The manifold hypotheses and theories on the evolution of human upright posture and gait have their own intriguing history. Until the early 1990s, the evolutionary development of human bipedalism was placed into a savannah scenario (e.g., Rose 1976; Blumenshine and Cavallo 1992). Later, most authors agreed that the evolution from a quadrupedal to a bipedal ancestor of extant humans occurred in a forested landscape (e.g., Haile-Selassie 2001; Sénut and Pickford 2004). As early as in 1989, Marean had even postulated that fully erect members of the genus Homo left woodland habitats in order to use more open vegetation not longer ago than after 1.6 Ma. The sheer number of different hypotheses proposed to the scientific community show that each of them was published, because their respective author was not convinced by the hitherto known theories, adding another one her- or himself, which, again, did not satisfy other researchers.

During the last 4 years, two articles appeared which discussed the evolution of the upright human posture and locomotion from a perspective different from the one chosen here. While Preuschoft (2004) concentrates on biomechanics, Crompton et al. (2008) focus on paleoanatomy. Moreover, two new explanations for the evolution of human bipedalism appeared (Sylvester 2006; Skoyles 2006), which will also be discussed later in this text. But still, most of the recently emerged theories did not automatically devalue former theories in part or as a whole, and many of them produced respectable grounds, or at least partly, uncontested arguments. These hypotheses may serve as constructive elements for the synthesis of a conciliatory, updated theoretical construction. After a theoretical outlook and a discussion of some Miocene and later hominoid and hominid fossils, this article presents an evaluation and discussion of the more influential, and finally, the more recent hypotheses, aiming to reconcile quite a number of aspects of the various theories.

The functional anatomy of about 20-Ma-old Morotopithecus from Uganda indicates “features of modern ape locomotion, reflecting climbing and/or suspensory activity” (Nakatsukasa 2004, see also MacLatchy et al. 2000). However, this fossil genus is even more derived than the gibbons (Hylobates; Young and MacLatchy 2004) and for this reason, cannot be a stepping-stone in the evolution of human bipedalism.

Nacholapithecus, a further early hominoid from the early middle Miocene of northern Kenya (Nakatsukasa et al. 2000), is one of the most complete hominoid skeletons ever found (Ishida et al. 2004). Quite a number of postcranial anatomical traits of this hardly 18 or up to 22 kg weighing hominoid were discussed comprehensively by Nakatsukasa (2004). The axial skeleton, as well as functional limb anatomy, “suggest that Nacholapithecus with arboreal climbing abilities (Sénut et al. 2004) engaged in orthograde behaviours (e.g., vertical climbing, hoisting, bridging) more frequently than did Proconsul” (Nakatsukasa 2004). Although less well documented by fossil remains, another hominoid of about the same age, Kenyapithecus africanus was suggested to have shown scansorial activities combined with a forelimb-dominated positional repertoire and with a considerable terrestrial component (McCrossin et al. 1998).

In contrast to Proconsul, and especially to Nacholapithecus, Pierolapithecus catalaunicus, a fossil from the Middle Miocene from Spain being some 12.5 to 13 Ma old and “probably close to the last common ancestor of great apes and humans” (Moyà-Solà et al. 2004), has a rather primitive hand, which is more similar to that of man than to extant apes. According to the same authors, the morphology of the Pierolapithecus hand indicates a considerable portion of palmigrade quadrupedalism rather than vertical climbing or suspension, and suggests that vertical climbing may be an “original modern ape adaptation”. Although the interpretation of Pierolapithecus is still a matter of debate, Schrenk (2008) stated that the combination of its robust lumbar spine in this fossil primate with the abovementioned features was essential for the ancestors of later hominids. Stated with some prudence, all this could perhaps make it a suitable prototype for a later hominid branch in primate phylogeny.

