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Timeline for the evolution of apes and homo ancestors
Charles Darwin said Natura non facit saltus: nature doesn’t make leaps. In other words, evolution happens gradually, in small steps, each of which has to be viable. It follows, therefore, that when tree-living monkey-like primates (arboreal) evolved into human ancestors who spent a lot of time in the water (semi-aquatic), there had to be an intermediate phase where they lived in trees as well as water (aquarboreal, from the Latin aqu-=water and arbor=tree).
Old World Monkeys to the first hominoids
According to DNA comparisons, Old World monkeys (cercopithecoids) and human–ape ancestors (hominoids) split around 25–30 Ma (million years ago); and lesser apes (gibbon ancestors) split from the great apes about 18 Ma during the Miocene epoch (which spanned the period 25–5 Ma). The Miocene and the Pliocene (5–2.6 Ma) were warm and wet compared to the later Pleistocene (the Ice Age epoch 2.6–0.01 Ma), and hot swamp forests were therefore much more common. Most, if not all, Mio-Pliocene hominoid fossils have been discovered in former swamp forests, wetlands and even coastal forests (this is despite coastal forests being poor places for fossilisation, because wave action tends to prevent fossilisation, which occurs more commonly in stagnant or slow moving waters).
All great apes still spend some time in forest swamps, wading bipedally for aquatic herbaceous vegetation (AHV). Lowland gorillas, for example, are known to wade one or two hours per day for aquatic plants such as sedges and frogbit in forest bais (swampy clearings), while smaller individuals climb vertically in the branches above the swamp. According to the aquarboreal theory, this lifestyle of upright wading and vertical climbing can explain most of the features that differentiate apes from monkeys: much larger bodies (from less than 10 kg in gibbons to more than 100 kg in gorillas); below-branch locomotion (including arm hanging and a vertical body) with a shortened lumbar region and habitually upright spine; a very broad thorax and breast-bone with the arms at the side of the body (e.g. for collecting AHV around the body) instead of underneath as in monkeys; and tail loss (tails are useful for fast movements in the branches, but not for wading in swamps).
Hominoid expansion into Eurasia and Asia
The earliest ape-like fossils, such as the 25 Ma Rukwapithecus, come from Africa. About 18 Ma, however, Africa (which was then joined to Arabia) collided with Eurasia, splitting the Tethys Sea, which separated the two landmasses, into a western and eastern branch. After this time, apes are found all over southern Eurasia, from Spain to Turkey to China. Some of the earliest apes from this time period come from coastal forest deposits, such as Heliopithecus from the Arabian Gulf (18 Ma) and Austriacopithecus from Slovakia (14 Ma). From 14 to 7 Ma, in southern Europe, apes are commonly found in swamp forests and reed marshes, including Pierolapithecus and Hispanopithecus from Catalonia, Rudapithecus from Hungary, Dryopithecus from France, and Oreopithecus from the then island of Tosco-Sardinian. In China, Lufengpithecus is known from swamp forests from about 10–5 Ma.
Morphological variation among Miocene hominoids was far greater than in today’s great apes. Some apes from this time period might have spent more time wading and climbing in mangrove forests feeding on hard-shelled foods such as mangrove oysters or coconuts, others may have hung from branches eating more fruits or leaves, while still others may have floated or swam in freshwater swamps collecting soft AHV or harder waterside grasses or cattails, or hard-shelled invertebrates. Early great hominoids seem to have inhabited the Tethys coastal forests and moved inland along lagoons, gallery forests and wetlands. From about 15 Ma, the pongids (orangutans and relatives) appear to have inhabited the southern Asian coasts (Eastern Tethys), while the hominids (early relatives of Gorilla, Pan and Homo) inhabited the Mediterranean coasts (Western Tethys).Late-Miocene hominids along the southern Mediterranean coasts might have followed the same water route to mega-lake Chad as the semi-aquatic anthracotheres (fossil relatives of hippopotamuses), since both they and Sahelanthropus appear there about 7 Ma.
