Environment

The Quaternary Period, the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), follows the Neogene Period and spans from 2.588 ± 0.005 million years ago to the present.

Typically defined by the cyclic growth and decay of continental ice sheets driven by Milankovitch cycles and the associated climate and environmental changes that occurred, the Quaternary Period is divided into two epochs: the Pleistocene and the Holocene (eleven thousand seven hundred years ago to today).

The Pleistocene spans the world's recent period of repeated glaciations.

With the onset of the Quaternary glaciation, the first of the several ice ages to follow, decreasing oceanic evaporation results in a drier climate in East Africa and an expansion of the savanna at the expense of forests.

Reduced availability of fruits forces some Australopithecines to unlock new food sources found in the drier savanna climate, representing a move from the mostly frugivorous or omnivorous diet of Australopithecus to the carnivorous scavenging lifestyle of early Homo.

Paranthropus species are still present in the beginning of the Pleistocene, along with early human ancestors, but they disappear during the lower Paleolithic.

The Lower Paleolithic, the earliest subdivision of the Paleolithic or Old Stone Age, begins around two and a half million million years ago when the first evidence of craft and use of stone tools by hominids appears in the current archaeological record.

The genus Homo, which includes modern humans and species closely related to them, is estimated to be about two point three to two point four million years old, evolving from australopithecine ancestors with the appearance of Homo habilis.

Specifically, H. habilis is considered the direct descendant of Australopithecus garhi, a gracile species that lived about two and a half million years ago.

The most salient physiological development between the two species is the increase in cranial capacity, from about four hundred and fifty cubic centimeters (twenty-seven cubic inches) in A. garhi to six hundred cubic centimeters (thirty-seven cubic inches) in H. habilis.

The earliest known occupation of Britain by humans occurs in the Palaeolithic—the Old Stone Age.

A huge length of time, it sees many changes in the environment, encompassing several glacial and interglacial periods that greatly affect human settlement in the region.

Providing dating for this distant period is difficult and contentious.

The inhabitants of the region at this time are bands of hunter-gatherers who roam northern Europe following herds of animals, or who support themselves by fishing.

Recent (2006) scientific evidence regarding mitochondrial DNA sequences from ancient and modern Europe has shown a distinct pattern for the different time periods sampled in the course of the study.

Despite some limitations regarding sample sizes, the results were found to be non-random.

As such, the results indicate that, in addition to populations in Europe expanding from southern refugia after the last glacial maximum (especially the Franco-Cantabrian region), evidence also exists for various northern refugia.

Southern and eastern Britain at this time are linked to continental Europe by a wide land bridge allowing humans to move freely.

The current position of the English Channel is a large river flowing westwards and fed by tributaries that will later become the Thames and Seine.

Reconstructing this ancient environment has provided clues to the route first visitors took to arrive at what was then a peninsula of the Eurasian continent.

Archaeologists have found a string of early sites located close to the route of a now lost watercourse named the Bytham River, which indicate that it was exploited as the earliest route west into Britain.

There is evidence from bones and flint tools found in coastal deposits near Happisburgh in Norfolk and Pakefield in Suffolk that a species of Homo was present in what is now Britain around seven hundred thousand years ago.

Cranial capacity had again doubled within the Homo genus by six hundred thousand years ago, from H. habilis to an archaic Homo species called Homo heidelbergensis, the second human wave to be pumped from Africa into the Middle East and Western Europe.

Sites such as Boxgrove in Sussex illustrate the later arrival in the archaeological record of heidelbergensis around five hundred thousand years ago.

The cranial capacity of H. heidelbergensis overlaps with the range found in modern humans; these early peoples make Acheulean flint tools (hand axes) and hunt the large native mammals of the period.

They are thought to have driven elephants, rhinoceroses, and hippopotamuses over the tops of cliffs or into bogs to kill them more easily.

Up until the 1970s, these kill sites, often at waterholes where animals would gather to drink, were interpreted as being where Acheulean tool users killed game, butchered their carcasses, and then discarded the tools they had used.

