Savannehypothese

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De savannehypothese is een hypothese die menselijke bipedie verklaart als gevolg van de overgang die vroege hominini maakten van een omgeving van tropisch bos naar savanne. Op die savanne moesten langere afstanden worden afgelegd om aan voedsel te komen en rechtop lopen is daarvoor efficiënter dan knokkellopen zoals andere homininae doen. In de eerste helft van de twintigste eeuw werd hierbij gedacht aan open grasland, maar sindsdien wordt er in toenemende mate rekening mee gehouden dat een savanne een rijke begroeiing heeft met bomen en bestaat uit een mozaïek van biomen.

Het idee dat een klimaatgedreven achteruitgang van tropische bossen de vroege hominini dwong tot bipedie is er al lange tijd, vaak impliciet. Sommige vroege auteurs zagen savannes als open graslanden, terwijl anderen een mozaïek van omgevingen zagen, van bosgebieden tot graslanden. Sinds de jaren 1990 krijgt de hypothese meer kritiek. De open-graslanden-versie wordt meestal afgewezen, terwijl de mozaïekversie nog steeds relatief breed gedragen wordt, hoewel de overgang van bos naar savanne waarschijnlijk geleidelijker is verlopen dan eerder werd gedacht.

Vroege hypotheses[bewerken]

De vroege hypothese was vooral gebaseerd op aannames, aangezien het bij gebrek aan fossielen ontbrak aan empirische onderbouwing. Dat het proces zich had afgespeeld in open grasland werd vooral zijdelings genoemd, terwijl de nadruk niet lag op waar, maar vooral hoe het zich had afgespeeld.

Charles Darwin wordt wel genoemd als een vroege proponent van deze hypothese, omdat hij een verandering van omstandigheden zag als drijfveer van de ontwikkeling van tweevoetigheid, aangezien daarmee de handen vrij kwamen om wapens en werktuigen te hanteren.[1] Darwin noemde de overgang naar van tropisch bos naar savanne echter niet als mechanisme. Lamarck had al voor Darwin een vergelijkbare vaststelling gedaan.[2]

Alfred Russel Wallace verwoordde in 1889 waarom de oorsprong van de mens in de open vlakte gezocht moest worden. Het zou zeer onwaarschijnlijk zijn dat tweevoetigheid ontwikkeld zou zijn in tropische bossen waar fruit het belangrijkste voedsel is en vooral verkregen moet worden door in bomen te klimmen. Afrika leek hem echter onwaarschijnlijk, omdat dit continent afgescheiden was geweest van Eurazië en waar de hogere zoogdieren zich pas na hereniging van de continenten zouden vestigen.[3]

Gustav Steinmann zag in 1908 klimaatverandering als aanleiding tot tweevoetigheid doordat bossen door verminderde regenval veranderde in savanne.[4] Dat een deel aapachtig bleef, verklaarde hij doordat een deel zich terug zou trekken in het kleiner wordende bos, terwijl zij die aan de randen woonden op de savanne tweebenigheid zouden ontwikkelen. Op die manieren zouden op verschillende tijdstippen meerdere menstypes geëvolueerd kunnen zijn.[5]

Darwin zag Afrika als waarschijnlijke plaats waar de evolutie van de mens begon.[6] Henry Fairfield Osborn kwam echter op vergelijkbare wijze tot de bossen en overstromingsvlakten van Zuid-Azië.[7] Op Java was in 1891 ook de tot dan toe oudste mensachtige gevonden, de Javamens. Naar Lamarck dacht Osborn dat het gebruik van handen de groei van de hersenen stimuleerde en naar Grafton Elliot Smith dat dit op zijn beurt bijdroeg aan tweevoetigheid.[8]

Max Hilzheimer stelde in 1921 zelfs dat het bos een remmend effect op de ontwikkeling heeft en dat bewoners van open landschappen bijna altijd geavanceerder zijn.[9]

Raymond Dart vond in 1924 in Zuid-Afrika het Taungkind, een Australopithecus africanus. De omgeving van Taung was niet bosrijk en Dart stelde dat een opener omgeving dan het bos waarin apen gedijen dan ook nodig was om tot de evolutie van de mens te komen en de sporadisch van bomen voorziene open vlaktes van Zuid-Afrika waren daarvoor de aangewezen plaats.[10] Robert Broom zag de vondst van het Taungkind ook als bevestiging dat een antropoïde aap de bossen had verruild voor de open vlaktes.[11]

Zo stelde ook Hans Weinert in 1932 dat mensapen zich alleen in geval van nood in open terrein begeven waar zij met hun relatief onhandige manier van lopen op de grond in het nadeel zijn ten opzichte van vijanden. De voorouders van de mens zouden de bomen dan ook niet verlaten hebben, maar de bomen verlieten hen.[12] Dat stelde ook Amadeus William Grabau in 1944.[13] Daarmee zou een einde zijn gekomen aan het zoete nietsdoen.[14]

Vanuit antropocentrische en eurocentrische ideeën kon de menselijke evolutie nog wel eens gepresenteerd worden als een onvermijdelijk proces, een unilineaire evolutie van vooruitgang. Zo zag Ellsworth Huntington met zijn ecologisch determinisme het klimaat als de uiteindelijk bepalende factor richting its present flowering in civilization. Ook bij hem was het verdwijnen van de bossen de grote aanjager tot tweevoetigheid.[15]

Dit idee werd breed gedragen, maar niet door iedereen geaccepteerd. Zo wees Franz Weidenreich dit widely spread belief af in zijn beschrijving uit 1939 van Sinanthropus, beter bekend als de Pekingmens. De voorouder van de Pekingmens zou volgens hem in het vroege Mioceen hebben geleefd en het was volgens Weidenreich onwaarschijnlijk dat deze de overgang van miljoenen jaren overleefd zou hebben in deze omgeving. De evolutie zou dan ook plaats hebben gevonden voordat deze op de grond ging leven. Volgens Weidenreich kwam het misverstand voort uit het idee dat er een enkele wieg van de mensheid was, terwijl hij een vroeg multiregionaal model voorstond.[16]

Ondanks deze kanttekening werd het verdwijnen van het bos veelal impliciet aangenomen voor de hypotheses die volgden. Of de aanzet tot tweevoetigheid nu lag in de mogelijkheid van het dragen van kinderen, voedsel of wapens, of dat het een waakzame rechtopstaande houding mogelijk maakte, het speelde zich af op de savanne. Bij het grote publiek werden deze ideeën bekend door schrijver Robert Ardrey die veel contact met Dart onderhield.

