User:Squeamish Ossifrage/sandbox/land

Definitions
Land is "the solid, dry surface of the Earth", either as a collective whole or in part. Land includes continents and islands. Continents are large, continuous masses of land, while islands are smaller and surrounded by water. The distinction between the two is one of convention: Australia is the smallest continent, and Greenland is the largest island.

Wetlands include a variety of habitats in which an excess of water allows the growth of hydrophytes or the development of hydric soil. However, it is difficult to precisely define wetlands, and no single definition is universally accepted. Some may be considered land, albeit land that is recurrently or permanently flooded. Other definitions of wetlands are very broad and include areas unlikely to be thought of as land. The Ramsar Convention, an international conservation treaty, includes marine waters up to 6 m deep in its definition. Some scientists consider wetlands to be ecotones, or transition areas, but not all can be described as transitional. Ecologists William J. Mitsch and Jim Gosselink describe wetlands as a separate from land and water, writing that they "combine attributes of both aquatic and terrestrial ecosystems but are neither".

The boundary where land meets a large body of water, such as an ocean, sea, or lake, is called the shoreline. Short-term processes, such as waves and tides, and longer-term geologic changes move and reshape the shoreline; shoreline mapping represents a complex engineering problem affected by the coastline paradox. The fringe of land alongside the shoreline is the shore. In physical oceanography, the shore is specifically the area of land altered by the actions of the adjacent water. The coast is the shore of the sea, although it may also refer to a broader area in some contexts.

Etymology
The origins of the word land can be traced through Old English (land or lond) to Proto-Germanic (*landom), in turn derived from the Proto-Indo-European *lendh-. This early form referred to open land or heath, with some scholars suggesting a more specific sense of low-lying or sunken land. Allan Bomhard has proposed an earlier reconstruction, *lamd-/*ləmd-, as part of the hypothetical Proto-Nostratic language.

Planetary geology
Earth is not unique in having land, nor does the presence of land imply the presence of liquid surface water. The surface area of dry terrestrial planets, such as Venus and Mars, has been called land, as has the surface of other rocky bodies such as the Moon.

Solid exoplanets may possess a wide range of conditions, from dry worlds that are entirely land, to Earth analogs partially covered by water, to ocean planets without any land. Under extreme circumstances, rocky planets such as COROT-7b may even be lava planets whose surface consists of an ocean of molten rock. Astronomical spectroscopy may allow for the detection and characterization of exoplanetary land.

Colonization of land
Although scientists differ regarding some aspects of the origin of life, it is generally agreed that living organisms emerged in some type of marine environment. Some prokaryotes evolved desiccation tolerance to survive in shallow-water environments that occasionally dried, and such microbes established the first communities of terrestrial life during the Mesoarchean or Neoarchean. Soil formation by these microbial mats would have been slow, and the terrestrial environment remained harsh.

It is not clear when eukaryotes first colonized land. Fossils of fungi are known from the Ordovician, but molecular clock estimates suggest eukaryotes were present in terrestrial environments in the late Precambrian or earlier. These probably took the form of symbiosis relationships between a fungus and a cyanobacterium, possibly similar to modern lichens or arbuscular mycorrhizae.

The first land animals were likely small arthropod detritivores in the Neoproterozoic. The oldest known fossil of a terrestrial animal is that of Pneumodesmus newmani, a millipede from the Late Silurian. Tetrapods, the group of four-limbed vertebrates that includes amphibians, reptiles, birds, and mammals, evolved during the Late Devonian.

Habitats
Several systems have been proposed to categorize terrestrial habitats or ecosystems. No single scheme is universally accepted. One widely-used approach to habitat classification was created by the World Wildlife Fund, based on earlier work by Miklos Udvardy. It divides the world into eight large biogeographic divisions called ecozones. Also identified are fourteen major terrestrial habitat types, called biomes. These groupings are in turn divided into ecoregions. Originally, 867 terrestrial ecoregions were defined; 15 additional Antarctic ecoregions were proposed in 2012. Other approaches identify "large areas with largely homogenous plant species composition", called phytochoria, or zoogeographical regions defined in a similar manner based on animal populations.

Tropical rainforests, a type of tropical moist broadleaf forest, occupy only about 6 percent of the Earth's land area, but are exceptionally biodiverse, containing between half and two-thirds of the world's terrestrial species. The Amazon rainforest alone may hold between 10 and 25 percent of terrestrial species, and its plants perform 15 percent of global photosynthesis occurring on land.

Terrestrial plants
Some algae exist in terrestrial environments as part of the soil biota or as components of lichens, but their contribution to the ecology of terrestrial plants is small. Botanist Karl J. Niklas defines "bona fide" land plants as "any photosynthetic eukaryote that can survive and sexually reproduce on land," a set of conditions that exclusively describes terrestrial embryophytes. This subkingdom includes all modern terrestrial plants: hornworts, liverworts, mosses, ferns, lycopods, gymnosperms, and angiosperms (flowering plants). Assemblages of plant species and the ground cover they provide are known as vegetation.