Soon, hominids appeared in the rest of Africa: Orrorin in Kenya about 6 Ma in lakeside forests, and the australopithecines from about 4–1 Ma in east- and later south-African swamp forests and wetlands. Their typical combination of curved hand-bones (vertical climbing) and flat human-like feet (wading-swimming) suggests most or all australopithecines were still aquarboreal. According to paleo-environmental data, the Pliocene gracile australopiths lived in swamp forests, and the Pleistocene robust australopiths in more open wetlands, papyrus swamps and lagoons. Isotopic as well as dental micro-wear evidence (e.g. glossy enamel polished by wet plants) suggests at least some of these species had AHV such as papyrus sedges in their diet.
Pan and Gorilla Ancestors
According to DNA comparisons, gorillas split from the ancestors of chimps, bonobos and humans about 8 million years ago. If we assume this event occurred in Africa, then it follows that the ancestors of gorillas, chimps and bonobos have lived in Africa since this time. But where are their fossils? Fossil-hunters have found thousands of fossils that they assume are human ancestors (who wouldn’t prefer to dig up a human rather than a chimp ancestor?), but virtually no ancestral Pan or Gorilla fossils have ever been found. However, if Pan and Gorilla had more bipedal ancestors as suggested by ontological as well as comparative evidence, their ancestors would have had very australopith-like bodies. Although all great apes still occasionally or habitually wade for AHV, possibly the Pleistocene cooling and drying made the African ape ancestors leave the swamps and become less bipedal, and live in tropical forests, where their semi-upright posture preadapted them to knuckle-walking on the dry forest floor (explaining why Gorilla and Pan evolved knuckle-walking in parallel).
Homo’s peculiar evolution (what we might call the ‘classic’ phase of AAT) might have started when Pleistocene sea-levels dropped and coastal forests dwindled, and vast, tree-poor but shellfish-rich territories became available for handy “apes” who could use stone tools to open hard-shelled littoral foods such as coconuts, crabs and mangrove oysters (which are rich in brain-specific nutrients such as DHA). They could have transitioned from aquarborealism in coastal forests to a littoral lifestyle in less forested coasts, which included beach-combing, wading and diving in shallow water, and waterside food collection. This might help explain why our ancestors acquired diving skills, subcutaneous fat tissues, naked bodies, poor sense of smell, and – visible in the fossil record of Pleistocene archaic Homo – external noses, bigger brains, long low flat skulls, and heavy skeletons, features which are all typical of littoral and (semi)aquatic animals. Thanks to this new niche, they “soon” (probably early-Pleistocene) dispersed intercontinentally along the Indian Ocean and Mediterranean Sea coasts. Homo erectus-like fossils or tools are typically found near abundant edible shellfish, probably as early as 1.8 Ma as far as Java (Mojokerto, deltaic & coastal sediments with barnacles), Algeria (Aïn-Hanech, coastal plain), Georgia (Dmanisi, big rivers near the then Black–Caspian Sea connection) and Kenya (Turkana Lake, with the stingray Dasyatis indicating a marine connection).
Gradually, Homo populations ventured inland along rivers, at first seasonally, later sometimes permanently. Neanderthals are typically found not only at sea-coasts (from Petralona in Greece to Gibraltar), but also in river valleys, oxbow lakes and beaver ponds, and their dental calculus sometimes shows traces of waterlilies, and their tools, of cattails.
Archaic Homo to Modern Humans
Meanwhile, if Homo sapiens’ ancestors in Africa evolved more complex distance weapons such as spears (harpoons), fish-traps and nets for collecting more mobile aquatic prey, this could explain the shift from archaic to modern humans: very long legs (esp. shin bones) for wading in very shallow water, lighter skeletons for more efficient movement out of the water, longer and downward pointing spinous processes in the back (stabilising the upright spine), higher skulls (loss of supra-orbital torus) and basicranial flexion for spotting shallow water foods from above etc. Eventually, the habit of carrying tools and weapons in combination with long legs and a vertical spine, allowed our ancestors to transition to a more terrestrial lifestyle as efficient upright bipeds, walking long distances, running and moving further from the water’s edge.
Modern-day Semi-Aquatic Homo groups
The Aquatic Ape Evolves: Common Misconceptions and Unproven Assumptions About the So-Called Aquatic Ape Hypothesis
Early Hominoids: Orthograde Aquarboreals in Flooded Forests?
Was the Swamp Ape Bipedal 
|Website: F. Mansfield, 2015|
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