Ecologische benaderingen[bewerken]

George A. Bartholomew en Joseph Birdsell stelden in 1959 vast dat een leven op de grond niet noodzakelijk tot tweevoetigheid leidt, aangezien apen van de Oude Wereld die deze levenswijze volgden, dit deden op vier poten. Hun werk zette aan tot een meer ecologische benadering en de rol van bevolkingsdruk bij het onderzoek naar de evolutie van de Australopithecus.[17]

In 1961 werd op Burg Wartenstein het African Ecology and Human Evolution-symposium gehouden door het Wenner-Gren Foundation for Anthropological Research. De resultaten werden in 1963 gepubliceerd. John Talbot Robinson voerde de ecologische aanpak door en onderzocht de adaptieve radiatie van de Australopithecus, de soortvorming door adaptie aan verschillende ecologische niches. Robinson stelde dat tweevoetigheid juist vooraf ging aan de groei van de hersenen en zocht dan ook naar een andere oorzaak van die bipedie. Hij zocht daarbij naar de verschillen in selectiedruk tussen Australopithecus en Paranthropus en vond die in verschillen in dieet. Klimaatverandering zag hij daarbij als mogelijke verklaring, waarbij de habitat veranderde van bossen naar savanne.[18] Théodore Monod beschreef de rol in de menselijke evolutie van natte periodes waarin de Sahara een beboste savanne was.[19]

Clifford Jolly wees op het gevaar van cirkelredeneringen die uiteindelijk niet verklaren waarom de mensachtigen zich afsplitsten van de overige mensapen. Hij wees de theorie af dat het gebruik van wapens en gereedschappen, de jacht en het eten van vlees zouden hebben bijgedragen aan de verandering naar bipedie. In het open landschap zag hij de zaden van grassen en kruiden als het hoofdvoedsel en hier zou de evolutie van de mens zijn begonnen, in eerste instantie in een dambo-drasland. Jolly richtte zich daarbij vooral op aanwijzingen van fossiele gebitten. Tweevoetigheid zou dan ontstaan zijn vanuit een rechtopzittende positie.[20]

C. Owen Lovejoy wees in 1981 de stelling van Jolly af dat de eethouding de aanleiding tot tweevoetigheid zou zijn geweest. Het zou waarschijnlijker zijn dat de vroege hominini al tweevoetig was toen deze zich op de savanne waagde. Ook directere savannehypotheses wees hij af. Hij stelde dat de toegenomen seizoensinvloeden door klimaatverandering bijdroegen aan de monogamie van de vroege hominini, waarbij de man een belangrijke rol speelde in de zorg voor de kinderen. Anders dan bij andere dieren voldeed de mond niet om voedsel in voldoende hoeveelheden te dragen, zodat het vrijkomen van de handen een selectiedruk richting tweevoetigheid zou geven.[21] Na de vondst van Ardipithecus ramidus stelde Lovejoy opnieuw dat vroege homonini monogaam waren, onder meer op basis van de geringe seksuele dimorfie, de kleine hoektanden en een geringe spermacompetitie. Vrouwen zouden de voorkeur geven aan niet-agressieve mannen die voedsel meebrachten.[22] Dat Ardi monogaam zou zijn, was echter onder meer volgens Frans de Waal een voorbarige conclusie.[23]

Aangezien geografische soortvorming de enige vorm van soortvorming is bij hogere diersoorten, ging Adriaan Kortlandt op zoek naar welke barrière dit had kunnen zijn. Aangezien apen slechte zwemmers zijn en weinig voedsel en water met zich mee kunnen dragen, zocht hij naar waterbarrières en droge gebieden. Kortlandt zag de Grote Slenk met de Nijl en de Zambezi als een dubbele barrière, waarbij desiccatie van Oost-Afrika vervolgens de aanzet tot tweevoetigheid zou hebben gegeven.[24] Dit kwam overeen met de locatie van enkele belangrijke fossielen die tot dan toe gevonden waren, zoals in 1939 de Australopithecus afarensis in Laetoli door Ludwig Kohl-Larsen en de Paranthropus boisei in de Olduvaikloof in 1959 door Mary Leakey. Deze Rift Valley theory werd bekend als de East Side Story van Yves Coppens.[25]

Consensus in beweging[bewerken]

Tot de jaren 1990 bevestigden de fossielen grotendeels de savanne als leefgebied, mede door gebitten waarvan de tanden met een dikkere laag glazuur zijn bedekt, wat wijst op een dieet met meer vezel dan het fruit in tropische bossen. Sindsdien werden er fossielen van oudere hominini gevonden die de consensus in beweging brachten. Bij deze vroege hominini waren een aantal veranderingen richting de moderne mens te zien. Zo wijzen kleinere hoektanden op een verschuiving in de voeding. De positie en oriëntatie van het foramen magnum of achterhoofdsgat, de gang en houding met andere postcraniële elementen zoals het bekken en de onderste ledematen wijzen op een rechtopstaande houding en aanzet tot tweevoetigheid. Andere eigenschappen laten niet of nauwelijks veranderingen zien, zoals de voeten, de bovenste ledematen en de schedelcapaciteit.