Since the advent of zooarchaeology, which has placed greater emphasis on studying animal bones from archaeological sites, this view has changed.

Many of the animals at these kill sites have been found to have been killed by other predator animals, so it is likely that humans of the period supplemented hunting with scavenging from already dead animals.

The extreme cold of the Anglian Stage, from four hundred and seventy-eight thousand to four hundred and twenty-four thousand years ago, is likely to have driven humans out of Britain altogether and the region will apparently be unoccupied until the ice recedes during the Hoxnian Stage.

The Wolstonian Stage, a Middle Pleistocene stage of the geological history of earth that precedes the Ipswichian Stage (Eemian Stage in Europe) and follows the Hoxnian Stage in the British Isles, apparently includes three periods of glaciation.

Commencing three hundred and fifty-two thousand years ago and ending one hundred and thirty thousand years ago, it is temporally analogous to the Warthe Stage and Saalian Stage in northern Europe and the Riss glaciation in the Alps, and temporally equivalent to all of the Illinoian Stage and the youngest part of the Pre-Illinoian Stage in North America.

It is contemporaneous with the North American Pre-Illinoian A, Early Illinoian, and Late Illinoian glaciations.

Britain first becomes an island about three hundred and fifty thousand years ago, near the beginning of the Wolstonian stage, named after the site of Wolston in the English county of Warwickshire where corresponding deposits were first identified.

This period sees the introduction of Levallois flint tools, possibly by humans arriving from Africa, although finds from Swanscombe and Botany Pit in Purfleet support Levallois technology as a European rather than African introduction.

This more advanced flint technology, which made hunting more efficient, therefore made Britain a more worthwhile place to remain until the increasingly cool climate of the Wolstonian Stage, which apparently includes three periods of glaciation, made continued habitation unattractive, if not impossible.

Acheulean flint tools, typically found with Homo erectus remains, have been found in Wolstonian deposits.

Neanderthals probably diverge from Homo heidelbergensis some three hundred thousand years ago in Europe during the Wolstonian Stage, from three hundred and fifty-two thousand to one hundred and thirty thousand years ago.

The beginning of the last Ice Age is conventionally dated at about 120,000 BCE.

The Eemian, an interglacial period which begins about one hundred and thirty thousand years ago and ends about one hundred and fourteen thousand years ago, is the second-to-latest interglacial period of the current Ice Age, the most recent being the Holocene which extends to the present day.

The prevailing Eemian climate is believed to have been about as stable as that of the Holocene.

Changes in the earth's orbital parameters from today (greater obliquity and eccentricity, and perihelion), known as the Milankovitch cycle, probably lead to greater seasonal temperature variations in the Northern Hemisphere, although global annual mean temperatures are probably similar to those of the Holocene.

The warmest peak of the Eemian is around one hundred and twenty-five thousand years ago, when forests reach as far north as North Cape (which is now tundra) in northern Norway well above the Arctic Circle.

Hardwood trees like hazel and oak grow as far north as Oulu, Finland.

At the peak of the Eemian, the northern hemisphere winters are generally warmer and wetter than now, though some areas are actually slightly cooler than today.

The hippopotamus is distributed as far north as the rivers Rhine and Thames.

Trees grow as far north as southern Baffin Island in the Canadian Arctic Archipelago instead of only as far north as Kuujjuaq in northern Quebec, and the prairie-forest boundary in the Great Plains of the United States lies further west—near Lubbock, Texas, instead of near Dallas, Texas, where the boundary now exists.

Sea level at peak is probably four to six meters (thirteen to twenty feet) higher than today (references in Overpeck et al., 2006), with much of this extra water coming from Greenland but some likely to have come from Antarctica.

Global mean sea surface temperatures are thought to have been higher than in the Holocene, but not by enough to explain the rise in sea level through thermal expansion alone, and so melting of polar ice caps must also have occurred.