Plants play an important role in the nitrogen cycle, through nitrogen assimilation, and in the terrestrial biological carbon cycle, as primary producers through photosynthesis. Estimates of the total biomass of terrestrial plants, measured in terms of their total organic carbon, vary between 500 and(-), about 90% of which is found in the Earth's forests. The annual net primary production of this biomass is approximately 56 Pg; the rate at which ecosystems and vegetation types contribute varies widely.

Evenly distributed, the dry mass of the world's terrestrial plants would cover the Earth's land areas with a layer 1 cm thick.

Terrestrial animals
Terrestrial animals are those adapted to life on land. Living in air instead of water allows for easier and more efficient respiration, but requires mechanisms to cope with the dehydrating environment. There is no clear demarcation between aquatic and terrestrial animals. A gradation of environments exists between water and land, and animal species similarly vary in their degree of independence from aquatic life. Some small animals, although living in soil or in association with moss such as Sphagnum, depend on a thin film of water to function normally. Biologist Johanna Laybourn-Parry describes them as "essentially aquatic organisms living in a hazardous environment". Such animals include a variety of protozoa, nematodes, rotifers, and tardigrades; many of them undergo encystment or cryptobiosis to survive dry conditions. By excluding these groups, as well as internal parasites, Colin Little identifies seven phyla with terrestrial representatives.

Two vermiform phyla possess only a few terrestrial species. Platyhelminthes, the phylum of flatworms, contains many parasites, including the flukes and tapeworms, as well as non-parasitic aquatic planarians. However, one family, the Geoplanidae, is terrestrial. Land planarians as a group have a cosmopolitan distribution, although most of the over 800 known species are native to the Southern Hemisphere; many tropical species are brightly colored. The ribbon worms of phylym Nemertea are mostly marine organisms, although about a dozen fresh water species are known. There are also fifteen known terrestrial or semi-terrestrial species. Native to diverse oceanic islands, they are believed to have directly evolved to colonize land from the sea.

Annelida

Mollusca

Arthropoda

Onychophora

Chordata

Exploration and cartography
Exploration is the process of travel to new areas for information or access to resources. The motivation for exploration may be colonization, conflict, the economic value of new resources or trade routes, or the scientific desire for geographical knowledge.

Human exploration began before the evolution of anatomically modern humans. Homo erectus originated in East Africa and expanded beyond the continent in at least three waves. The earliest confirmed hominin site outside of Africa is at Dmanisi, in modern-day Georgia, dating back to. Modern humans began to spread beyond Africa perhaps as early as 125 ka, spreading along the coast into Asia, inland into Europe, and across the Bering land bridge into the Americas. From this early travel, multiple civilizations developed, largely isolated and ignorant of each other.

Exploration for the purpose of long-distance trade in luxuries such as ochre probably began before the widespread advent of sedentism, but the details are difficult to determine from surviving physical evidence. The development of cartography allowed the knowledge gained from exploration to be recorded in permanent form. It is unclear when cartography developed, and the oldest extant map cannot be identified with certainty. Purported maps of hunting grounds, from a cave in Navarre, and of the community at Çatalhöyük, remain controversial. More records exist for Ancient Egyptian exploration, both southward along the Nile and in the Eastern Desert. The Turin Papyrus Map of Wadi Hammamat, drawn about 1160 BCE, is the oldest known topographic map and geologic map. The Babylonian Imago Mundi is the earliest survivng world map, dating from the 7th or 6th century BCE. Like most ancient world maps, it depicted the country of origin and nearby regions, surrounded by ocean.

During classical antiquity, explorers frequently mapped coastlines by sea, such as the travels of Hanno the Navigator and Pytheas, although Herodotus described the existence of an overland trade route from Libya, southwest across the Sahara Desert. Scylax of Caryanda was the first person from the Western world to navigate the Indus River and describe the interior of India; his descriptions likely influenced Alexander the Great's 4th century BCE expansion eastward. In 139 BCE, China's Han Dynasty also engaged in overland exploration of Asia; Zhang Qian set out to the west and ultimately visited remnants of Alexander's conquests, the Greco-Bactrian lands of Dayuan and Daxia, and described the travels of others to the Indus Valley and the Parthian Empire. This travel led to the establishment of the Silk Road and subsequent exploration of central Asia. Ptolemy's Geographia, written about 150 CE, compiled known geographical information and established new principles for cartography, including the representation of unknown areas—terra incognita—such as undiscovered continents believed to exist antipodal to Europe and West Asia.

During the Pax Mongolica of the 13th and 14th centuries, trade along Silk Road routes flourished, and Europeans were able to travel to China. The best known of these was Venetian merchant Marco Polo, whose detailed chronicle was widely read. This book, along with John Mandeville's fictitious travelogue, provided inspiration for the later expeditions of the European Age of Discovery. Prince Henry the Navigator directed a period of Portuguese exploration in the 15th century, including the discovery (or rediscovery) of Madeira in 1419 by João Gonçalves Zarco and of the Azores about 1427, as well as a slowly-advancing knowledge of the African coast. By 1488, Bartolomeu Dias passed the southern tip of Africa, and Pêro da Covilhã reached Ethiopia by overland travel, demonstrating that the Indian Ocean was not land-locked, as had long been believed.