In 1993 werden in Aramis in Ethiopië 4,4 miljoen jaar oude fossiele tanden gevonden door een groep onder leiding van Tim D. White die toegeschreven werden aan een nieuwe soort, Australopithecus ramidus, later Ardipithecus ramidus genoemd. De leeftijd was daarmee een half miljoen jaar ouder dan tot dan toe bekende A. afarensis en had een aapachtiger voorkomen.[26] In 2009 werd er na uitgebreid onderzoek in een serie van elf artikelen in Science meer gepubliceerd over Ardi. Hierin werd de conclusie getrokken dat Ar. ramidus meer beboste gebieden prefereerde in plaats van open grasland, waarmee de klimaatgedreven savannehypothese niet houdbaar zou zijn.[27][28][29]

Daarop kwam een jaar later de kritiek dat er in Aramis wel degelijk sprake was geweest van savanne.[30]

Voor Phillip Tobias droeg de vondst in 1994 van Little Foot er toe bij om de savannehypothese als achterhaald te betitelen. Tot die tijd was hij steeds uitgegaan van die hypothese.[31]

In 2000 vonden Brigitte Senut en Martin Pickford in Kenia de zo'n 6 miljoen jaar oude Orrorin tugenensis. Het skelet lijkt te wijzen op zowel tweevoetigheid als een goede klimvaardigheid. Dit laatste wijst op een bosachtige omgeving, net als de vondst van zwartwitte franjeapen. De vondst van impala's wijst dan weer meer richting een opener landschap.[32] Het bracht Senut later tot de conclusie dat de savannehypothese niet langer houdbaar was.[33] Als deze fossielen inderdaad vroege voorouders zijn van de moderne mens, dan is de leefomgeving van de latere Australopithecus minder relevant.

In 2001 werd in Tsjaad de 7 miljoen jaar oude Sahelanthropus tchadensis ontdekt. Gebaseerd op dierlijke vondsten in de buurt was dit een mozaïek van omgevingen met savannes, graslanden en galerijbossen langs meren, al was er meer onderzoek nodig om dit precies vast te stellen.[34] De in 1997 ontdekte en zo'n 5,6 miljoen jaar oude Ardipithecus kadabba werd gevonden in een vergelijkbaar terrein.[35]

Definitie van savanne[bewerken]

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.[36]

Door onderscheid te maken tussen de C3-planten van de tropische bossen en de mix van bomen en C4-grassen van de savanne, onderzochten zij de stabiele koolstof-isotoop van paleosollen van enkele sites in Oost-Afrika. Zo beschreven zij landschappen variërend van 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.[37]

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.[38] Een andere factor is die van schaal. Paleontologen onderzoeken veelal alleen de site zelf, een gebied van enkele honderden tot duizenden meters. Deze habitats worden wel aangeduid als biomen, maar deze laatsten omvatten vele honderden kilometers.[39] 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.[40]

Noten[bewerken]