According to Stauffer et al. (2001), molecular phylogeny suggests that the lineages leading to extant gorillas on the one hand and to humans on the other, started to diverge about 6.4 ± 1.5 Ma ago, while the bifurcation of the human and chimpanzee lineages may be dated about 5.4 ± 1.1 Ma bp. This agrees at least roughly with three rather recent paleontological discoveries, which are all claimed to belong to this phase of human evolution. The oldest is Sahelanthropus tchadensis and has been redated recently between 6.5 and 7.4 Ma (Brunet et al. 2005). Its authors (Brunet et al. 2004) state: “The new hominid is probably temporally close to the common ancestor of chimpanzees and humans but displays a unique combination of primitive and derived characters that clearly shows a close relationship to later hominids rather than with chimpanzees or gorilla”, adding (2005) that supplementing findings “confirm the morphological differences between S. tchadensis and African apes”. Zollikofer et al. (2005) suggest: “Analysis of the basicranium further indicates that S. tchadensis might have been an upright biped, suggesting that bipedalism was present in the earliest known hominids, and probably arose soon after the divergence of the chimpanzee and human lineages.” But they qualify: “However, postcranial evidence will be necessary to test more rigorously the hypothesis that S. tchadensis... was a biped”. According to genetic evidence, the human–chimpanzee speciation was estimated, after a rather long genetic exchange between the two lineages, to have occurred only <5.4 Ma ago (Patterson et al. 2006). Especially, as their results have been contested (Wakeley 2008; Patterson et al. 2008), a reconciliation of paleontological and genetic estimations has still to be achieved.

Orrorin tugenensis was dated to be approximately 6 Ma old (Fig. 1; Sénut et al. 2001; contra Patterson et al. 2006, cf. above). With respect to locomotor adaptations, Sénut (2003) notes: “The position of the femoral head, the morphology of the femoral neck, the presence of a groove for the M. obturator externus and the cortical deposits on the femoral neck suggest that this hominid was a biped. Its traits are also closer to those of later Homo than of Australopithecines in the position of the lesser trochanter, the morphology of the femoral neck and the proximal shaft. However, the humerus clearly resembles those of Australopithecines and suggests climbing adaptations” (see also Pickford et al. 2002). This notion has recently been qualified by a morphological study contradicting Sénut and at the same time supporting her interpretation, “indicating that O. tugenensis was bipedal but is not more closely related to Homo than to Australopithecus” (Richmond and Jungers 2008).

Ardipithecus ramidus kadabba fossils were recovered from strata of the Middle Awash area of Ethiopia that date, according to paleontological methods, “to 5.2−5.8 Ma and are associated with a wooded paleoenvironment. These Late Miocene fossils... represent the earliest definitive evidence of the hominid clade” (Haile-Selassie 2001). Postcranial remains consisting of fragmentary humerus and ulna, as well as few hand and foot phalanges, have not been interpreted yet with respect to locomotion and show a mosaic of ape and hominid characters.

All this indicates that the posture and locomotion of the first hominoid ancestors of human beings were probably neither specialised for arboreal nor for terrestrial posture and locomotion. When becoming definitive bipeds, at the latest, however, they started to reduce the arboreal portion of their lives to varying percentages in different species (Niemitz 2004). Gebo (1996) concluded from his comparative anatomical studies that our ancestor was “quite capable as a climber” and yet, “...hominid bipedalism simply continues the terrestrial travelling trend evident in African apes”. This is in general agreement with Hunt's interpretation of Australopithecus afarensis (1998), stating terrestrial and arboreal functions. Stern (2000), resuming decades of experience, noted: “... ever increasing numbers of anthropologists are accepting the arboreal behavior as an adaptively significant component of early australopithecine behavior... I was never as certain about the nature of A. afarensis bipedalism as I was about its retained adaptations for movement in the trees. I am no more or less certain now.” As later fossils of Australopithecus and Homo are bipedal as well, they are considered elsewhere in the text for various reasons given there.

The Watching Out Hypothesis In 1959, Dart suggested that the visual advantage of being able to survey the surrounding might have favoured the adoption of a human erect posture. Rose (1976) took the behaviour of free-ranging baboons as a nonhuman model for our quadruped ancestors. Besides feeding posture (see below), he argued that baboons may sometimes stand up in order to watch out for various reasons including for dangerous predators such as hyenas or leopards. However, in a later study on bipedal behaviour of habituated chimpanzees (Hunt 1994), scanning the environment was recorded only twice out of 97 bipedal events, which is certainly insufficient to support the Watching Out Hypothesis. Including baboons, the same author stated: “Chimpanzees stood bipedally more often than baboons (0.3% versus never)” (Hunt 1996).

