Article 32: The overall pattern of biological change is one of expanding open vegetation systems, at the expense of diminishing closed vegetation. The apes arise and diversify, becoming widespread in the Old World.
The Miocene Period of the Earth’s history spans 18 million years, from 23 to 5 million years ago. Together with the next period, the Pliocene, it is called the Neogene (23 – 2,5 mya).
The overall pattern of biological change for the Miocene is one of expanding open vegetation systems, such as deserts, tundra and grasslands, at the expense of diminishing closed vegetation, such as forests. This leads to a re-diversification of temperate ecosystems and to many morphological changes in animals. More open landscapes allow animals to grow to larger sizes than they had earlier. Grasslands begin to spread, followed by an evolutionary radiation of open-habitat herbivores and carnivores.
This mural at the National Museum of Natural History presents an idealized picture of this period, about 25 million years ago in what is now Nebraska.
Mammals and birds in particular develop new forms, whether as fast-running herbivores, large predatory mammals and birds or small quick birds and rodents. Unequivocally recognizable dabbling ducks, plovers, typical owls, cockatoos and crows appear during the Miocene. During the later Miocene mammals are more modern, with easily recognizable dogs, bears, raccoons, horses, beavers, deer, camels and whales.
By the epoch’s end, all or almost all modern bird families are believed to have been present.
The apes arise and diversify during the Miocene epoch, becoming widespread in the Old World. Approximately 100 species of them live during this time, ranging throughout Africa, Asia and Europe and varying widely in size, diet and anatomy. Due to scanty fossil evidence it is unclear which ape or apes contribute to the modern hominid clade, but molecular evidence indicates this ape lives between 7 and 8 million years ago.
The first hominins (homo and pan) appear in Africa at the very end of the Miocene. By the end of this epoch, the ancestors of humans have split away from the ancestors of the chimpanzees to follow their own evolutionary path. We are on our way. We have always been.
As in the Oligocene before it, grasslands continue to expand and forests to dwindle in extent. In the Miocene seas, kelp forests make their first appearance and soon become one of Earth’s most productive ecosystems.
The dragon blood tree is considered a remnant of the Mio-Pliocene subtropical forests, that are now almost extinct in North Africa
The Miocene sees a change in global circulation patterns, due to slight position changes of the continents and globally warmer climates.
Conditions on each continent change somewhat, because of these positional changes, however, it is an overall increase in aridity through mountain-building that favors the expansion of grasslands. Climates remain moderately warm, although the slow global cooling, that eventually will lead to the Pleistocene glaciations, continues.
Although this long-term cooling trend is well underway, there is evidence of a warm period during the Miocene. This Miocene warming begins 21 million years ago and continues until 14 million years ago, when global temperatures take a sharp drop, the Middle Miocene Climate Transition, MMCT.
By 8 million years ago, temperatures drop sharply once again. The Antarctic ice sheet is already approaching its present-day size and thickness. Greenland begins to have large glaciers as early as 7 to 8 million years ago, although the climate for the most part remains warm enough to support forests there well into the next period, the Pliocene.
20 million years ago, Antarctica is covered by ice and the northern continents are cooling rapidly. The world has taken on a ‘modern’ look, but notice that Florida and parts of Asia are flooded by the sea.
The first of the major periods of immigration via the Bering land connection between Siberia and Alaska occurs in the middle of the Miocene. By the end of the Miocene, the Panama isthmus begins to form, between Central and South America. India continues to collide with Asia, creating dramatic new mountain ranges.
Thanks to the University of California, Berkeley, Museum of Paleontology, to Christopher R. Scotese and to Wikipedia, Miocene