  1. As soon as some ancient member in the great series of the Primates came, owing to a change in its manner of procuring subsistence, or to a change in the conditions of its native country, to live somewhat less on trees and more on the ground, its manner of progression would have been modified; and in this case it would have had to become either more strictly quadrupedal or bipedal. Baboons frequent hilly and rocky districts, and only from necessity climb up high trees; and they have acquired almost the gait of a dog. Man alone has become a biped; and we can, I think, partly see how he has come to assume his erect attitude, which forms one of the most conspicuous differences between him and his nearest allies. Man could not have attained his present dominant position in the world without the use of his hands, which are so admirably adapted to act in obedience to his will. Darwin, C.R. (1871): The Descent of Man, and Selection in Relation to Sex, John Murray
  2. Effectivement, si une race quelconque de quadrumanes, surtout la plus perfectionnée d’entre elles, perdoit, par la nécessité des circonstances, ou par quelqu’autre cause, l’habitude de grimper sur les arbres, et d’en empoigner les branches avec les pieds, comme avec les mains, pour s’y accrocher; et si les individus de cette race, pendant une suite de générations, étoient forcés de ne se servir de leurs pieds que pour marcher, et cessoient d’employer leurs mains comme des pieds; il n’est pas douteux, d’après les observations exposées dans le chapitre précédent, que ces quadrumanes ne fussent à la fin transformés en bimanes, et que les pouces de leurs pieds ne cessassent d’être écartés des doigts, ces pieds ne leur servant plus qu’à marcher.
    En outre, si les individus dont je parle, mus par le besoin de dominer, et de voir à la fois au loin et au large, s’efforçoient de se tenir debout, et en prenoient constamment l’habitude de génération en génération ; il n’est pas douteux encore que leurs pieds ne prissent insensiblement une conformation propre à les tenir dans une attitude redressée, que leurs jambes n’acquissent des mollets, et que ces animaux ne pussent alors marcher que péniblement sur les pieds et les mains à la fois. Lamarck, J.B. de (1809): Philosophie zoologique, ou Exposition des considérations relative à l'histoire naturelle des animaux, Dentu
  3. It has usually been considered that the ancestral form of man originated in the tropics, where vegetation is most abundant and the climate most equable. But there are some important objections to this view. The anthropoid apes, as well as most of the monkey tribe, are essentially arboreal in their structure, whereas the great distinctive character of man is his special adaptation to terrestrial locomotion. We can hardly suppose, therefore, that he originated in a forest region, where fruits to be obtained by climbing are the chief vegetable food. It is more probable that he began his existence on the open plains or high plateaux of the temperate or sub-tropical zone, where the seeds of indigenous cereals and numerous herbivora, rodents, and game-birds, with fishes and molluscs in the lakes, rivers, and seas supplied him with an abundance of varied food. In such a region he would develop skill as a hunter, trapper, or fisherman, and later as a herdsman and cultivator,—a succession of which we find indications in the palaeolithic and neolithic races of Europe.
    In seeking to determine the particular areas in which his earliest traces are likely to be found, we are restricted to some portion of the Eastern hemisphere, where alone the anthropoid apes exist, or have apparently ever existed.
    There is good reason to believe, also, that Africa must be excluded, because it is known to have been separated from the northern continent in early tertiary times, and to have acquired its existing fauna of the higher mammalia by a later union with that continent after the separation from it of Madagascar, an island which has preserved for us a sample, as it were, of the early African mammalian fauna, from which not only the anthropoid apes, but all the higher quadrumana are absent. There remains only the great Euro-Asiatic continent; and its enormous plateaux, extending from Persia right across Tibet and Siberia to Manchuria, afford an area, some part or other of which probably offered suitable conditions, in late Miocene or early Pliocene times, for the development of ancestral man. Wallace, A.R. (1889): Darwinism. An Exposition of the Theory of Natural Selection with Some of Its Applications, Macmillan and Co.
  4. Als Vorstufe der Menschen haben wir uns pithekoide Wesen zu denken, die durch Annahme des aufrechten Ganges eine allmähliche Umbildung zur Stufe des Menschen erfahren haben. Die Ursachen für das Verlassen der ursprünglich vierfüßigen Fortbewegung können wir am besten in klimatischen Vorgängen suchen. Denn wenn ein Waldgebiet, in dem pithekoide Wesen etwa von der Fortbewegungsart der heutigen Menschenaffen wohnen, durch allmähliche Abnahme der Niederschläge sich lichtet, in eine Savannengegend sich umwandelt oder gar zum Buschwald wird, so sind die pithekoiden Bewohner genötigt, sich diesen geänderten Verhältnissen anzubequemen, und wenn sie sich schon, ähnlich wie die heutigen Menschenaffen, gelegentlich aufrecht oder halbaufrecht bewegten, so bedeutet es auch keine erhebliche Änderung, nach und nach zum dauernd aufrechten Gange überzugehen. Alle weiteren Umbildungen, im besonderen die zunehmende Entwicklung der Sinne, die Ausgestaltung der Hände zu einem vielseitig verwendbaren Organ und die aus diesen beiden Änderungen resultierende Zunahme der geistigen Fähigkeiten und der Hirnmasse folgen naturgemäß aus diesem ersten, wichtigsten Schritte zur Menschwerdung. Steinmann, G. (1908): Die geologischen Grundlagen der Abstammungslehre, W. Engelmann
  5. Denken wir uns nun das Verbreitungsgebiet solcher Pithekoiden mit verschiedenen nahestehenden Arten besetzt, die wir systematisch vielleicht zu einer »Gattung« vereinigen würden, und sich in diesem Gebiet allmählich einen Klimawechsel in angegebenem Sinne vollziehen, so würden einige Arten, die etwa nahe bei oder in sehr feuchten Flußniederungen leben, sich in diese zurückziehen und dabei ihre bisherigen Gewohnheiten beibehalten, andere, die näher der Waldgrenze wohnen, würden dagegen zur zweibeinigen Gangart übergehen und so allmählich befähigt werden, sich in den neu entstehenden und in den schon vorhandenen Savannen- und Buschwaldgebieten auszubreiten. Die Trennung in epistatische Pithekoiden und progressive Urmenschen wäre damit vollzogen. Da aber die Vorgänge, die diese Spaltung verursacht haben, keineswegs ungewöhnlich sind, sondern sich in der gleichen oder in einer anderen Gegend in späteren Zeiten ganz ähnlich wiederholen können, so folgt daraus, daß aus einer zusammengesetzten Pithekoiden-Gattung oder auch aus mehreren Gattungen wiederholt unabhängig und zu sehr verschiedenen Zeiten Menschenarten entstanden sein können. Steinmann (1908)