The Freeing of the Hands Hypothesis The second hypothesis to be discussed here goes even further back in time and is based on thoughts Charles Darwin wrote down in his Descent of Man (1871). When the fossil documentation of our past and the knowledge about the behaviour of monkeys and apes were still poor, Hewes (1961) noted: “A long list of names can be cited for the notion that 'freeing of the hands' for tool-using, weapon-handling, food-gathering, and self-defense was the main cause of hominid bipedalism (Darwin 1871; Haeckel 1900; Carter 1953; Hill 1954; Shapiro 1956; Washburn 1959: 24; Oakley 1960: 322).” This hypothesis has received widespread acceptance and was to be found in scientific as well as in popular literature (Ardey 1961). But, Hewes (1961, p. 694) also stated that all primates—including humans—do not assume bipedal postures for manipulation (see Carrying Food Hypothesis). Thus, while food transport may be considered as one of the possible contributing factors, the behaviour of man and of all nonhuman primate models indicates that manipulation is neither a trigger nor a promoter of standing or walking upright. Therefore, the freed hands were certainly used for quite a variety of purposes later in our biohistory.

The Throwing Hypothesis (Kirschmann 1999) The motor control and skill of throwing requires numerous anatomical adaptations, not only in the shoulder girdle and arm, but in the brain as well (e.g., Hore et al. 1995, 1999; Young 2003). In an unpublished manuscript, Kirschmann and Young write: “We suggest, that the specialization to use weapons... was a key adaptation for early hominids. The upright gait was developed as a trailing adaptation.” Also, they declare the oldest stone tools of humankind to be 2.6 Ma old, while the first indications for an upright walk have been shown to be approximately 6 Ma old (Richmond and Jungers 2008). Since this is an unbridgeable time span of more than 3 Ma, the Throwing Hypothesis may contribute to the issue of human evolution elsewhere but not with respect to human bipedalism. Also, Dunsworth et al. (2003) found that the evolution of throwing “... possibly occurred with the emergence of Homo erectus...”.We know today that hominin orthograde locomotion evolved at least almost two if not several million years before tool production and before an accelerated brain growth is documented by hominin fossils. While the acquisition of upright locomotion is suggested to extend between at least 4.2 and about 6 Ma back in time (e.g., Pickford et al. 2002), intensified and elaborated tool use beyond the chimpanzee scale appeared only 2 or 2.5 Ma before present (Semaw et al. 1997). The absence of evidence in the fossil record for tool use does, indeed, not necessarily mean the evidence for absence. But chimpanzees, as an extant model, use decayable materials such as wood as well as stone (e.g., Boesch 1999), and it seems most unlikely to the present author that, for at least 1.8 Ma, our ancestors selected decayable materials for their first tools exclusively.

The Infant Carrying Hypothesis Several hypotheses with relation to bipedal transport of loads were published at different times, each of them stressing other causes or parameters. The belief that women might have carried their babies was discussed by Etkin (1954), who argued that an upright locomotion in a hunting society might have been more effective if the females stood up and walked bipedally carrying their offspring on their waists. Also, Washburn (1967) stated the opinion that carrying babies might have elicited bipedal locomotion.From their observations of infant carrying in savannah baboons of the Amboseli National Park, Kenya, Altman and Samuels (1992) calculated that infant transport by the mother consumes an approximate average of almost 350 kJ/day. However, for several behavioural reasons (necessary detours for picking the baby up, etc.), an almost double average value of 670 kJ/day has to be estimated. Only recently, Etkin's child-carrying hypothesis was tested in human beings quantifying the energy expended by measuring O₂ consumption (Watson et al. 2008). These authors found that “...asymmetrical weights, such as a mannequin carried on the hip... are more energetically costly... and are...the most metabolically expensive methods of carrying”. Wall-Scheffler et al. (2007) calculated that “the burden of carrying an infant in one's arms... seems to have the potential to be a greater energetic burden even than lactation”. They found that, after bipedalism had been established, tool use like that of a sling could reduce the caloric energy needed for passive child transport significantly.Only as a matter of completeness, it may be added here that the energetic costs of carrying an infant has not been calculated yet for any ape. Erect bipedal infant transport seems to happen only when wading (Fig. 4e), although rare special situations of upright infant carrying on the dry land cannot be precluded. As “all Old World monkeys (Cercopithecidae), e.g. baboons, have solved the problem of an easy, fast and safe transport of their infants in an optimal fashion” (Fig. 2; Niemitz 2007), an evolutionary invention of a new, specifically human way of carrying little children was completely unnecessary, and after the calculations presented here, the Infant Carrying Hypothesis for the evolution of an upright posture and gait can be regarded as refuted.

