Savannah hypothesis

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The savannah hypothesis is the assumption that the evolution of hominini from ape-like tree dwellers to the upright representatives of the genus Homo started around 7 to 8 million years ago because the quadruped ancestors of the modern, still living in forests at that time People moved their habitat to open, treeless savannahs and gradually developed locomotion there on two legs.

The savannah hypothesis is now considered disproved. The current state of research is that the early hominini were able to walk upright as a pre- adaptation for hundreds of thousands of years before the savannah was settled.

Another perspective can be found in the aquatic monkey theory .

Main features

According to this savannah hypothesis (also known as the "free-range hypothesis"), the ancestors of today's humans left the declining tropical rainforests during a period of dry climates and - in terms of their habitat comparable to today's steppe baboons - switched to life on the ground. In this way, the characteristic features of humans would ultimately have resulted:

  • Upright gait to have a better view of the grassland, similar to an ostrich . In addition, this has freed the hands to carry.
  • Great enlargement of the brain as a later adaptation to a hunting lifestyle.
  • Loss of hair in order to be able to dissipate heat better through sweating .

history

Charles Darwin already derived from the comparison of the African great apes with humans - long before the discovery of the African Australopithecus fossils - an origin of humans in Africa. The early ancestors of humans have become less arboreal ("less arboreal") due to a change in lifestyle or the environment. This led to two-legged locomotion, which made it easier to use the hands to transport objects and to use tools.

In 1925, Raymond Dart described the " child of Taung ", the oldest specimen of a human ancestor to date. Since the finding of this fossil belonging to Australopithecus africanus had been unforested for millions of years according to the doctrine of the time, the savannah hypothesis arose from this assessment. While the forest as a habitat, according to Dart, made it easy to acquire food, the open habitat required intelligence and skill.

During the following decades, the savannah hypothesis spread particularly in popular scientific literature. A critical scientific analysis was not carried out, but there was still broad consensus on this model in anthropology.

objection

The savannah hypothesis was based primarily on the fact that the finds of early species of hominini were mainly made in savannah areas and that the existence of savannas was also assumed as a landscape form during the lifetime of the fossils. However, this turned out to be a misjudgment. In addition, no analogous adaptations were found in other ape species that inhabit a similar habitat: All ape species in the savannahs and other open terrains move on four legs (quadruped). This also applies to species, some of which are predatory. Mammals of the savannah are usually hairy; The only exceptions are rhinos and elephants , which, however, have a significantly higher body volume compared to the body surface. The savanna hypothesis is therefore now considered outdated.

From the 1970s , Elisabeth Vrba , who at that time still lived in South Africa and later taught at Yale University , examined the paleoclimate of South Africa in more detail for the first time - particularly on the basis of fossil fauna . She found that the climate in South Africa changed dramatically 2.5 to 2 million years ago. The global climate cooled, Africa became drier. Changes can also be recorded for Tanzania , Kenya and Ethiopia . The first signs of cooling and dehydration can already be seen 5 million years ago on the Miocene - Pliocene border. Vrba sees in the climate change 2.5 million years ago a possible selection pressure, which led to adaptive radiation (Vrba 1993: explosive radiation ). For the first time in the long history of the savannah hypothesis, falsifiability was possible.

Just one year later, in 1994, however, reported John D. Kingston, Andrew Hill and Bruno D. Marino (Yale and Harvard) using carbon - isotope -Untersuchungen after that it's in the Tugen Hills , Kenya, within the last 15, 5 million years ago there were no noticeable shifts between C3 plants and C4 plants . Since C3 plants are typical for forests, while grasslands are characterized by a higher proportion of C4 plants, there could not have been a dramatic change in habitat from forest to savannah. Accordingly, Kingston, Hill and Marino concluded: When the hominini developed in East Africa in the late Miocene, the ecological conditions were different from the savannah hypothesis.

More recent finds of early Australopithecus fossils raised further doubts about the savannah hypothesis, since accompanying finds of other vertebrate species consistently indicated a habitat that consisted primarily of sparse forests and gallery forests . Thus, the findings of were Australopithecus afarensis from Hadar in a heavily wooded context in the field of water supplies. Australopithecus bahrelghazali , which was discovered at the site KT 12 in Chad, lived in an alluvial forest -like biotope. At 4.4 million years old, the oldest of the significant finds, Ardipithecus ramidus , was found together with typical forest-dwelling slippery monkeys and colobus monkeys as well as pollen from a mixed forest biotope, and even for Sahelanthropus from the site TM 266 a wooded biotope was found between a sandy desert and a reconstructed large lake.

According to Friedemann Schrenk, a reconstruction of the climatic conditions in Africa shows the following course of the tribal history based on current knowledge :

  • The first great apes lived in the African rainforests 30 million years ago; some populations spread to Asia and Europe 15 million years ago.
  • Geological processes in connection with the formation of the African Rift led, starting around 10 million years ago, to climate changes, as a result of which the extensive rainforests were displaced by tree-lined savannas and bushland.
  • About 8 million years ago, "when the climatic conditions in the late Miocene worsened due to increasing drought, some great ape populations found themselves on the eastern periphery of the rainforest along the nutrient-rich riparian zones in the rain shadow of the developing African rift." At the edge of the tropical rainforest, the line of development of the hominini separated from that of the other great apes.
  • According to Friedemann Schrenk, the individuals leading to the hominini “experimented with locomotion on the ground”; the upright gait developed - “with a body structure suitable for hand-handing” - because “the path from tree to tree was obviously covered on the ground”. Such behavior can also be observed in other great apes (still living today): " Ardipithecus ramidus was obviously the most successful here."
  • The body construction known from Ardipithecus ramidus , whose accompanying finds suggested a varied landscape of forests, bushes, wetlands and savannah-like areas, could therefore have been the starting point for the development of the two-legged climbing locomotion of the Australopithecines. It was not until two million years later, when the savannah areas were actually settled, that the upright gait, which had already been developed in sparse forests, proved to be advantageous for overseeing large areas and for carrying loads.

"Delicate" and "robust" Australopithecus species

The ramification of the already upright representatives of the Australopithecus group to "graceful" and "robust" Australopithecus species is interpreted today as a consequence of climate change 2.5 to 2 million years ago in southern Africa and with the open ones that actually developed there at the time Savannah landscapes associated.

Individual evidence

  1. ^ Charles Darwin: The Descent of Man and Selection in Relation to Sex. 2nd edition from 1882, p. 51
  2. ↑ Tim D. White et al .: Macrovertebrate Paleontology and the Pliocene Habitat of Ardipithecus ramidus. Science, Volume 326, 2009, p. 67, doi : 10.1126 / science.1175822
  3. ↑ Elizabeth S. Vrba: The Pulse That Produced Us. Natural History, 5/93, pp. 47-51
  4. ↑ John D. Kingston, Andrew Hill, Bruno D. Marino: Isotopic Evidence for Neogene Hominid Paleoenvironments in the Kenya Rift Valley. Science Volume 264, No. 5161, 1994, pp. 955-959, doi : 10.1126 / science.264.5161.955
  5. ↑ Friedemann Schrenk : The early days of man. The way to Homo sapiens . CH Beck, 1997, pp. 30-32

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