  6. We are naturally led to enquire, where was the birthplace of man at that stage of descent when our progenitors diverged from the Catarrhine stock? The fact that they belonged to this stock clearly shews that they inhabited the Old World; but not Australia nor any oceanic island, as we may infer from the laws of geographical distribution. In each great region of the world the living mammals are closely related to the extinct species of the same region. It is therefore probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man's nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere. But it is useless to speculate on this subject; for two or three anthropomorphous apes, one the Dryopithecus (17. Dr. C. Forsyth Major, 'Sur les Singes fossiles trouves en Italie:' 'Soc. Ital. des Sc. Nat.' tom. xv. 1872.) of Lartet, nearly as large as a man, and closely allied to Hylobates, existed in Europe during the Miocene age; and since so remote a period the earth has certainly undergone many great revolutions, and there has been ample time for migration on the largest scale. Darwin (1871)
  7. The partly known ancestors of the anthropoid apes and the unknown ancestors of man probably originated among the forests and flood-plains of southern Asia and early began to migrate westward into northern Africa and western Europe.
    [...]
    It is possible that within the next decade one or more of the Tertiary ancestors of man may be discovered in northern India among the foot-hills known as the Siwaliks. Such discoveries have been heralded, but none have thus far been actually made. Yet Asia will probably prove to be the centre of the human race. We have now discovered in southern Asia primitive representatives or relatives of the four existing types of anthropoid apes, namely, the gibbon, the orang, the chimpanzee, and the gorilla, and since the extinct Indian apes are related to those of Africa and of Europe, it appears probable that southern Asia is near the centre of the evolution of the higher primates and that we may look there for the ancestors not only of prehuman stages like the Trinil race but of the higher and truly human types. Osborn, H.F. (1915): Men of the Old Stone Age. Their Environment, Life and Art, Charles Scribner's Sons
  8. Elliot Smith's argument that the steady growth and specialization of the brain itself has been the chief factor in leading the ancestors of man step by step upward indicates that such an advance as the erect attitude was brought about because the brain had made possible the skilled movements of the hands. Osborn (1915)
  9. Es scheint, als ob den Randrassen die gleiche Weiterentwicklung, wie den Bewohnern der offenen Landschaften, nicht beschieden war. Wirkt doch in allen Zeitaltern und Klimaten der Wald entwicklungshemmend und sind fast regelmäßig innerhalb einer Art die Bewohner der offenen Landschaften fortgeschrittener als die der Wälder. Hilzheimer, O.J.M. (1921): 'Aphoristische Gedanken über einen Zusammenhang zwischen Erdgeschichte, Biologie, Menschheitsgeschichte und Kulturgeschichte' in Zeitschrift für Morphologie und Anthropologie, 21, p. 185-208
  10. In anticipating the discovery of the true links between the apes and man in tropical countries, there has been a tendency to overlook the fact that in the luxuriant forests of the tropical belts, Nature was supplying with profligate and lavish hand an easy and sluggish solution, by adaptive specialization, of the problem of existence in creatures so well equipped mentally as living anthropoids are. 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. Dart, R.A. (1925): 'Australopithecus africanus: the man-ape of South Africa' in Nature, Volume 115, p. 195-199
  11. Before Australopithecus was discovered some of us believed that the ancestor of man would be found in an anthropoid ape which had left the forest and taken to living on the plains and among the rocks; and here we have just such a form. Broom, F.R.S. (1933): The Coming of Man. Was it Accident or Design?, H. F. and G. Witherby
  12. [...] der gefahr aus, in ungewohntem, freien Gelände mit seiner verhältnissmässigen Unbeholfenheit beim Bodengehen Feinden gegenüber im Nachteil zu sein, denen er im Baum besser begegnen oder entgehen kann. [...]
    Die Menschenaffen, die zu Menschen wurden, sind nicht deshalb vom Baum heruntergestiegen und haben nicht deshalb aufrechtgehen gelernt, sondern umgekehrt: der Baum ist unter dem Affen fortgelaufen und hat ihn dadurch auf den Boden gesetzt, ob er wollte oder nicht. Weinert, H. (1932): Ursprung der Menschheit. Über den engeren Anschluss des Menschengeschlechts an die Menschenaffen, Ferdinand Enke
  13. The only conceivable change that would bring about this new condition is the disappearance of the forests themselves. Instead of the apes leaving the trees, the trees left the apes. Then, when the trees were gone and no escape from the tree-less area was possible, it became a struggle to continue under such new conditions, a struggle for existence, where survivors would be few, while those that were doomed to extinction, because they were incapable of adaptation, formed the majority. Grabau, A.W. (1961): The World We Live in. A New Interpretation of Earth History, Geological Society of China
  14. Der ganze Übergang vom Tier zur Menschheit war eine Antwort auf neue Lebensverhältnisse, unter denen das dolce far niente eines tropischen oder subtropischen Urwaldlebens mit hinreichender Nahrungsfülle nicht mehr bestand. Weinert, H. (1940): Der geistige Aufstieg der Menschheit vom Ursprung bis zur Gegenwart, Ferdinand Enke
  15. Reasoning in this way, geologists and anthropologists are generally agreed that our ancestors came down from the trees as the result of still another period of climatic stress. This presumably took place somewhere in Asia. Two or three million years ago Tibet and the neighboring regions constituted a well-watered and well-forested area only slightly elevated above the sea. There, presumably, our nimble ancestors with their unique combination of eye, hand, muscle, and brain jumped from tree to tree and probably chattered, as busily as school girls. Little by little life became more difficult; the distances from tree to tree became longer; some of the trees that supplied food disappeared; many watering places dried up. These things happened because the climate became drier by reason of the gradual uplifting of the Himalayas and other great mountains. As the dryness increased, the forest doubtless assumed an open, orchardlike appearance, with abundant grass between the trees and many open, grassy glades, as is normal in such climates. [...]