The Reaching for Food Hypothesis Jolly (1970) favoured a more postural hypothesis for the evolution of early bipedalism suggesting that our ancestors, being placed in a savannah scenario, were forced to pick mainly high growing ears of grass or other kinds of high growing food. Rose (1976) presented a variety of triggers for baboons to assume erect postures. The most important ones to assume upright postures were fresh sprouts or inviting buds on a high bush. Wrangham (1980), observing gelada baboons (Theropithecus gelada), noted that they would sometimes move bipedally in a somewhat squatting position when picking seeds and other small food items from the ground. However, distances longer than 1 m were always covered in a quadrupedal manner. Therefore, these latter observations cannot serve as explanation neither for habitual upright postures nor for bipedal walking locomotion.However, recording the behaviour of habituated wild chimpanzees for about 700 h, Hunt (1996) found that “eighty per cent of chimpanzee bipedalism was during feeding; 86% of all bipedal activity arboreally and 70% terrestrially, and an overwhelming percentage of these bouts were postural (95%)”. Only once, he observed a chimpanzee moving bipedally (Hunt 1994, 1996), describing it as “short distance within-site shuffling” and elsewhere, as “...shifting one contact point, while minutely displacing the body's center of gravity...” (1994). In contrast, bipedal walking in the context of feeding behaviour was never observed. Yet, Stern (2000) refers to Rose (1984, 1991) and Hunt (1994, 1998) resuming: “Along with others, I believe the bipedal adaptation first arose to improve access to food sources close to the ground, movement between such sources, or both.”In such a situation of harvesting food, bipedality is often assisted by pulling or support of the upper extremity (Wrangham 1980; Hunt 1994). Besides adaptations for standing up, reaching for high food often and repeatedly may also contribute to the evolutionary acquisition of greater excursions in the shoulder joint: “The food gathering function of chimpanzee bipedalism suggests that hominid bipedalism may have evolved in conjunction with arm-hinging as a specialized feeding adaptation that allowed for efficient harvesting of fruits among open-forest-woodland trees” (Hunt 1994). This notion can be reconciled in certain aspects with the hypotheses of food carrying, canopy scrambling, and wading (see below).

The Carrying Food or Provisioning Hypothesis According to the title of his article “Food transport and the origin of hominoid bipedalism”, Hewes (1961) notes: “My thesis is that the arms and hands were needed for something other that locomotion: the carrying of food, and that only a bipedal gait could permit them to fulfill this need with real efficiency.... Köhler mentions that his chimpanzees walked upright when their hands were full (with food, CN) (1959:278-79)... Leakey (1959) observed such behaviour in his pet baboon when carrying ears of maize”. Hewes presents a drawing of a macaque carrying some bulky food bipedally, and I remember a photograph of Macaca fuscata on Koshima Island carrying a sweet potato in this fashion. However, to the knowledge of this author, wild great apes rather avoid bipedal locomotion when transporting food. Captive gorillas and chimpanzees may even transport up to four food items in both hands, one foot, and with their mouth simultaneously in a quadrupedal fashion (own observation).Nevertheless, bipedal food carrying is certainly performed sometimes and may similarly have occurred in our ancestors. Although bipedalism is not often performed in the context of food carrying, it may perhaps remain a possible contributing factor to the evolution of habitual upright bipedalism. If an erect posture is accepted as a behavioural stage before the evolution of the upright walk, feeding adaptations may well have contributed to the emergence of later modern human bipedalism. However, further functional factors were necessary to establish the habitual erect posture and especially, habitual upright walking behaviour.In an early article, Lovejoy (1981) sketches a behavioural sequence: “It is likely that the need to carry significant amounts of food was a strong selective factor in favour of primitive material culture. Although it is not a significant shift from primitive tools of the type chimpanzees use today, such as ‘termite sticks’... to simple and readily available natural articles that could be used to enhance carrying ability, it is a significant shift from such primitive and occasional tool use to the stone tools of basal Pleistocene.” He argues that provisioning was necessary, being one feature of the intensified parental care of early human antecedents increasing the survival rate of infants. According to this idea, male provisioning mediates between the evolution of bipedalism, tool-use and human monogamy (Lovejoy 1981).

