Quaternary

The Quaternary Period is the geologic time period after the Neogene Period, spanning 1.805 +/- 0.005 million years ago to the present. The Quaternary includes two geologic epochs: the Pleistocene and the Holocene Epoch.

There is an ongoing debate of the status of Quaternary &mdash; a recent proposal from International Commission on Stratigraphy (ICS) was to make Quaternary a subperiod under Neogene, but that was retracted after criticism from International Union for Quaternary Research (INQUA), so instead ICS and INQUA agreed to erect Quaternary as an Era, above Neogene, and to place the base for Quaternary at 2.588 ± 3.005, the base for Gelasian Stage. However IUGS decided that Quaternary could not start within the Pliocene Epoch thereby splitting it in two, so the decision is still awaiting settlement.

Overview
The term Quaternary ("fourth") was proposed by Giovanni Arduino in 1759 for alluvial deposits in the Po river valley in northern Italy. It was introduced by Jules Desnoyers in 1829 for sediments of France's Seine Basin that seemed clearly to be younger than Tertiary Period rocks. The Quaternary Period follows the Tertiary Period and extends to the present. The Quaternary covers the time span of glaciations classified as the Pleistocene, and includes the present interglacial period, the Holocene. The alternative usage places the start of the Quaternary at the onset of Northern Hemisphere glaciation approximately 2.6 million years ago and includes portions of what has been classified as the upper Pliocene. This definition is that favoured by the vast majority of Quaternary scientists. However, some people object to the term Quaternary, instead considering it an informal term for time included in the Neogene Period. This latter definition was included in the 2003 edition of the International Stratigraphic Chart, published by the International Commission on Stratigraphy. See discussion of this topic on the International Subcommission of Quaternary Stratigraphy website.

The 1.8–1.6 million years of the Quaternary represents the time during which recognizable humans existed. Over this short a time period, the total amount of continental drift was less than 100 km, which is largely irrelevant to palaeontology. Nonetheless, the geological record is preserved in greater detail than that for earlier periods, and is most relatable to the maps of today, revealing in the second half of the twentieth century its own series of extraordinary landform changes. The major geographical changes during this time period included emergence of the Strait of Bosphorus and Skagerrak during glacial epochs, which respectively turned the Black Sea and Baltic Sea into fresh water, followed by their flooding (and return to salt water) by rising sea level; the periodic filling of the English Channel, forming a land bridge between Britain and the European mainland; the periodic closing of the Bering Strait, forming the land bridge between Asia and North America; and the periodic flash flooding of Scablands of the American Northwest by glacial water. The Great Lakes and other major lakes of Canada, and Hudson Bay, are also just the results of the last cycle, and are temporary. Following every other ice age within the Quaternary, there was a different pattern of lakes and bays.

The climate was one of periodic glaciations with continental glaciers moving as far from the poles as 40 degrees latitude. Few major new animals evolved, again presumably because of the short&mdash;in geologic terms&mdash;duration of the period. There was a major extinction of large mammals in Northern areas at the end of the Pleistocene Epoch. Many forms such as saber-toothed cats, mammoths, mastodons, glyptodonts, etc., became extinct worldwide. Others, including horses, camels and cheetahs became extinct in North America.

Quaternary glaciation
Glaciation took place repeatedly during the Quaternary the Ice Age – a term coined by Schimper in 1839 that began with the start of the Quaternary about 2.58 Ma and continues to the present-day.

Last glaciation
In 1821, a Swiss engineer, Ignaz Venetz, presented an article in which he suggested the presence of traces of the passage of a glacier at a considerable distance from the Alps. This idea was initially disputed by another Swiss scientist, Louis Agassiz when he undertook to disprove it, he ended up affirming his colleague's hypothesis. A year later, Agassiz raised the hypothesis of a great glacial period that would have had long-reaching general effects. This idea gained him international fame and led to the establishment of the Glacial Theory.

In time, thanks to the refinement of geology, it has been demonstrated that there were several periods of forward and backward movement of the glaciers and that past temperatures on Earth were very different from today. In particular, the Milankovitch cycles of Milutin Milankovitch are based on the premise that variations in incoming solar radiation are a fundamental factor controlling Earth's climate.

During this time, substantial glaciers advanced and retreated over much of North America and Europe, parts of South America and Asia, and all of Antarctica. The Great Lakes formed and giant mammals flourished in parts of North America and Eurasia not covered in ice. These mammals became extinct when the last Ice Age ended about 11,700 years ago. Modern humans evolved about 190,000 years ago (source: Leakey). During the Quaternary period, mammals, flowering plants, and insects dominated the land.