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arbovirus Summary
aphasia Summary
livestock
livestock Summary
endoplasmic reticulum
endoplasmic reticulum Summary
attitude Summary
hydrologic cycle
water cycle Summary
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life summary

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Learn about the origin and evolution of life

Written and fact-checked by
The Editors of Encyclopaedia Britannica
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.
The Editors of Encyclopaedia Britannica
Below is the article summary. For the full article, see life.
Streptococcus pyogenes
Streptococcus pyogenesScanning electron micrograph of Streptococcus pyogenes, associated with strep throat and scarlet fever.

life, Matter characterized by the ability to metabolize nutrients (process materials for energy and tissue building), grow, reproduce, and respond and adapt to environmental stimuli. The oldest fossil evidence of Earth’s first living organisms, bacteria and cyanobacteria, dates to about 3.5–3.7 billion years ago. All known life-forms possess either DNA or RNA. Viruses, which possess DNA and RNA, cannot reproduce without a host cell and do not metabolize nutrients, and it is uncertain whether they should be classified as living or nonliving. Scientists disagree on the likelihood of extraterrestrial life. See also Drake equation.

arbovirus Summary
Arbovirus, acronym derived from arthropod-borne virus, any of a group of RNA viruses that develop in arthropods (chiefly blood-sucking mosquitoes and ticks), in which they cause no apparent harm, and are subsequently transmitted by bites to vertebrate hosts, in which they establish infections and
aphasia Summary
Aphasia, defect in the expression and comprehension of language caused by damage to the temporal and the frontal lobes of the brain. Aphasia can be caused by a head injury, a tumour, a stroke, or an infection. Symptoms vary with the location and extent of the brain tissues involved. Damage to the
livestock
livestock Summary
Livestock, farm animals, with the exception of poultry. In Western countries the category encompasses primarily cattle, sheep, pigs, goats, horses, donkeys, and mules; other animals, such as buffalo, oxen, llamas, or camels, may predominate in the agriculture of other areas. By the 21st century,
endoplasmic reticulum
endoplasmic reticulum Summary
Endoplasmic reticulum (ER), in biology, a continuous membrane system that forms a series of flattened sacs within the cytoplasm of eukaryotic cells and serves multiple functions, being important particularly in the synthesis, folding, modification, and transport of proteins . All eukaryotic cells