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Earth is home to millions of species, including the dominant terrestrial species, humans. The planet formed 4.54 billion years ago, and life appeared on its surface within a billion years.

Evolution of lifeEdit

Main article: Evolutionary history of life

At present, Earth provides the only example of an environment that can sustain the evolution of life.[1] Highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later the last common ancestor of all life existed.[2] The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed in a layer of ozone (a form of molecular oxygen [O3]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[3] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.[4]

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 Ma, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.[5]

Following the Cambrian explosion, about 535 Ma, there have been five mass extinctions.[6] The last extinction event was 65 Ma, when a meteorite collision probably triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews. Over the past 65 million years, mammalian life has diversified, and several million years ago, an African ape-like animal gained the ability to stand upright.[7] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,[8] affecting both the nature and quantity of other life forms.

The present pattern of ice ages began about 40 Ma and then intensified during the Pleistocene about 3 Ma. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last ice age ended 10,000 years ago.[9]


  1. ^ Purves, William Kirkwood; Sadava, David; Orians, Gordon H.; Heller, Craig (2001). Life, the Science of Biology: The Science of Biology. Macmillan. p. 455. ISBN 0716738732. 
  2. ^ Doolittle, W. Ford (February 2000). "Uprooting the tree of life". Scientific American 282 (6): 90–95. doi:10.1038/nature03582. 
  3. ^ Berkner, L. V.; Marshall, L. C. (1965). "On the Origin and Rise of Oxygen Concentration in the Earth's Atmosphere". Journal of Atmospheric Sciences 22 (3): 225–261. doi:10.1175/1520-0469(1965)022<0225:OTOARO>2.0.CO;2. Retrieved on 2007-03-05. 
  4. ^ Burton, Kathleen (2002-11-29). "Astrobiologists Find Evidence of Early Life on Land". NASA. Retrieved on 2007-03-05. 
  5. ^ Kirschvink, J. L. (1992). Schopf, J.W.; Klein, C. & Des Maris, D.. ed. Late Proterozoic low-latitude global glaciation: the Snowball Earth. The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge University Press. pp. 51–52. ISBN 0521366151. 
  6. ^ Raup, D. M.; Sepkoski, J. J. (1982). "Mass Extinctions in the Marine Fossil Record". Science 215 (4539): 1501–1503. doi:10.1126/science.215.4539.1501. PMID 17788674. Retrieved on 2007-03-05. 
  7. ^ Gould, Stephan J. (October 1994). "The Evolution of Life on Earth". Scientific American. Retrieved on 2007-03-05. 
  8. ^ Wilkinson, B. H.; McElroy, B. J. (2007). "The impact of humans on continental erosion and sedimentation". Bulletin of the Geological Society of America 119 (1–2): 140–156. doi:10.1130/B25899.1. Retrieved on 2007-04-22. 
  9. ^ Staff. "Paleoclimatology - The Study of Ancient Climates". Page Paleontology Science Center. Retrieved on 2007-03-02. 

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