    In their forested grasslands, then, our primate ancestors were confronted by another of those unconscious but real choices which repeatedly arise in the progress of evolution. Should they remain in the old kind of forest as its borders were slowly pushed southward, or should they boldly attempt a new mode of life? Should they depend on new kinds of food and face dangers hitherto unknown? Of course the choice depended on the physical make-up of the animals. Those that did not differ significantly from their ancestors were obliged to remain in the old kind of forest or perish. Those that had advanced sufficiently could at least attempt to live in the new environment. Many, however, must have been destroyed. The new kind of life was possible only if the anthropoids, as we may now call these progressive primates, walked on the ground in the open grassy spaces. Except in small clumps the trees were now too far apart for jumping. As aridity increased, the anthropoids were obliged to change their food, eating a much larger proportion of meat, especially such kinds as young antelopes, small rodents, and ground birds, and also substituting hard, dry seeds of cereals for the fruits and nuts of the forest. They were also forced to be far more wary than formerly, for the big carnivores doubtless found anthropoids as good eating as sheep or colts. Huntington, E. (1945): Mainsprings of Civilization, Wiley
  16. [...] there undoubtedly exists a strong contrast between the appearance of the skull of Sinanthropus and that of the skeletal parts of his body. This phenomenon can only imply that within the phylogenetic body probably preceded the head, or, in other words, man had already adopted the erect posture while his brain and skull still remained in a more anthropoid-like phase. If such an assumption is correct, then the specific human differentiation mentioned above must again be transferred. Brain or brain case cannot be considered as the organs supplying the decisive impulses for further evolution but the erect posture seems to have done so, freeing the hand from locomotorial functions and, at the same time, supplying the brain with the opportunity of gaining sufficient space in which to expand. Now a new question arises: what circumstances forced the ancestor of man to assume an erect posture?
    Since the ancestor was a climbing primate, it suggests itself that the direct cause may have been the disappearance of forests due to changes in climatic conditions. This, indeed, is a widely spread belief. However, I must say, that I cannot accept such an explanation. Of course, discussions on this problem are pure speculations on account of the complete lack of real knowledge as to time and place where the event in question may have occurred. In addition, there are certain suppositions of this theory which are rather precarious. The character of the Sinanthropus dentition indicates that his forerunner and with him the early hominids must have branched off from the general stem of the before the differentiation of the Dryohitpecus line occurred (cf. Lecture II). This suggests the ancestor to Sinanthropus having lived within the Lower Miocene at least. Can it be surmised that during the long period of time required by the transformation from a climbing foot into one adapted to standing and walking, a hominid was able to survive within an area which not only was lacking in opportunity for using his foot in the accustomed way but also was barren of shelter and food? It is much more likely that this hominid deliberately or voluntarily assumed the habit of living on the ground because the proportions of his upper and low limbs already in existence then favored such a change.
    All the ideas of compulsory influence of the environment upon the development of the hominids are based more or less on the conception that the evolution evolved within one locality only and was confined to a few individuals. In other words, that there should have been one cradle of mankind fixed geographically and temporally. Such a presupposition in the first place overlooks that human evolution cannot have been restricted to a certain time. For this evolution was not an event taking place but once but rather represents a process which extended over a period of millions of years. Weidenreich, F. (1939): 'Six Lectures on Sinanthropus Pekinensis and Related Problems' in Bulletin of the Geological Society of China, Volume 19, p. 1-92
  17. Bartholomew, G.A.; Birdsell, J.B. (1959): 'Ecology and the Protohomininds' in American Anthropologist, Volume 55, Issue 4, p. 481–498
  18. One may conclude from this that suitable habitats for the vegetarian, original Australopithecines (Paranthropus) line will have become increasingly scarce through the later Tertiary. This will have been as true for other forms requiring forest or broken forest habitat and reasonably moist conditions, hence it could be expected that competition for such environments may have been more severe than usual. On the other hand grass savanna and other more arid environments will have expanded at this time, thus providing increased opportunity for animals adapted to, or capable of adapting to, such conditions. Robinson J.T. (1963): 'Adaptive radiation in the Australopithecines and the origin of man' in Howell, F.C.; Bourlière, F. African Ecology and Human Evolution, Aldine, p. 385-416
  19. Clearly, we must suppose wet cycles during which the Sahara was covered with steppes, savannas, and lakes, to appreciate its role in human evolution. In such periods it is not the Sahara itself as such which was benefitted, but the habitat. One supposes this type of environment was extended over most of Africa aside from the dense forest.
    We have often stressed the advantages which a Primate in the process of hominization would find in a wooded savanna, while acquiring the erect posture necessary to the development of the hand as well as the vision. It had neither sufficient speed for the steppe nor a quadrumanal adaptation to tree life Monod, T. (1963): 'Late Tertiary and Pleistocene in the Sahara' in Howell; Bourlière, p. 117–229
  20. Recently Walker and Rose (1968) and Walker (personal communication) have detected signs of locomotor adaptations very like those of the living African pongids in the fragmentary (and largely undescribed) postcranial remains of the African Dryopithecinae. In the basal hominid, therefore, the 'gelada' specialisations would be superimposed upon a behavioural repertoire and post-cranial structure already attuned to some degree of truncal erectness. This combination of heritage and adaptation may have been the elusive determinant of terrestrial bipedalism. a gait that is inherently 'unlikely', and which would thus have begun as a gelada-like shuffel. Locomotion of any kind is infrequent during gelada-like foraging, so that (unlike hunting!) it is an ideal apprenticeship for an adapting biped. [...]
    The anatomical evidence seems to suggest that at some time during the Tertiary, the populations of Dryopithecinae destined to become hominids began to exploit more and more exclusively a habitat in which grass and seeds constituted most of the available resources while trees were scarce or absent. However, the great majority of contemporary tropical grasslands and open savannahs (especially those immediately surrounding patches of evergreen rainforest in all-year rainfall areas), are believed to be recent artefacts of burning and clearance by agricultural man [...] Under climatic climax conditions the vegetation of the seasonal rainfall tropics would almost always include at least one well-developed tree stratum, ranging from semi-deciduous forests through woodlands to sahel where paucity of rainfall inhibits both herb and tree strata [...]
