Savannehypothese: verschil tussen versies

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Niet iedereen ging mee in het afschrijven van de savannehypothese. Een gebrekkige definitie van wat een savanne is, speelde daarbij een rol. Critici van de hypothese zagen in veel gevallen de savanne als open graslanden met slecht sporadische boomgroei. Savannes kunnen echter een hoge boomdichtheid hebben en ook vochtig zijn. De huidige Afrikaanse savannes bevatten vooral bomen van de soort ''[[Brachystegia]]'', ''[[Isoberlinia]]'' en ''[[Julbernardia]]''. Het grote onderscheid tussen savannes en bossen is dan ook het ontbreken van grassen in de laatste. [[Thure E. Cerling]] ontwikkelde een methode om de bosachtige bedekking te bepalen van oude landschappen, waarmee een definitie van wat een savanne is niet meer nodig is.<ref>[...] ''an imprecise and often overly simplistic application of the definition of savannahs hinders progress in the debate over the timing and nature of their role in human evolution. To move past this persistent problem, we develop a relationship between the modern carbon isotope ratio in soils and the amount of woody cover in tropical environments and show that this can be used as a calibration for estimating woody cover of past environments. By using this relationship we can focus on the degree to which habitats were wooded, thereby circumventing any need to apply a functional definition of savannah to past environments where only structure can be inferred.'' {{aut|Thure E. Cerling|Cerling, T.E.}}; {{aut|Jonathan G. Wynn|Wynn, J.G.}}; {{aut|Samuel Andanje|Andanje, S.A.}}; {{aut|Michael I. Bird|Bird, M.I.}}; {{aut|David Kimutai Korir|Korir, D.K.}}; {{aut|Naomi E. Levin|Levin, N.E.}}; {{aut|William Mace|Mace, W.}}; {{aut|Anthony Macharia|Macharia, A.N.}}; {{aut|Jay Quade|Quade, J.}}; {{aut|Christopher Remien|Remien, C.H.}} (2011): [https://www.researchgate.net/profile/Jonathan_Wynn/publication/51545105_Woody_Cover_and_Hominin_Environments_in_the_Past_6_Million_Years/links/02e7e51fff8776647a000000/Woody-Cover-and-Hominin-Environments-in-the-Past-6-Million-Years.pdf?origin=publication_detail 'Woody cover and hominin environments in the past 6 million years'] in ''Nature'', Volume 476, p. 51-56</ref>
 
Door onderscheid te maken tussen de [[C3-plant|C<sub>3</sub>-planten]] van de tropische bossen en de mix van bomen en [[C4-plant|C<sub>4</sub>-grassen]] van de savanne, onderzochten zij de stabiele [[Isotopen van koolstof|koolstof-isotoop]] van [[paleosol]]len van enkele sites in Oost-Afrika. Zo beschreven zij landschappen variërend van [[Bos (vegetatie)|bos]], open bos/ [[struweel]], beboste graslanden tot graslanden. Zij kwamen tot de conclusie dat de vroegere hominini in een opener omgeving leefden dan ''Australopithecus'', waarmee de savannehypothese een plausibele mogelijkheid blijft.<ref>''Our observations of the environment of some of the earliest hominins do not contradict the longstanding hypothesis that savannahs in Africa may have had a role in the development of bipedal locomotion, or other key defining characteristics of hominins post-dating the LCA. If either species of ''Ardipithecus'' (''Ar. ramidus'' or ''Ar. kadabba'') is validated as the ‘‘long-sought potential root species for the Hominidae’’<sup>42</sup> then the soil carbonate data now make it clear that both species were surrounded by more open environments than ''Australopithecus'', which was more efficiently bipedal and occurred in more wooded environments of both the Omo-Turkana Basin and the Awash Valley (Fig. 6). Thus, the combined results from two of the most significant hominid-bearing regions in eastern Africa leave the savannah hypothesis as a viable scenario for explaining the context of earliest bipedalism, as well as potentially later evolutionary innovations within the hominin clade.'' {{aut|Thure E. Cerling|Cerling}} ''et al.'' (2011)</ref>
 
