The Cenozoic era, 65 million years ago to the present, is divided into two periods, the Tertiary and the Quaternary. The Tertiary period, 65 to 2 million years ago, encompasses the rebuilding of the animal kingdom at the end of the great Cretaceous extinction. From an unpromising beginning as small, nocturnal opportunists, mammals, along with the surviving birds, radiated into the most numerous and diverse life forms to inhabit Earth.
During the Quaternary, 2 million years ago to the present, dramatic climate changes reduced the overall diversity of animal life, yet saw the rise of another remarkable evolutionary opportunist—the bipedal hominid .
The earlier phase of the Quaternary, the Pleistocene epoch (2 million to 10,000 years ago), covers the alternating periods of glacial advance and retreat across the Northern Hemisphere and the corresponding effect on plant and animal life. The current chapter of Earth history, the Holocene epoch, deals with the interglacial period which saw the rise to dominance of Homo sapiens, the first mammal to shape its environment as well as be shaped by it.
During the Pleistocene, the continents continued their leisurely drift northward. Antarctica remained in place over the South Pole with a steadily increasing ice cap. The land bridge that formed between North and South America continued to disrupt ocean currents by sending colder water into the tropics. These geographic events, coupled with an overall cooling trend of about 10°F, seem to have tipped the planet into a series of "ice ages" interspersed with more brief, temperate periods. During glaciation, ice sheets formed at the North Pole moved across the Northern Hemisphere, locking up huge amounts of global water. Sea levels dropped by as much as 90 meters (300 feet), connecting previously separated land masses. Europe and Britain became contiguous, as did Siberia and Alaska. The more important effect of the glaciers was the decreasing of worldwide humidity. As the tropics became deserts, the larger mammals of Madagascar and Australia were pushed to extinction. As forests declined into grasslands, animals worldwide were pushed into shrinking refuges and competition with one another. Herds of herbivores that thrived on grasslands were unable to cope when the temperate forests expanded northward during the warmer periods. The grasslands in the colder latitudes closer to the pole were not as lush or varied and were nutritionally poorer. The episodic advance and retreat of habitat may have led to the contemporary patterns of migration in warm-blooded animals.
Continual climate stresses and the mingling of species across what had once been barriers contributed to overall decreased diversity. By the end of the Pleistocene 12,000 years ago, the "megafauna"—mammoths, mastodons, 2.5-metric-ton (3-ton) sloths, and Aepyornis, the 400-kilogram (900-pound) bird—had all but disappeared. Almost unnoticed among the giants (mammoths, mastodons, and sloths) of the time was a new evolutionary player. Arboreal primates resembling small tree shrews had flourished 60 million years ago giving rise to several groups. These were the placental mammals, which had hands with the ability to grasp, eyes that faced forward, and teeth adapted to an omnivorous diet. Hand-to-eye coordination, encouraged by a tree-climbing lifestyle, also forced the development of the "thinking" part of the brain.
As environmental stresses worked on the prosimians, they developed into the brainier anthropoids, and finally hominoids, the apelike family that spread throughout Africa, Europe and Asia 25-10 million years ago. Around 4 million years ago, driven by need or lured by opportunity, certain primates took up a new behavior, walking on their hind legs. Bone fossils show that their originally upright posture alternated with four-legged running and climbing. But gradually, the hominid became more and more bipedal, freeing its hands for carrying and manipulating its environment. Upright walking required dramatic changes in anatomy, and these changes further widened the anatomical gap between this protohuman and its closest relative, the quadripedal ape.
Pelvic changes limited the size of the young at birth, creating a longer period of infant dependency. This in turn encouraged the development of a social organization to protect and rear the young. Other benefits of this cooperation fostered more complex arrangements such as foraging together, using of communal shelters, tool making, the specialization of labor, and sharing resources. Thus began the long journey from the tropical treetops into the remotest regions of the globe, by the mammal which possesses the power to affect all life on Earth.
The history of life can be seen as an unending cycle of environmental pressure → change and decline → adaptive radiation → and more pressure. Homo sapiens, the first species ever to create its own environmental pressures, now stands at the threshold. Will we adapt or decline?
see also Geological Time Scale.
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Fortey, Richard. Fossils: The Key to the Past. Cambridge, MA: Harvard University Press, 1991.
———. Life: A Natural History of the First Four Billion Years of Life on Earth. New York: Viking Press, 1998.
Gould, Stephen Jay, ed. The Book of Life. New York: W. W. Norton & Company, 1993.
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McLoughlan, John C. Synapsida: A New Look Into the Origin of Mammals. New York: Viking Press, 1980.
Steele, Rodney, and Anthony Harvey, eds. The Encyclopedia of Prehistoric Life. New York: McGraw Hill, 1979.
