Life - a characteristic that distinguishes physical entities that do have biological processes, such as signaling and self-sustaining processes, from those that do not, either because such functions have ceased, or because they never had such functions and are classified as inanimate.

The definition of life is controversial, can at times be ambiguous, and may or may not define viruses, viroids, or potential synthetic life as "living". The current definition is that organisms are open systems that maintain homeostasis, are composed of cells, have a life cycle, undergo metabolism, can grow, adapt to their environment, respond to stimuli, reproduce and evolve. Several other biological definitions have been proposed, and there are some borderline cases of life, such as viruses or viroids.

The "Beginning" of Life on Earth

LUCA   →

• Prokaryotes
  • Bacteria domain
  • Archaea domain
• Eukaryota domain

A rough timeline of much of the Earth's Living Lineage

Domain: Eukaryota - The cells of Eukaryotes have a nucleus enclosed within membranes, unlike Prokaryotes (Bacteria and other Archaea). Eukaryotic cells also contain other membrane-bound organelles such as mitochondria and the Golgi apparatus, and in addition, some cells of plants and algae contain chloroplasts. Unlike unicellular archaea and bacteria, eukaryotes may also be multicellular and include organisms consisting of many cell types forming different kinds of tissue.

Kingdom: Animalia - are multicellular eukaryotic organisms. With few exceptions, animals consume organic material, breathe oxygen, are able to move, reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development.

Phylum: Chordata - chordates possess a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail, for at least some period of their life cycle.

Class: Mammalia - a clade of endothermic amniotes distinguished from reptiles (including birds) by the possession of a neocortex (a region of the brain), hair, three middle ear bones, and mammary glands. Females of all mammal species nurse their young with milk, secreted from the mammary glands.

Order: Primates - include two distinct lineages, strepsirrhines and haplorhines. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging environment. Most primate species remain at least partly arboreal.

Suborder: Haplorhini - the "dry-nosed" primates is a suborder of primates containing the tarsiers and the simians (Simiiformes or anthropoids)

Infraorder: Simiformes - The simians are monkeys and apes, cladistically including: the New World monkeys or platyrrhines, and the catarrhine clade consisting of the Old World monkeys and apes (including humans).

Family: Hominidae - known as great apes or hominids, are a taxonomic family of primates that includes eight extant species in four genera: Pongo, the Bornean, Sumatran and Tapanuli orangutan; Gorilla, the eastern and western gorilla; Pan, the common chimpanzee and the bonobo; and Homo, which includes modern humans and its extinct relatives (e.g., the Neanderthal), and ancestors, such as Homo erectus.

Subfamily: Homininae - a subfamily of Hominidae. It includes two tribes, with their extant as well as extinct species: the Hominini tribe (with the genus Homo including modern humans, Australopithecina, comprising at least three extinct (or, fossil) genera and the genus Pan including chimpanzees and bonobos), and the Gorillini tribe (gorillas). It comprises all hominids that arose after orangutans (subfamily Ponginae) split from the line of great apes.

Tribe: Hominini - form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes genus Homo (humans), but excludes genus Gorilla (gorillas). There is at present (as of 2018) no consensus on whether it should include genus Pan (the chimpanzees), the question being closely tied to the complex speciation process connecting humans and chimpanzees and the development of bipedalism in proto-humans.

Genus: Homo - The genus that encompasses the extant species Homo sapiens (modern humans), plus several extinct species classified as ancestral to or closely related to modern humans, most notably Homo erectus. The genus is taken to emerge with the appearance of Homo habilis, just more than two million years ago. Homo is derived from the genus Australopithecus, which itself had previously split from the lineage of Pan, the chimpanzees. Taxonomically, Homo is the only genus assigned to the subtribe Hominina which, with the subtribes Australopithecina and Panina, comprise the tribe Hominini.

Species: H. sapiens - Humans (taxonomically Homo sapiens) are the only extant members of the subtribe Hominina. The Hominina are sister of the Chimpanzees with which they form the Hominini belonging to the family of great apes. They are characterized by erect posture and bipedal locomotion; high manual dexterity and heavy tool use compared to other animals; open-ended and complex language use compared to other animal communications; and a general trend toward larger, more complex brains and societies.

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