Aansluitend bij Cerling stelt [[Manuel Domínguez-Rodrigo]] dat de gebruikelijke onderverdeling van landschappen in grassig, bosrijk en bebost weinig zinvol is, omdat dit niets zegt over de selectiedruk op zoogdieren. Zo is de selectiedruk van grasvelden in tropische bossen onvergelijkbaar met de graslanden van de savannes. Ook kennen tropische bossen vele verschillende soorten bomen, terwijl savannes slechts enkele soorten kennen, die ook nog eens nauwelijks fruit dragen.<ref>''A strict [[Fysionomie|physiognomic]] definition of a landscape (grassy, wooded, forested) is evolutionarily irrelevant, because it contains no meaning regarding the selective factors that it represents. For example, rain forests also contain patches (sometimes quite extensive) of grasslands, and the natural selection factors in them are utterly unrelated to those of open grasslands in savannas.''<br />[...] ''those factors were not dependent on the degree of openness or closeness of the landscape but on the presence of specific plant and animal resources and their ecological interaction.''<br />[...] ''Savannas (and more specifically, savanna woodlands) are very different from tropical forests in terms of productivity'' [...] ''due to different taxonomic distributions of plants and animals. Tropical rainforests may contain hundreds of tree species per square hectare'' [...]'', whereas in savannas, tree diversity per square hectare may be as low as one or two species, and they produce fewer fruits'' [...] ''Selective pressures are, subsequently, also radically different,thus conditioning the adaptation of primates to them. This explains why it is critical to differentiate between savannas and other biome types when trying to understand early human evolution. The relevant question here is this: were ''A. ramidus'' and the other early hominins living in a wooded savanna (and, therefore, a savanna biome) or in a denser woodland or forest?'' {{aut|Manuel Domínguez-Rodrigo|Domínguez-Rodrigo, M.}} (2014): [https://www.researchgate.net/profile/Doris_Barboni/publication/260052724_Comment_on_a_Forum_paper_on_Theory_in_Anthropology_entitled_Is_the_Savanna_Hypothesis_a_dead_concept_for_explaining_the_emergence_of_the_earliest_hominins_by_M_Dominguez-Rodrigo/links/568bbc9608aebccc4e1c080d/Comment-on-a-Forum-paper-on-Theory-in-Anthropology-entitled-Is-the-Savanna-Hypothesis-a-dead-concept-for-explaining-the-emergence-of-the-earliest-hominins-by-M-Dominguez-Rodrigo.pdf 'Is the “Savanna Hypothesis” a Dead Concept for Explaining the Emergence of the Earliest Hominins?'] in ''Current Anthropology'', Volume 55, Number 1, p. 59-81</ref> Een andere factor is die van schaal. Paleontologen onderzoeken veelal alleen de site zelf, een gebied van enkele honderden tot duizenden meters. Deze [[habitat]]s worden wel aangeduid als [[Bioom|biomen]], maar deze laatsten omvatten vele honderden kilometers.<ref>''One problem in defining the ecosystemic context of early hominins is that of scale. Paleontologists often use terms that refer to biomes or ecosystems to describe palaeoenvironments that derive from highly localized data: those windows that exhibit fossil remains and that should more appropriately be initially identified as habitats or geons'' [...] ''instead of ecosystems and later placed into a broader ecosystemic context'' [...] ''The latter may span hundreds of kilometers, whereas paleontological localities usually span a few hundred meters. In the case of A. ramidus, the paleolandscape targeted in the Middle Awash area, because of exposure, has a length of 9 km.'' {{aut|Manuel Domínguez-Rodrigo|Domínguez-Rodrigo}} (2014)</ref> Ook Domínguez-Rodrigo stelt dat de savannehypothese nog steeds een goede verklaring kan geven, al is de overgang van omgeving waarschijnlijk minder abrut geweest dan enkele vroegere auteurs dachten.<ref>''The philosophy of the original “savanna hypothesis” posited that human evolution (as initially identified by the emergence of bipedism) was triggered by a change in the environment, involving increasing openness of the landscape and decreasing feeding resources, forcing hominins to move longer distances across more diverse biotopes, with increasing predatory risks. Initially conceived as a sharp change from forest to grassland, the mounting evidence derived from paleontological sites, as we have seen above, has shown that Pliocene savanna environments were more humid than modern savannas and more wooded, thus rendering the transition from forest to savanna more gradual than previously thought. However, this transition is not trivial. If it were, apes would be equally populating the semideciduous, more open forests peripheral to the rainforests where they usually live or other forests (e.g., the so-called [[West-Afrikaanse chimpansee|savanna chimpanzees]]). Wooded environments in the form of woodlands should not be mistaken with forests; natural selection for primates is different in both environments.''<br />[...] ''None of the interpretations derived from the Middle Awash ''Ardipithecus'' data refutes the “savanna hypothesis,” unless this refers to its popular simple “grassland” version. Adaptation to a dry deciduous woodland such as the one described would require adaptive skills different from adaptation to a forest. In essence, it would require dealing with less diverse and more spaced seasonally available feeding resources and move across more diverse habitats in a mosaic environment with a higher predatory pressure. This is the core of the old “savanna hypothesis.”'' {{aut|Manuel Domínguez-Rodrigo|Domínguez-Rodrigo}} (2014)</ref>
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