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Cell Theory

Cell Theory

Cell theory states that[3]:

  • The cell is the fundamental unit of life.
  • All living things are composed of one or more cells or the secreted products of those cells, such as shells.
  • Cells arise from other cells through cell division
  • In multicellular organisms, every cell in the organism's body is produced from a single cell in a fertilized egg.
  • The cell is considered to be the basic part of the pathological processes of an organism.

[edit] Evolution

A central organizing concept in biology is that life changes and develops through evolution and that all life-forms known have a common origin. Introduced into the scientific lexicon by Jean-Baptiste de Lamarck in 1809, Charles Darwin established evolution fifty years later as a viable theory by articulating its driving force: natural selection (Alfred Russel Wallace is recognized as the co-discoverer of this concept as he helped research and experiment with the concept of evolution). Darwin theorized that species and breeds developed through the processes of natural selection and artificial selection or selective breeding.[4] Genetic drift was embraced as an additional mechanism of evolutionary development in the modern synthesis of the theory.

The evolutionary history of the species— which describes the characteristics of the various species from which it descended— together with its genealogical relationship to every other species is called its phylogeny. Widely varied approaches to biology generate information about phylogeny. These include the comparisons of DNA sequences conducted within molecular biology or genomics, and comparisons of fossils or other records of ancient organisms in paleontology. Biologists organize and analyze evolutionary relationships through various methods, including phylogenetics, phenetics, and cladistics. For a summary of major events in the evolution of life as currently understood by biologists, see evolutionary timeline.

Up into the 19th century, spontaneous generation, the belief that life forms could appear spontaneously under certain conditions, was widely believed. This misconception was challenged by William Harvey's diction that "all life [is] from [an] egg" (from the Latin "Omne vivum ex ovo"), a foundational concept of modern biology. It means that there is an unbroken continuity of life from its initial origin to the present time.

A group of organisms share a common descent if they share a common ancestor. All organisms on the Earth both living and extinct have been or are descended from a common ancestor or an ancestral gene pool. This last universal common ancestor of all organisms is believed to have appeared about 3.5 billion years ago.[5] Biologists generally regard the universality of the genetic code as definitive evidence in favor of the theory of universal common descent for all bacteria, archaea, and eukaryotes (see: origin of life)[6].

Evolution does not always give rise to progressively more complex organisms. For example, the process of dysgenics has been observed among the human population.[7]

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