    What, then, would have been the biotope of the grain-eating, basal hominids? The obvious answer is provided by the areas of treeless edaphic grassland which exist, even under natural conditions, within woodland or seasonal forest zones, wherever local drainage conditions cause periodic flooding, and hence lead to perpetual sub-climax conditions by inhibiting the growth of trees and shrubs [...] The remains of Villafranchian hominids are often found in deposits formed in such seasonal waters, as is Pleistocene Theropithecus [...], adding some circumstantial evidence that this was their preferred habitat. [...]
    The first stages of grain-feeding adaptation probably took place in a dambo-like environment, later shifting to wider floodplains. [...]
    They would thus have attained a stable, adaptive plateau upon which they could have persisted for millions of years, peacefully accumulating the physiological adaptations of a terrestrial, 'open-country' species. Jolly, C.J. (1970): 'The Seed-Eaters: A New Model of Hominid Differentiation Based on a Baboon Analogy' in Man, Volume 5, p. 5–26
  21. A model of hominid origin proposed by Jolly (8) uses analogy to anatomical and behavioral characters shared by Theropithecus gelada and some early hominids. He suggests that early hominid populations relied on small-object feeding, that this dietary specialization led to a suite of adaptations to the grassland savannah, and that bipedality developed in response to feeding posture. Yet geladas, which do rely on small-object feeding, are not bipedal and show no significant adaptations to bipedality. Bipedal locomotion is clearly not required for extensive small-object feeding especially on grasslands where speed and agility are of great value in animals who also lack wide visual fields and sensitive olfaction (36). Furthermore, the dental morphology of A. afarensis is considerably more generalized than that of later hominids. The dietary specialization seen in A. robustus is possibly accountable by Jolly's model, but the more generalized dentition of A. afarensis is not (37). It is more likely that hominids venturing into open habitats were already bipedal and that their regular occupation of savannahs was not possible until intensified social behavior was well developed.
    [...] it would appear that late Miocene habitat mosaics would allow adoption of bipedality (in forests and transition mosaics) rather than directly select for it. All present evidence therefore indicates that hominid clade evolved in forest or mosaic conditions, or both (43), rather than only on grassland or savannahs, and that bipedal locomotion was not a response to feeding posture, material culture, or predator avoidance.
    [...]
    One critical difference separates provisioning in birds and canids from that suggested for early hominids. Birds and canids can carry in their mouths or regurgitate (or both) a significant proportion of their body weight. Oral carrying would have been inadequate for early hominids, however, and a strong selection for bipedality, which would allow provisions to be carried "by hand," would thus accompany provisioning behavior (74). Lovejoy, C.O. (1981): 'The Origin of Man' in Science, Volume 211, Number 4480, p. 341-350
  22. Comparisons of the Ar. ramidus dentition with those of all other higher primates indicate that the species retained virtually no anatomical correlates of male-to-male conflict. Consistent with a diminished role of such agonism, the body size of Ar. ramidus males was only slightly larger than that of females.
    Breakthrough adaptations can transform life-history by deviating from typical reproductive strategy. Early hominids show feminized male canines (left) and primitive bipedality (right). These suggest that females preferred nonaggressive males who gained reproductive success by obtaining copulation in exchange for valuable foods (vested provisioning). Success would depend on copulatory frequency with mates whose fertility remained cryptic (e.g., absence of cycling in mammary size). Lovejoy, C.O. (2009): 'Reexamining Human Origins in Light of Ardipithecus ramidus' in Science, Volume 326
  23. Whereas the chief anthropologist on the Ardi team goes by the bonobo-like name of Owen Lovejoy, he focuses all of this attention on the chimpanzee, as is tradition in his field. Since chimps are violent and Ardi probably wasn't, he argues that we have a totally unique creature on our hands. His pet theory is that this must mean that Ardi and her contemporaries were monogamous, but unless the diggers come up with a male and female fossil holding hands and having wedding rings, the idea that these ancestors avoided conflict through pair-bonding remains pure speculation. There is no evidence for it, and the only pair-bonded primate we have in our direct lineage (the gibbon) has in fact huge canine teeth. Waal, F.B.M. de (2009): 'Was “Ardi” a Liberal?'
  24. In a subsequent phase, the tectonic formation of the western branch of the African Great Rift Valley system, in combination with the Nile and Zambezi River systems, constituted a double set of barriers to creatures that could neither swim nor cross arid rain-shadow zones. Consequently, the progressive desiccation of East Africa caused by the steady rise of the Rift mountain ridges, together with the general climatic trends during the Pliocene, must have converted the last-surviving dryopithecine (Proconsul) ape there into an upright-walking, drought-adapted, and "humanoid" type of bush and grassland ape, i.e., in all probability the Homininae, strictly speaking. Kortlandt, A. (1972): New Perspectives on Ape and Human Evolution, Stichting voor Psychobiologie
  25. Coppens, Y. (1994): 'East Side Story: The Origin of Humankind' in Scientific American, Volume 270, no. 5, p. 88-95
  26. White, T.D.; Suwa, G.; Asfaw, B. (1994): 'Australopithecus ramidus, a new species of early hominid from Aramis, Ethiopia', Nature, Volume 371, p. 306–312
  27. Along the northern slope of the CAC, all localities of the Lower Aramis Member yielded tragelaphine bovids, monkeys, and other data indicative of more wooded conditions. Carbon isotopes from the teeth of five Ardipithecus individuals found here imply that they fed largely on C3 plants in woodlands and/or among the small patches of forests in the vicinity. We interpret the combined contextual data to indicate that Ar. ramidus preferred a woodland-to-forest habitat (29, 30) rather than open grasslands. This finding is inconsistent with hypotheses positing hominid origins via climate-driven savanna expansion. White, T.D.; Asfaw, B.; Beyene, Y.; Haile-Selassie, Y.; Lovejoy, C.O.; Suwa, G.; WoldeGabriel, G. (2009): 'Ardipithecus ramidus and the Paleobiology of Early Hominids' in Science, Volume 326, p. 75-86
