Phase I:

Oreopithecus-like 'Swamp-apes' ancestral to all African great apes arrive in North & East Africa and becomes established in coastal mangrove swamp & gallery forest environment.

Key:

Swamp Apes - African Ape / Homo ancestor Dryopithecus / Oreopithecus-like ape.  

Orang-utan ancestor 

Pros:

• Offers a plausible mechanism for bipedal origins (wading) in an arboreal context.

• Consistent with Sahelanthropus and Orrorin findings and molecular data estimation of Pan/Homo split. (i.e. a 6 mya putative biped found before the assumed date of the last common ancestor - indicating that bipedalism was the ancestral condition.)

Cons:

• No clear fossil evidence unambiguously placing the last common ancestor of the African Hominoidae although Dryopithecus has often been considered for that roll.
• Climbing-Wading lifestyle rare in primates. Proboscis monkey is the only notable exception.

This phase of the model is based on the assumption that the earliest apes lived in rather wet habitats dominated by coastal, and probably swampy mangrove forests. Their putative location (what today is the Eastern Mediterranean, north-east Africa and Arabia) underwent dramatic physical changes between 30 and 10 million years ago. Whole coast lines disappeared and re-appeared several times and the Tethys Sea and its derivatives of the region (e.g. the Mediterranean) were alternatively undergoing flooding when reconnected to the oceans and desiccations when they were cut off again. What we now call the Mediterranean was, apparently, flooded and dried up 14 times in the Miocene.

Maps Taken from from Dixon et al 2001

Europe 20 mya

Europe 10 mya

Oreopithecus bambolii, a long studied primate that was a putative biped lived around 10 mya in this region (on an island in the Tethys Sea) has been associated with a swamp habitat. Although this 'monkey' is not thought to be part of the Hominoid (ape) line it is interesting to postulate that one of the earliest bipeds may have done so because of the need for wading. See Moyà-Solà et al 1999 for more about fossils of this primate.

Various 'Miocene Apes' that lived in less aquatic habitats have been postulated as ancestral to the African apes (including Dryopithecus brancoi) found, at least in one site in "a subtropical, forested, swamp margin environment" Kordos & Begun (2001) but there is a significant gap in the fossil record here. The AHAH model makes the (admittedly rather large) assumption that the species ancestral to the African apes was a climbing-wading ('aquarboreal') swamp ape and that they were already well adapted to mangrove/swamp forests and adopted a climbing/swinging locomotor repertoire in the trees and bipedal wading in water.

The model proposes that some of these coastal mangrove apes began to migrate inland, along the rift valley rivers and lakes. It is proposed that Sahelanthropus tchadensis and Orrorin tugenensis the earliest putative bipedal hominids  were example of this inward migration possibly on the way to becoming Gorilla. 

Later, another smaller and  more arboreal ape that was more omnivorous - the model proposes - would eventually split from the remaining swamp apes and eventually evolve into the progenitors of Pan and Homo. This model is not prescriptive here about the exact semi-aquatic environment which was responsible for the divergence of the Homo lineage from Pan, but it is proposed that it could have been a combination of coastal mangrove forests and in-land gallery forests, shrinking ever closer to rivers as climate change in East Africa reduced forestation there.

Predictions:
If this phase of the AHAH model is correct a number of observations/discoveries should be made in the future.

Specifically:

• The LCA of the African Hominoidae should be unambiguously bipedal, although, like Australopithecus afarensis, that bipedality is unlikely to be fully upright and human-like. This should become clear as earlier fossils are found in Africa earlier than 6mya (e.g. Orrorin.)

• Earlier Hominoid fossil ancestors should be found in more coastal locations.

• It is suggested that these earliest great apes (ancestors of Pan, Gorilla, Homo and Pongo) lived in a mangrove forest habitat. It is therefore expected that more evidence for this will emerge in the next few years.

Specifically:

• If clear evidence arose showing that the earliest bipeds lived in habitats where wading could not have taken place then the wading origins model for bipedalism would have to be rejected.

• If a combination of fossil evidence and/or molecular data showed that bipedalism emerged purely on the Homo line then the specific element of this model which suggests that Pan/Gorilla reverted to quadrupedalism would have to be rejected. It should be noted that if this did happen it would not necessarily argue against a wading origin for bipedality, merely that this was the ancestral condition of all African apes.

It is proposed that phase I of the AHAH model ended after inter-glacial flooding or glacial desiccation occurred causing the vast majority of these putative climbing-wading apes to become extinct.

This is a difficulty because the flood/desiccation cycles apparently occurred several times.

It is assumed that the last time it happened a group of these climbing-wading apes found themselves in inland forests, forming a radiation of migration of semi-bipedal apes - the Australopithecines.. Those living west of the rift would remain in relatively stable equatorial rain forest habitats, whereas those on the east of the rift would increasingly be exposed to the process of aridification, causing forests to shrink closer to rivers and lakes. 

That migration and radiation is dealt with in Phase II.