Bipedalism is a discerning trait of humans and pre-dated brain enlargement by millions of years. Its importance was perhaps best encapsulated by Montgomery: “Man’s habitual and irrevocable bipedalism is unique in the mammal kingdom. Its uniqueness is possibly more strange and noteworthy than our extraordinary intelligence. Our intelligence is a matter of degree, bipedalism is absolute” [30]. Obligate bipedalism is rare among animals, pursued only by the extinct Tyrannosaurus rex, kangaroos, penguins, ostriches, gorillas, and hopping mice. There is mounting fossil, nutritional, and biogeographic evidence that bipedalism among hominoids occurred in the littoral setting. Wading engages bipedal walking among extant primates such as bonobo chimpanzees, gorillas in the Congo, and Okavango Swamp baboons [31]. Limited bipedality is also evident among gibbons, atel- ids, and sifakas. Conventional belief for many years was that East African drying and
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desiccation of the forest environment forced arboreal hominoids to become terrestrial – the so- called Savannah hypothesis, succinctly presented as the East Side Story [32]. This hypothesis has since been jettisoned, most notably by long- time supporter Phillip Tobias, revered researcher of australopithecines, who declared ruefully at a prominent lecture to the Royal College in London in 1995 that the Savannah hypothesis was dead [33,34].
Littoral zone evolution now seems much more likely, as detailed by Verhaegen, an avid proponent of the aquatic ape hypothesis, first presented by Hardy [35] and subsequently elaborated further by Morgan and Verhaegen [36,37]. Verhaegen developed his view and version, summarized as an aqua–arboreal, littoral, and wading phase of our evolution. In brief, the theory proposed is that our Miocene and Pliocene ape ancestors may have pursued an aqua–arboreal existence within swampy mangrove- type forests and wetlands, feeding on plants, hard- shelled foods, crustaceans, fish, and shellfish in littoral zones. Indian Ocean coastal regions would have been the most likely littoral zone for Pleistocene Homo ances- tors. A subsequent wading phase is proposed in the later Pleistocene epoch with migration inland from the coast, presumably along the river valleys imbued with convenient gallery forest, associated with less diving, but with predominant wading activities assisted by an orthograde posture and longer legs. After the initial ocean habitat, then riverine occupa- tion, subsequent lacustrine habitation in the East African Rift Valley is postulated, still pur- suing shallow- water fish and riverine or lacustrine aquatic foods [38].
Without the need for mutations, Verhaegen’s view presumes a sequence of events termed mosaic evolution, characterizing the transition from the ape to the human form.
Primates are postulated to have evolved from a so- called above- branch pronograde (walking with the body parallel to the ground) arborealism during the Miocene/Pliocene epochs to an orthograde (upright, vertical) below- branch aqua- arborealism, and subse- quently to archaic Homo species that partook in shallow diving within a littoral environ- ment. Wading in shallow waters presumably favored bipedality developing in early Homo species. Walking and running was subsequently favored in the predominant land dwell- ers [39]. This is counter to the bipedal origin views of Bramble and Lieberman’s Savannah hypothesis with human running on the plains occupying an endurance running niche and “born to run” concept [40]. Verhaegen contends that the aqua–arboreal hypothesis is a more parsimonious explanation, with savannah mammals far exceeding our relatively limited sprint speed of ~20 km/h. In his view, Homo locomotion most likely sequentially evolved from tree climbing to swimming to diving and wading and eventually walking and running.
Evidence of Miocene 21–16 mya fossil primates with orthograde posture with both arboreal and terrestrial abilities appeared in apes such as Nacholapithecus (orthograde climbing ability) and Kenyapithecus, with scansorial activities (adapted or specialized for climbing). The shift from quadrupedalism to bipedality may best be perceived as an initial quadruped ape that was part- arboreal, part- terrestrial quadruped, that engaged in bipedal wading after descending from gallery forests adjoining riverine, lacustrine, or marine envi- ronments. Bipedalism is likely to have evolved soon after our ancestral separation from the gorilla and chimpanzee clades [41]. Further support for bipedal evolution comes from the study of extant Atelids, who use their tails for suspension but have the features of lumbar lordosis and full extension of their hind limbs with upright stance. Their anatomy reveals the tell tale reduction of iliac height and broad sacral alae, viewed as an early component of hominid bipedality. Both the tail less Proconsul and Ekembo nyanzae had broad alae. This supports some primitive Miocene hominoids as being in- between suspensory activity with extension ability as well as early bipedal capabilities [42].
