Biodiversity, a contraction of "biological diversity," generally refers to the variety and variability of
life on Earth. One of the most widely used definitions defines it in terms of the variability within species, between species, and between ecosystems.
 It is a measure of the variety of
organisms present in different
ecosystems. This can refer to
genetic variation, ecosystem variation, or
species variation (number of species)
 within an area,
planet. Terrestrial biodiversity tends to be greater near the
 which seems to be the result of the warm
climate and high
 Biodiversity is not distributed evenly on
Earth, and is richest in the tropics. These tropical forest ecosystems cover less than 10 per cent of earths surface, and contain about 90 percent of the worlds species.
Marine biodiversity tends to be highest along coasts in the Western
sea surface temperature is highest and in the mid-latitudinal band in all oceans. There are
latitudinal gradients in species diversity.
 Biodiversity generally tends to cluster in hotspots,
 and has been increasing through time,
 but will be likely to slow in the future.
The number and variety of plants, animals and other organisms that exist is known as biodiversity. It is an essential component of nature and it ensures the survival of human species by providing food, fuel, shelter, medicines and other resources to mankind. The richness of biodiversity depends on the climatic conditions and area of the region. All
species of plants taken together are known as flora and about 300,000 species of plants are known to date. All species of animals taken together are known as fauna which includes birds, mammals, fish, reptiles, insects, crustaceans, molluscs, etc.
environmental changes typically cause
 More than 99 percent of all species, amounting to over five billion species,
 that ever lived on Earth are estimated to be
 Estimates on the number of Earth's current
species range from 10 million to 14 million,
 of which about 1.2 million have been documented and over 86 percent have not yet been described.
 More recently, in May 2016, scientists reported that 1 trillion species are estimated to be on Earth currently with only one-thousandth of one percent described.
 The total amount of related
base pairs on Earth is estimated at 5.0 x 1037 and weighs 50 billion
 In comparison, the total
mass of the
biosphere has been estimated to be as much as 4
TtC (trillion tons of
 In July 2016, scientists reported identifying a set of 355
genes from the
Last Universal Common Ancestor (LUCA) of all
organisms living on Earth.
age of the Earth is about 4.54 billion years old.
 The earliest undisputed evidence of
life on Earth dates at least from 3.5 billion years ago,
 during the
Eoarchean Era after a geological
crust started to solidify following the earlier molten
Hadean Eon. There are
fossils found in 3.48 billion-year-old
sandstone discovered in
 Other early physical evidence of a
biogenic substance is
graphite in 3.7 billion-year-old
meta-sedimentary rocks discovered in
 More recently, in 2015, "remains of
biotic life" were found in 4.1 billion-year-old rocks in Western Australia.
 According to one of the researchers, "If life arose relatively quickly on Earth .. then it could be common in the
life began on Earth, five major
mass extinctions and several minor events have led to large and sudden drops in biodiversity. The
Phanerozoic eon (the last 540 million years) marked a rapid growth in biodiversity via the
Cambrian explosion—a period during which the majority of
phyla first appeared.
 The next 400 million years included repeated, massive
biodiversity losses classified as
mass extinction events. In the
rainforest collapse led to a great loss of
Permian–Triassic extinction event, 251 million years ago, was the worst; vertebrate recovery took 30 million years.
 The most recent, the
Cretaceous–Paleogene extinction event, occurred 65 million years ago and has often attracted more attention than others because it resulted in the extinction of the
The period since the emergence of
humans has displayed an ongoing biodiversity reduction and an accompanying loss of
genetic diversity. Named the
Holocene extinction, the reduction is caused primarily by
human impacts, particularly
 Conversely, biodiversity impacts
human health in a number of ways, both positively and negatively.
United Nations designated 2011–2020 as the
United Nations Decade on Biodiversity.