Wade, Nicholas, ed. The Science Times Book of Fossils and Evolution. New York: The Lyons Press, 1998.
In geologic time , the Quaternary Period (also termed the Anthropogene Period), the second geologic period in the Cenozoic Era , spans the time between roughly 2.6 million years ago (mya) and present day. On the geologic time scale, Earth is currently in the Quaternary Period of the Cenezoic Era of the Phanerozoic Eon .
The Quaternary Period contains two geologic epochs. The earliest epoch, the Pleistocene Epoch ranges from approximately 2.6 mya to 10,000 years ago. The Pleistocene Epoch is further subdivided into (from earliest to most recent) Gelasian and Calabrian stages. The Calabrian stage is also frequently replaced by a series of geologic stages, from earliest to most recent, including the Danau, Donau-Günz, Günzian, Günz-Mindel, Mindelian, Mindel-Riss, Rissian, Riss-Würm, and Würmian stages. The latest, most recent, and current epoch, the Holocene Epoch ranges from approximately 10,000 years ago until present day. According to geologic time, Earth is currently in the Holocene Epoch.
During the Quaternary Period, Earth's continents assumed their modern configuration. The Pacific Ocean separated Asia and Australia from North America and South America ; the Atlantic Ocean separated North and South America from Europe (Euro-Asia) and Africa . The Indian Ocean filled the basin between Africa, India, Asia, and Australia. The Arabian Plate wedged between the Eurasian and African plates continues to provide high levels of tectonic activity (e.g., earthquakes) in the area of modern day Turkey. The Indian plate driving against and under the Eurasian plate uplifts both in rapid mountain building. As a result of the ongoing collision, ancient oceanic crust bearing marine fossils was uplifted into the Himalayan chain. The collision between the Indian and Eurasian plate continues with a resulting slow—but measurable—increase in the altitude of the highest Himalayan mountains (e.g., Mt. Everest) each year.
Glaciation (e.g., ice ages ), and fluctuating climatic conditions—possibly at least partially explainable by Milankovitch cycles—during both the Tertiary and Quaternary Periods brought about sweeping changes in the landscape evident in modern topographical features.
The fossil record provides evidence that by the end of the Tertiary Period (also known as the Neogene period), the species Ardipithecus ramidus walked upright in an area now encompassing modern Ethiopa. Near the start of the Quaternary Period, a number of species lived and became extinct before modern humankind (Homo sapiens ) appeared. Many of these species, including Australopithecus anamensis, Australopithecus afarensis, Australopithecus garhi, and Australopithecus africanus were only collateral rungs on the ladder of evolution to Homo sapiens, and do not provide a direct evolutionary link to humans. Although these species became extinct near the start of the Quaternary Period, they at least co-existed with the direct ancestors of humans. Early in the Quaternary Period Homo habilis and Homo rudolfensis lived and became extinct. Their extinctions are dated to approximately the appearance of Homo ergaster, a species some anthropologists argue is one of the earliest identifiable direct ancestors of Homo erectus, Homo heidelbergensis, Homo neanderthalensis, and Homo sapiens.
The last major impact crater with a diameter over 31 mi (50 km) struck Earth near what is now Kara-Kul, Tajikistan at the Pliocene Epoch and Pleistocene Epoch geologic time boundary that established the start of the Quaternary Period.
See also Archean; Cambrian Period; Cretaceous Period; Dating methods; Devonian Period; Eocene Epoch; Evolution, evidence of; Fossil record; Fossils and fossilization; Historical geology; Jurassic Period; Mesozoic Era; Miocene Epoch; Mississippian Period; Oligocene Epoch; Ordovician Period; Paleocene Epoch; Paleozoic Era; Pennsylvanian Period; Precambrian; Proterozoic Era; Silurian Period; Supercontinents; Triassic Period
quat·er·nar·y / ˈkwätərˌnerē/ • adj. 1. fourth in order or rank; belonging to the fourth order. 2. (Quaternary) Geol. of, relating to, or denoting the most recent period in the Cenozoic era, following the Tertiary period and comprising the Pleistocene and Holocene epochs (and thus including the present). 3. Chem. denoting an ammonium compound containing a cation of the form NR4+, where R represents organic groups or atoms other than hydrogen. ∎ (of a carbon atom) bonded to four other carbon atoms.• n. (the Quaternary) Geol. the Quaternary period or the system of deposits laid down during it.
four things collectively ; a group of four facts or circumstances. Also quaternion.
Examples : quaternion of earth, air, water, and fire, 1695; of English writers—Shakespeare, Hooker, Bacon, Jeremy Taylor, 1868; of topics, 1648.