Tethys Ocean and the Origins of Bipedalism
Several researchers support the view that habitual terrestrial bipedality, together with arboreal hand- assisted bipedality seems kinetically more parsimonious [43]. Likewise, the aqua–arboreal (semi- arboreal, semi- aquatic) theory of Miocene hominoids descend- ing from large branches into mangrove- like swamp forests appears similar to what Congo lowland gorillas and bonobo chimpanzees practice nowadays [44]. The vertical aqua- arborealism phase gains support from findings of broad thoraces, dorsal scapu- lae, tail loss, and more vertical spines of Miocene hominoids such as Morotopithecus and Griphopithecus 18 mya, which have lumbar vertebrae more similar to human lumbar verte- brae to support orthogrady [45]. Review of human lower limb features such as a wide pel- vis (platypelloidy), flattened femurs (platymeria), and vertebral changes (platyspondyly) and generally heavy skeletons are not optimal features for running and are best explained by wading activity. Many other archaic hominoid fossil features such as platy cephaly, platymeria, brain expansion, ear exostoses, pachyosteosclerotic bones (more suited to ballast control), protruding nose (mid- facial prognathism), wide bodies, and enlarged paranasal sinuses (perhaps for buoyancy) support shallow littoral diving and wading rather than a cursorial lifestyle. There is also a craniocaudal subcutaneous fatty tissue arrangement in humans for thermoregulatory exchange in the upper part of the body and insulation of the abdomen and lower limbs that may have evolved for thermoregulation while wading [46]. Bipedalism antedated australopithecines fossils discovered from ~3.5 mya onwards, all found in water- based environments but with dental and cranial support of a predominantly vegetarian diet. Evolutionary stasis of Australopithecines, with their relatively small brain size of 400–500 cc, has been attributed to their land- based diet.
Meat eating is presumed to have evolved 1–2 million years later, but the partially arboreal australopithecines were not suited to compete with larger carnivores (felines, canid, hyae- nid) on the savannah. They also were not suited physiologically, without sun- reflective fur, with a subcutaneous fat layer about tenfold thicker than extant chimpanzees, and their sweat gland method of cooling being more wasteful of water and sodium, as well with low urine concentration competence, none of which are traits suited to the challeng- ing hot savannah habitat [39]. Marine aquatic animals, as well as human infants, feature reniculated kidneys, characterized by multipyramidal medullas and adaptation for avoid- ing saline water dehydration by facilitating first the concentration and then the excretion of urinary salts, which recedes in childhood [47].
The most likely site of this phase of evolution was within the early Mediterranean and/
or the Indian Ocean continental shelves and coastal littoral associated with prolific shell- fish and fish (Figure 2.5). Further support comes from the findings of numerous archaic Homo species discovered in the adjoining Turkana Basin (East Africa) and marine stingray evidence deposits, also in the last two million years, implying that the Turkana Basin was connected to the Indian Ocean [48]. Pachyosteosclerosis (POS) bones help with buoyancy control and are seen in semi- aquatic animals, and are also a feature in early Homo species, such as Homo erectus, suggesting this species dined on sessile foods in the littoral zone [49].
Paleo-Climatological Support for Bipedality
During the Late Miocene period of 7–11 mya, termed the Tortonian stage, aridification of North Africa and formation of the Sahara Desert occurred, with shrinking of the Tethys Sea and a weakened African summer monsoon, through orbital forcing (Milankovitch cycles). These predications were formulated through climate model simulations by Zhang
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et al. and these climatic events caused major African and Asian plant and animal fluxes that included the early hominins in North and East Africa [50].
