horse diversity declined, while tapirs and rhinos did fairly well. Cows and antelopes were successful, and some camel species crossed into Asia from North America. Hyenas and early saber toothed cats appeared, joining other predators including dogs, bears and weasels.

Africa was dominated by hoofed animals, and primates continued their evolution, with australopithecines (some of the first hominids) appearing in the late Pliocene. Rodents were successful, and elephant populations increased. Cows and antelopes continued diversification and overtaking pigs in numbers of species. Early giraffes appeared, and camels migrated via Asia from North America. Horses and modern rhinos came onto the scene. Bears, dogs and weasels (originally from North America) joined cats, hyenas and civets as the African predators, forcing hyenas to adapt as specialized scavengers.

South America was invaded by North American species for the first time since the Cretaceous, with North American rodents and primates mixing with southern forms. Litopterns and the notoungulates, South American natives, were mostly wiped out, except for the macrauchenids and toxodonts, which managed to survive. Small weasel - like carnivorous mustelids and coatis migrated from the north. Grazing glyptodonts, browsing giant ground sloths and smaller caviomorph rodents, pampatheres, and armadillos did the opposite, migrating to the north and thriving there.

The marsupials remained the dominant Australian mammals, with herbivore forms including wombats and kangaroos, and the huge diprotodon. Carnivorous marsupials continued hunting in the Pliocene, including dasyurids, the dog - like thylacine and cat - like Thylacoleo. The first rodents arrived in Australia. The modern platypus, a monotreme, appeared.

The predatory South American phorusrhacids were rare in this time; among the last was Titanis, a large phorusrhacid that migrated to North America and rivaled mammals as top predator. Its distinct feature was its claws, which had re-evolved for grasping prey, such as Hipparion. Other birds probably evolved at this time, some modern, some now extinct.

Alligators and crocodiles died out in Europe as the climate cooled. Venomous snake genera continued to increase as more rodents and birds evolved. Rattlesnakes first appeared in the Pliocene. The modern species Alligator mississippiensis, having evolved in the Miocene, continued into the Pliocene, except with a more northern range; specimens have been found in very late Miocene deposits of Tennessee. Giant tortoises still thrived in North America, with genera like Hesperotestudo. Madtsoid snakes were still present in Australia.

Oceans continued to be relatively warm during the Pliocene, though they continued cooling. The Arctic ice cap formed, drying the climate and increasing cool shallow currents in the North Atlantic. Deep cold currents flowed from the Antarctic.

The formation of the Isthmus of Panama about 3.5 million years ago cut off the final remnant of what was once essentially a circum - equatorial current that had existed since the Cretaceous and the early Cenozoic. This may have contributed to further cooling of the oceans worldwide.

The Pliocene seas were alive with sea cows, seals and sea lions.

In 2002, astronomers discovered that roughly 2 million years ago, around the end of the Pliocene epoch, a group of bright O and B stars called the Scorpius - Centaurus OB association passed within 150 light years of Earth and that one or more supernovae may have occurred in this group at that time. Such a close explosion could have damaged the Earth's ozone layer and caused the extinction of some ocean life (at its peak, a supernova of this size could have the same absolute magnitude as an entire galaxy of 200 billion stars).