Miocene Epoch
The Miocene Epoch is a significant geological period that lasted from approximately 23 million to 5.3 million years ago, forming the first major division of the Neogene period in the Cenozoic era, often referred to as the age of mammals. This epoch saw a gradual cooling of global temperatures, leading to the transformation of various ecosystems, such as the conversion of dense forests into expansive grasslands, which supported the proliferation of grazing mammals. During the Miocene, many modern fauna began to emerge, including early forms of horses, deer, elephants, and various carnivores like primitive hyenas and saber-toothed cats. It was also a pivotal time for avian evolution, with many modern bird species, as well as significantly larger species, appearing during this period.
The Miocene epoch played a crucial role in primate evolution, marking the divergence of early hominins from chimpanzees. This period included the emergence of the genus Dryopithecus, which is believed to be an ancestor to both modern apes and humans. Notably, the Miocene set the stage for substantial evolutionary developments that would continue into the subsequent Pliocene epoch, ultimately influencing the emergence of modern humans. The epoch concluded with significant ecological shifts, including a marked cooling that contributed to the extinction of some species and the expansion of grasslands, shaping the biodiversity of the planet.
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Miocene Epoch
The Miocene epoch is a geological time span that extended from about 23 million to 5.3 million years ago. The Miocene was one of seven epochs of the Cenozoic era, a larger period sometimes called the age of mammals. When the Miocene began, global climates were relatively warm and cooled as the epoch continued. Several modern ecosystems and many of the species that inhabit the present day first developed during the Miocene epoch. The ancestors of modern humans also split from the evolutionary line of chimpanzees near the end of the Miocene.
Background
Scientists have divided the geological history of Earth into a hierarchy of several periods. The divisions begin with the formation of Earth about 4.6 billion years ago and continue into present day. From largest timeframe to smallest, the classifications are eons, eras, periods, epochs, and ages. An eon comprises several eras, an era comprises several periods, a period comprises several epochs, and so on. The demarcation boundaries between periods are determined by major geological or evolutionary shifts, such as rock formation, temperature changes, biological proliferation, or mass extinctions.
The Phanerozoic eon began about 542 million years ago with the evolution of more advanced, hard-shelled organisms, and continues into the present day. The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic. The Paleozoic era began with a rapid increase in biological diversity in Earth's oceans called the Cambrian explosion. The era ended about 251 million years ago with a mass extinction that killed about 96 percent of the planet's life. It was followed by the Mesozoic era, or the age of dinosaurs.
The dinosaurs were the dominant form of life on Earth for almost 200 million years, until an asteroid slammed into the planet about 65 million years ago. The mass extinction event marked the end of the Mesozoic and ushered in the Cenozoic era. While mammals had evolved during the Mesozoic, it was during the Cenozoic that they began to flourish. For this reason, the Cenozoic is often referred to as the age of mammals, although birds, insects, and flowering plants also thrived during the era.
Overview
The Cenozoic is further divided into the Paleogene, Neogene, and Quaternary periods. The Miocene epoch is the first major division of the Neogene period, which began about 23 million years ago. The beginning of the Miocene coincided with a gradual drop in global temperatures from the previous epoch, the much warmer Oligocene. The Miocene is also broken down into six ages. These ages—the Aquitanian, Burdigalian, Langhian, Serravallian, Tortonian, and Messinian—are determined by difference in geological rock formations. The term Miocene was coined by nineteenth-century British geologist Charles Lyell. It evolved from the Greek words meion (less) and the suffix -cene (recent).
During the Miocene, Earth's landmasses continued to shift and began to resemble the modern continents. Geological activity also formed some of the planet's major mountain ranges, such as the Himalayas in Asia, the Andes in South America, and the Sierra Nevada and Cascade ranges in North America. The changing landscapes resulted in a drier climate, transforming forests into grasslands, an ecosystem better able to support grazing mammals. In the oceans, the climate fueled the development of kelp forests, which offered food and protection for many sea creatures.
The open grasslands led to the evolutionary rise of horses and the proliferation of the ancestors of modern deer and elephants. The Miocene saw the first primitive hyenas, bears, giraffes, and saber-toothed cats. Prehistoric dogs, such as the long-extinct Tomarctus, also began to resemble their current forms. Most of the modern species of whales developed during the epoch, as did early forms of seals and walruses.
Many bird and reptile species of the Miocene were much larger than their current counterparts are. The Quinkana was an Australian crocodile that could reach up to 20 feet (6.1 meters) in length. In South America, the Argentavis was a bird species with a 25-foot (7.6-meter) wingspan. By the end of the epoch, ducks, sparrows, hawks, eagles, and the majority of modern bird species had evolved. Scientists also estimate that about 95 percent of modern seed plants have survived from the Miocene.
The first true primates had evolved about 55 million years ago during the Eocene epoch. By the Miocene, primates were common in many environments, with hundreds of species estimated to have been living around the globe. The first monkeys evolved during the Oligocene, while the first apes split from monkeys in the early Miocene. A spike in global temperatures about 15 million years ago led to the evolution of Dryopithecus, a primate group similar to modern African apes.
Anthropologists believe the descendants of Dryopithecus split along evolutionary lines about 8 to 9 million years ago. One branch went on to become African gorillas, while the other became chimpanzees and bonobos. About 6 to 7 million years ago, humanlike primates called hominins diverged from the chimpanzees. These early hominins—such as Sahelanthropus, a species from West Central Africa—combined ape and humanlike features and had the ability to walk upright. While it is unknown if Sahelanthropus was a direct human ancestor, hominin development during the Miocene is thought to have eventually led to the evolution of modern humans.
The Miocene epoch gave way to the Pliocene epoch about 5.3 million years ago. The boundary between the epochs was marked by a more significant cooling of Earth's climate. In some areas, the global cooling resulted in a number of species becoming extinct. The cooler, drier temperatures continued to spur the growth of grasslands such as the Great Plains of North America. Many of the evolutionary trends of the Miocene also carried over into the Pliocene, including the rise of the first human ancestors to use tools.
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