The Display Hypothesis Although this hypothesis is one of the savannah theories, it shall be discussed, because some of their reasoning might also fit into a more wooded habitat. From a current perspective, this hypothesis is pleasant and somewhat disturbing at the same time: “We propose that the bipedal displays of pre-hominids led to resolution of intragroup conflict almost exclusively by ritual and gesture and only very rarely deteriorated into injurious attacks.” The attractive aspect is that bipedal threat displays are assumed to combine the evolution of bipedalism with the acquisition of a conflict-solving signal. On the other hand, the disturbing fact is that, it can definitely not be decided how often a bipedal display may have led to either an appeasement or to an open fight. Today, speculations of this kind on the putative communication of our ancestors can certainly not serve as a fundament for any evolutionary hypothesis.One of the central elements of the hypothesis is the transmission of females from group to group: “We propose that female transmission ultimately led to the successful establishment of habitual terrestrial bipedalism...” (Jablonski and Chaplin 1993). On the one hand, these authors point out: “It can be envisioned that the performance of unique bipedal displays, occasionally resulted, by dint of their sheer novelty, in the winning of encounters between males from different groups by their surprising effect” (my italics). They continue: “The offspring... may have stood up more and thus achieved higher status by this novel behaviour”. On the other hand, however, the authors do not consider how a novelty may maintain a surprising long-term effect, even if it is repeated innumerable times over many generations in order to establish a habitual bipedal posture and gait within the population.With the exception of a short yet not discussed remark on juvenile play, not a single other trigger for an increase of bipedal behaviour in quadrupedal forerunners is discussed. By focussing exclusively on their own explanation, Jablonski and Chaplin were unable to discover or to weigh the amount of a possible contribution of display behaviour to the evolution of bipedalism. Display behaviour is per definition associated with short behavioural episodes. With regard to the behaviour of wild free-ranging chimpanzees, Hunt (1996) found that “social display (1%)” was “rare”. Therefore, this hypothesis does not seem suitable to explain why displaying individuals—and especially the other members of their group—should have remained upright after the actual social situation had been solved.

