Savannah hypothesis

The Savannah hypothesis (or Savanna hypothesis) is a formerly widely held hypothesis that human bipedalism evolved as a direct result of human ancestors transition from an arboreal lifestyle to one on the savannas. It used to be a prominent hypothesis among Paleoanthropologists, but has been falling out of favor since the 1990s. According to this hypothesis, between 2 and 5 million years ago hominins left the woodlands that had previously been their natural habitat since about 5-6 million years ago, and adapted to their new habitat by learning first to stand, and then to walk upright.

Overview

The hypothesis was closely related to the knuckle-walking hypothesis; the claim that early human ancestors walked on all fours when they first emerged into the savannas. This was based on observations of morphological characteristics found in Australopithecus anamensis and Australopithecus afarensis, and posited that knuckle-walking was an example of convergent evolution in chimpanzees and gorillas, which was then lost by the genus Homo.[1] Paleoanthropologists also posited that the upright posture would have been advantageous to savanna-dwelling hominids, as it allowed them to peer over tall grasses for predators, or in search of prey.[2] P. E. Wheeler suggested that another advantage lay in reducing the amount of skin exposed to the sun, which helped regulate body temperatures.[3]

The turnover-pulse hypothesis, first described by Elizabeth Vrba seemed to support the savanna hypothesis by suggesting that climate change events resulting in the shrinking of forested areas forced animals out into the open grasslands.[4]

History

Though the fundamental ideas behind it date back to Darwin,[4] the hypothesis first came to prominence with the discovery of Australopithecus africanus by Raymond Dart in 1924. In an article on the discovery, published in the journal Nature, Dart wrote:

"For the production of man a different apprenticeship was needed to sharpen the wits and quicken the higher manifestations of intellect - a more open veldt country where competition was keener between swiftness and stealth, and where adroitness of thinking and movement played a preponderating role in the preservation of the species. Darwin has said, "no country in the world abounds in a greater degree with dangerous beasts than Southern Africa." and, in my opinion, Southern Africa, by providing a vast open country with occasional wooded belts and a relative scarcity of water, together with a fierce and bitter mammalian competition. furnished a laboratory such as was essential to this penultimate phase of human evolution."

— Raymond Dart, Australopithecus africanus: The Man-Ape of South Africa [5]

In the decades following Dart's discovery, more hominid fossils were found in Eastern and Southern Africa, leading researches to conclude that these were savanna dwellers, as well. Much of the academic discussion at the time took for granted that the transition to the savannas was responsible for the emergence of bipedalism, and focused instead on determining particular mechanisms by which this happened.[4]

Decline

In the latter parts of the 20th century, new fossil evidence began to emerge which called the savanna hypothesis into question. These newly-discovered remains showed indications that they were still well adapted to climbing trees, even after they had begun to walk upright.[6] Both humans and chimpanzees tend to walk upright when moving along long branches of trees, increasing their reach.[7] Both Lucy, the famous Australopithecus afarensis and "Little Foot," the collection of Australopithecus africanus foot bones demonstrated features consistent with tree-climbing as well as an upright gait; the former through curved fingers, and the latter via a divergent big toe. Additionally, ancient pollen found in the soil near where these fossils were discovered suggest that the area used to be much more wet and covered in thick vegetation, only recently becoming the arid desert it is now.[4]

See also

References

  1. ↑ Richmond, Brian; Strait, David (23 March 2000). "Evidence that humans evolved from a knuckle-walking ancestor". Nature. 404: 382–385. Retrieved 26 September 2017.
  2. ↑ Falk, Dean (22 February 2000). Primate Diversity (2 ed.). New York: W. W. Norton & Company. ISBN 0393974286. Retrieved 26 September 2017.
  3. ↑ Wheeler, P. E. (1 January 1984). "The evolution of bipedality and loss of functional body hair in hominids". Journal of Human Evolution. 13 (1): 91–98. Retrieved 26 September 2017.
  4. 1 2 3 4 Shreeve, James (1 July 1996). "Sunset on the Savanna". Discover. 17 (7). Retrieved 26 September 2017.
  5. ↑ Dart, Raymond (7 February 1925). "Australopithecus africanus: The Man-Ape of South Africa" (PDF). Nature. 115 (2884): 195–199. Retrieved 26 September 2017.
  6. ↑ Green, Alemseged, David, Zeresenay (2017). "Australopithecus afarensis Scapular Ontogeny, Function, and the Role of Climbing in Human Evolution". Science. 338 (6106): 514–517.
  7. ↑ Thorpe, S. K.; Holder, R.L; Crompton, R. H. (2007). "Origin of human bipedalism as an adaptation for locomotion on flexible branches". Science. 316: 1328–31. doi:10.1126/science.1140799. PMID 17540902.
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