Geophysical Support for Bipedality
Tectonic plate movement was also relevant, with the most plausible scenario for the evo- lution of human bipedalism to me, suggested by La Lumiere. He postulated a scenario whereby the complex tectonic plate movements in the tri- plate Afar region, compris- ing three converging crustal blocks, resulted in a geographic island around 14–9 mya – the Danakil Horst/Alps in the southern Red Sea during the later Miocene period. From
~7 mya, a marine basin was established until 70 kya, with sea flooding into Afar that has never left, instead having evaporated, with the northern part remaining as the Dead Sea in Israel and the eastern end, the Danakil Alps, previously the Danakil Island. Rather than being viewed as a geographic island, it was more appropriately seen as a biological island with regard to the animals caught up in the tectonic plate turmoil. This remains speculation, and the “biological island” may not have been Danakil Island specifically, but any other near shore island within the north- eastern part triplate area, during the Late Miocene period. Perhaps a group of ancestral hominoids stranded on such an island, encircled by ample littoral zones for about two million years, was the appropriate scenario for bipedalism and later surging brain growth (Figure 2.6) [51–53].
Geographic speciation due to geographic separation of this kind provides ideal condi- tions for rapid evolutionary change, with more rapid change corresponding to more abso- lute isolation. Support for such mercurial change comes from studies of fish speciation
Figure 2.5 Human brain evolution: a new wetlands scenario – humans evolved in the rich ecological niche of the land–water interface. Image: IndustryAndTravel/Shutterstock
Tethys Ocean and the Origins of Bipedalism
in isolated small bodies of water that have been recorded to occur ~1000 times faster than in the open ocean [54]. This theory gains support for a very similar scenario for Oreopithecus bambolii on the islands of Tuscany and Sardinia, adjacent to modern- day Italy. Notably, Australopithecus afarenesis or Lucy, dated to 3.2 mya, was subsequently found in the Afar region of north-east Africa by Donald Johanson in 1976 [55]. No other previously proposed theories of bipedalism explicate this unique human attribute. The many theories of bipedality proposed over the years remain unsubstantiated. Examples include the following hypotheses:
• watching out hypothesis;
• freeing of hands hypothesis;
• throwing hypothesis;
• infant crying hypothesis;
• reaching for food hypothesis;
• carrying of food or provisioning hypothesis;
• display hypothesis;
• orthograde scrambling hypothesis;
• scavenger hypothesis;
• thermoregulation hypothesis.
In summary, a number of features have been proposed in support of the aqua–arboreal bipedalism hypothesis by the proponents, with variable degrees of substantiation (Table 2.1).
EURASIAN PLATE
ARABIAN PLATE
NUBIAN PLATE
AFAR ISTHMUS SOMALIA DANAKIL HORST
(Microplate)
EURASIAN PLATE
DANAKIL ISLAND STRAIT OF BAB AL MANDAB ARABIAN
PLATE
NUBIAN PLATE
SOMALIAN PLATE ETHIOPIAN
ESCARPMENT
RIFT VALLEY DANAKIL STRAIT
AFAR GULF PROTO-
MEDITERRANEANSEA MEDITERRANEANSEA
PROTO-RED RED SEA
SEA
PROTO-GULF OF
GULF OF ADEN ADEN
PROTOGULF
OF PERSIA PERSIA
GUL F OF
Figure 2.6 Danakil Island and Danakil Alps during the Late Miocene (left) and Early Pliocene (right). The Afar isthmus land bridge that allowed animals to traverse between Africa and Arabia and the Danakil horst (540 km × 75 km wide) are both important aspects of the theory.
Source: La Lumiere LP. Evolution of human bipedalism: a hypothesis about where it happened. Phil Trans R Soc Lond B 1981;292:103–107. Reproduced with permission of the Royal Society (Great Britain); permission conveyed through Copyright Clearance Center, Inc.