  28. Our combined evidence indicates that Ar. ramidus did not live in the open savanna that was once envisioned to be the predominant habitat of the earliest hominids, but rather in an environment that was humid and cooler than it is today, containing habitats ranging from woodland to forest patches. WoldeGabriel, G.; Ambrose, S.H.; Barboni, D.; Bonnefille, R.; Bremond, L.; Currie, B.; DeGusta, D.; Hart, W.K.; Murray, A.M.; Renne, P.R.; Jolly-Saad, M.C.; Stewart, K.M.; White, T.D. (2009): 'The Geological, Isotopic, Botanical, Invertebrate, and Lower Vertebrate Surroundings of Ardipithecus ramidus' in Science, Volume 326, p. 65
  29. Isotopic data indicate that the Ar. ramidus diet was predominantly forest- to woodland-based. This interpretation is consistent with evidence of the dental and skeletal biology of this primate (1). The ecological context of 4.4 Ma Aramis hominids, combined with their absence or extreme rarity at Late Miocene and Early Pliocene sites, suggest that the anatomy and behavior of the earliest hominids did not evolve in response to open savanna or mosaic settings. Rather, this clade appears to have originated within more closed habitats favored by these peculiar primates until the origin of Australopithecus, and perhaps even beyond (50). White, T.D.; Ambrose, S.H.; Suwa, G.; Su, D.F.; DeGusta, D.; Bernor, R.L.; Boisserie, J.R.; Brunet, M.; Delson, E.; Frost, S.; Garcia, N.; Giaourtsakis, I.X.; Haile-Selassie, Y.; Howell, F.C.; Lehmann, T.; Likius, A.; Pehlevan, C.; Saegusa, H.; Semprebon, G.; Teaford, M.; Vrba, E. (2009): 'Macrovertebrate Paleontology and the Pliocene Habitat of Ardipithecus ramidus' in Science, Volume 326, p. 87-93
  30. In contrast, we find the environmental context of Ar. ramidus at Aramis to be represented by what is commonly referred to as “tree or bush savanna” with 25% or less woody canopy cover. The habitats involved probably ranged from riparian forest to grassland.
    [...]
    The Middle Awash reconstruction has been used to challenge the long-standing “savanna hypothesis” that states that bipedalism, among other human traits, evolved in response to the progressive expansion of grasslands and savannas (wooded grasslands) in Africa after 8 Ma. Although we do not judge the validity of the savanna hypothesis, we note that from the stable isotopic record, the connection between bipedalism and C4 grass expansion starting in the late Miocene and continuing in the Pliocene remains a viable idea. Previous work has suggested that the period 8 to 4 Ma records a major ecologic transition globally as well as locally in Ethiopia. The data from Aramis reinforce this view. Cerling, T.E.; Levin, N.E.; Quade, J.; Wynn, J.G.; Fox, D.L.; Kingston, J.D.; Klein, R.G.; Brown, F.H. (2010): 'Comment on the Paleoenvironment of Ardipithecus ramidus' in Science, Volume 328, 1105
  31. All of this fossil evidence adds up to the small-brained, bipedal hominins of 4.0 to 2.5 Ma, having lived in a woodland or forest niche, not in a savannah terrain. The evidence for the presence of big forest trees supports the idea we had gleaned from bones of 'Little Foot' that tree-climbing had been a part of the life ways of these early African hominins. At least, one could conclude, there had been trees big enough to bear the weight of the australopithecines (for which stunted acacias of the savannah would have been unsuitable).
    To a large audience at University College London, I cried, “Open the window and throw out the savannah hypothesis; it's dead and we need a new paradigm.” Following that, Bernard Wood in 1996 wrote, “The savannah 'hypothesis' of human origins ... is now discredited” [43]. I said: “All the former savannah supporters (including myself) must now swallow our earlier words in the light of the new results from the early hominid deposits ... Of course, if savannah is eliminated as a primary cause, or selective advantage of bipedalism, then we are back to square one”. Vaneechoutte, M.; Kuliukas, A.; Verhaegen, M. (2011): Was Man More Aquatic in the Past? Fifty Years After Alister Hardy - Waterside Hypotheses of Human Evolution, Bentham Science Publishers
  32. The predominance of impalas in the Kapsomin faunal assemblage suggests that the surroundings of the site were probably open woodland, while the presence of several specimens of colobus monkeys indicate that there were denser stands of trees in the vicinity, possibly fringing the lake margin and streams that drained into the lake. Pickford, M.; Senut, B. (2001): 'The geological and faunal context of Late Miocene hominid remains from Lukeino, Kenya' in Comptes Rendus de l’Academie des Sciences, Series IIA, Earth and Planetary Science, Volume 332, No. 2, p. 145-152
  33. The famous ‘savannah hypothesis’ is no longer tenable because the palaeontological data support a more vegetated environment for the origins of bipedal hominids. Senut, B. (2015): 'Morphology and environment in some fossil Hominoids and Pedetids (Mammalia)' in Journal of Anatomy, Volume 228, Issue 4
  34. [...] the TM 266 vertebrate fauna contemporary of the Toros-Menalla hominid suggests a mosaic of environments from gallery forest at the edge of a lake area to a dominance of large savannah and grassland. Determining the precise habitat of the TM 266 hominid locality among the mosaic of environments available to it constitutes a research challenge to be met by further laboratory and field studies currently in progress. Vignaud, P. et al. (2002): 'Geology and palaeontology of the Upper Miocene Toros-Menalla hominid locality, Chad' in Nature, Volume 418, p. 152-155
  35. The vertebrate faunas as well as the isotopic and geological evidence all indicate that aquatic habitats with large bodies of water were present. These would have been accompanied by riparian woodland and floodplain grassland that supported the terrestrial mammalian communities sampled by the fossil record. Arid savanna environments were unlikely to have been important parts of the overall ecological setting. Su, D.F.; Ambrose, S.H.; DeGusta, D.; Haile-Selassie, Y. (2009): 'Paleoenvironment' in Haile-Selassie, Y.; WoldeGabriel, G. Ardipithecus Kadabba. Late Miocene Evidence from the Middle Awash, Ethiopia, University of California Press
  36. [...] 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. Cerling, T.E.; Wynn, J.G.; Andanje, S.A.; Bird, M.I.; Korir, D.K.; Levin, N.E.; Mace, W.; Macharia, A.N.; Quade, J.; Remien, C.H. (2011): 'Woody cover and hominin environments in the past 6 million years' in Nature, Volume 476, p. 51-56
  37. 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’’42 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. Cerling et al. (2011)
  38. A strict 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.
    [...] 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.
    [...] 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? Domínguez-Rodrigo, M. (2014): '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
  39. 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. Domínguez-Rodrigo (2014)
  40. 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 savanna chimpanzees). Wooded environments in the form of woodlands should not be mistaken with forests; natural selection for primates is different in both environments.
    [...] 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.” Domínguez-Rodrigo (2014)