Because of the sea level drop since the Eemian, exposed fossil coral reefs are common in the tropics, especially in the Caribbean and along the Red Sea coastlines.

These reefs often contain internal erosion surfaces showing significant sea level instability during the Eemian.

A 2007 study found evidence that the Greenland ice core site Dye 3 was glaciated during the Eemian, which implies that Greenland could have contributed at most two meters (6.6 feet) to sea level rise.

Scandinavia was an island due to the inundation of vast areas of northern Europe and the West Siberian Plain.

Given that the shorelines of the islands of the present are at this time several feet higher, humans of this epoch must have used boats: the large island of Crete in the Aegean basin is settled as early as 128,000 BCE, during the Middle Paleolithic age.

The period closes as temperatures steadily fall to conditions cooler and drier than the present, with four hundred and sixty-eight-year long aridity pulse in central Europe, and by one hundred and fourteen thousand years before the present, a glacial era has returned.

The beginning of the last Ice Age is conventionally dated at about one hundred and twenty thousand BCE with the onset of the Abbassia Pluvial, an extended wet and rainy period in the climate history of North Africa.

The Abbassia Pluvial, which lasted approximately thirty thousand years, and ended around ninety thousand ybp, spanned the end of the Lower Paleolithic and the start of the Middle Paleolithic eras—an interval that is also sometimes identified as the Acheulean (two hundred and fifty to ninety kybp).

As with the subsequent Mousterian Pluvial (circa fifty to thirty kybp), the Abbassia was brought about by global climate changes associated with the ice ages and interglacials of the Pleistocene Epoch.

Like the Mousterian Pluvial, the Abbassia Pluvial brought wet and fertile conditions to what is now the Sahara Desert, which bloomed with lush vegetation fed by lakes, swamps, and river systems, many of which later disappeared in the drier climate that followed the pluvial.

During this period, African wildlife now associated with the grasslands and woodlands south of the Sahara penetrated the entire North African.

Human Stone Age cultures—notably the Mousterian and Aterian Industries—flourished in Africa during the Abbassia Pluvial.

The shift to harsher climate conditions that came with the end of the pluvial promoted the emigration of modern Homo sapiens out of Africa and over the rest of the globe.

This was the third wave of the Sahara pump cycle.

There is little evidence of human occupation in Britain during the Ipswichian Stage (Eemian Stage elsewhere) between around one hundred and thirty thousand and one hundred and ten thousand years ago.

Meltwaters from the previous glaciation cut Britain off from the continent during this period, which may explain the lack of activity.

Overall, there appears to be a gradual decline in population between the Hoxnian Stage and this time suggesting that the absence of humans in the archaeological record here was is the result of gradual depopulation.

The foundation population of the humans that today inhabit the world are the survivors of what appears to be an evolutionary bottleneck caused by a global catastrophe during the period that begins around 90,000 BCE.

The Toba supereruption (Youngest Toba Tuff or simply YTT), a supervolcanic eruption that occurs some time between sixty-nine thousand and seventy-seven thousand years ago at Lake Toba in Sumatra, Indonesia, is recognized as one of the Earth's largest known eruptions and is the most closely studied supereruption.

The related catastrophe hypothesis holds that this event plunged the planet into a six-to-ten-year volcanic winter and possibly an additional one thousand-year cooling episode.

This change in temperature results in the world's human population being reduced to ten thousand or even a mere one thousand breeding pairs, creating a bottleneck in human evolution.

Consistent with the Toba catastrophe theory, evolutionary biologist Richard Dawkins has postulated that human mitochondrial DNA (inherited only from one's mother) and Y chromosome DNA (from one's father) show coalescence at around one hundred and forty thousand and sixty thousand years ago, respectively.

In other words, all living humans' female line ancestry traces back to a single female (Mitochondrial Eve) at around one hundred and forty thousand years ago.

All humans can trace their ancestry with certainty via the male line back to a single male (Y-chromosomal Adam) at ninety thousand to sixty thousand years ago.