The Orthograde Scrambling Hypothesis Studying female orang-utans, Sugardjito and van Hooff (1986) as well as Cant (1987) point out that, most of the time, they observed a four-handed scrambling fashion for travelling in the canopy of the trees—not clearly specified as orthograde or pronograde in both sources—combined with brachiation and tree swaying, adding that quadrupedal walking and vertical climbing occurs much less often. The Orthograde Scrambling Hypothesis was first presented to a Congress of the Werner Reimers Stiftung and Forschungsinstitut Senckenberg in 1999 (Crompton et al. 2003) and appeared again in a revisited and supplemented new version (Thorpe et al. 2007a). Recently, it was comprehensively discussed in a somewhat different context by Crompton et al. (2008). In quite a number of statements and arguments, it repeats the considerations of Gebo (1996), who had concluded: “It is not climbing but traveling locomotion that best divides the living apes into discrete groups... Orangutans travel via scrambling, brachiation, and tree swaying. Among orang-utans, only males use the ground for traveling over long distances, and that only occasionally (Galdikas 1979, 1988; Rodman 1984)”. As its name implies, the Orthograde Scrambling Hypothesis points out, more than it has been done by Gebo, that orang-utans often adopt a more or less upright posture, especially when suspension is needed on rather thin branches in order to secure the individuals from falling down. Its advocates argue that this upright travelling locomotion of orang-utans in the crown of high trees represents an ancestral behaviour which has been retained in later hominin bipedalism. Thorpe et al. (2007b) observed that orang-utans extend hip and knee joints under circumstances that would be preadaptive to the fully extended hominin knees and hip when walking upright. The authors find that, “during the Miocene... orangutan ancestors became more specialized, and restricted to, shrinking closed canopy forest that could be traversed at canopy level”. Gebo (1996) underlines this notion of locomotor specialisation by comparing the adaptations of extant orang-utans to sloths, because of the far-reaching anatomical alterations of the upper extremity and especially of the hand of Pongo.The specialisations of extant orang-utans are manifold and not at all restricted to locomotor features, but also to marked ecological and behavioural differences from African apes. Some of them are mentioned by Thorpe et al. (2007a), but include also, e.g., enamel structures of the teeth and many further traits (cf. Niemitz 2002, 2004). It has been recognised from molecular data (e.g. Friday 1992) as well as from multivariate functional biometry that Pongo clusters, in many respects, much more closely and more easily with gibbons than with African apes: As early as in 1979, based on 23 dimensional data of total body proportions, Oxnard (1979) demonstrated this close functional and phyletic relationship (see also Oxnard 1983). The bifurcation of the orang-utan clade from the chimpanzee, gorilla, and human branch is commonly dated between some 11.3±1.3 Ma ago (Stauffer et al. 2001) and about 15 Ma (Fig. 1) resulting in, roughly, between 20 and almost 30 Ma of separate evolution in both the orang-utan and the human branch (e.g., Begun 2003; Moyà-Solà et al. 2004). This bears a variety of obstacles for the hypothesis discussed here, because the danger to misinterpret possible autapomorphic features of the specialised orang-utans after their long genetic separation from the African apes.In their predominantly superb anatomical review, Crompton et al. (2008) state: “Only one living great ape, the orang-utan, has been observed to engage in suspensory quadrupedalism: neither the panins nor the gorillines exhibit this behaviour (Thorpe and Crompton 2006). The absence of such gait in panins and gorillines might, however, be a simple statistical consequence of much more exclusive arboreality (my emphasis) in Pongo”. The locomotor similarities of orang-utans to man stated above, together with their relative phylogenetic distance to Homo in relation to the distance between Homo and the panins/gorillines, raise further questions about the origin of the similarities described.Also, both opinions, the one by Thorpe and colleagues on the one hand and the second by Gebo on the other, have much in common, although the former authors contradict the latter one decidedly. To mention only one example, both stress the appearance of a heel strike—which is also a feature of modern human walking—by the orang-utan while walking but interpret its function differently. While Gebo (1996), citing Cant (1987), calculated that “... we can assume that heel-strike plantigrady is rare in the arboreal locomotion in orang-utans”, Crompton et al. (2008) emphasise the occurrence of the heel-strike in the orang-utan and discuss this phenomenon together with “the highly extended hip and knee in voluntary bipedalism of orang-utans” (p. 506). The heel-strike shown in their photograph, however, occurs with flexed hip and extended knee, while the “clearly double humped curves in 25% of cases” exemplified by a graph, shows a clearly unimodal curve with only one hump.With reference to their comprehensive observations in the field, Thorpe et al. (2007b) point out: “Bipedalism was strongly associated with locomotion on multiple supports and with locomotion on the smallest support diameters” and simultaneous suspension with the forelimbs going along with an orthograde posture. This is in agreement with observations of Hunt in chimpanzees (Hunt 1996). Yet, Thorpe and coauthors (2007a) derive a later biped human locomotor behaviour from a kind of terminal branch feeding: “The advantage of hand-assisted bipedality is that the hand assistance ensures maximum safety while the bipedalism enables an unloaded hand to reach out for feeding... in the peripheral branches, where the majority of the preferred foods are situated”. While the lighter gibbons have optimised brachiating suspension for locomotion and terminal branch feeding, according to this proposition, the heavier orang-utans may have evolved bipedalism combined with an orthograde suspension for the same purposes. Nevertheless, this orthograde posture of orang-utans while foraging with suspension stress in the upper limb is paralleled—if evolved separately or not—by the feeding behaviour of chimpanzees discussed above (Hunt 1996).This common specialisation for crown dwelling and terminal branch feeding behaviour may underline the extraordinarily similar morphometrics of orang-utans and gibbons stated above. Their rather minor differences may mainly be associated with their body mass, substrate use for locomotion, and corresponding joint mobility (cf. Isler 2002; Isler and Thorpe 2003) and thus, ultimately, for their differences in body weight. Among mammals, orang-utans belong to the heaviest canopy dwellers of the world. They are an extraordinary exception requiring extraordinary specialisations.

The Scavenging Hypothesis Two publications, which appeared in the same year, postulate regular scavenging of our ancestors as important for human evolution (Eiseley 1953; Bartholomew and Birdsell 1953). From time to time, carrion robbery was proposed as a central factor of food acquisition at the early stages of human evolution. Some 40 years later, Blumenshine and Cavallo (1992) revived this theory adding a number of substantiations. However, the updated Scavenging Hypothesis was associated with the definitely bipedal australopithecines and separated from the evolution of an upright posture and locomotion. Thus, the hypothesis of carrion robbers is no longer considered in relation to the evolution of upright hominids but refers to later times in our biohistory. Nevertheless, this new version is relevant in so far as its authors located this hypothesis in a new scenario. They concluded that fresh carrion as an important food resource was available with greatest probability in gallery forests along the rivers and streams: “The earliest hominids may have come across defleshed kills while foraging for plants in thin ribbons of riparian woodlands” (Blumenshine and Cavallo 1992). This remark is in agreement with the Amphibian Generalist Theory (see below).

The Aquatic Ancestor Hypothesis In his book on the specifically human evolution, Westenhöfer (1942) dedicated one short chapter to “The hypothetical aquatic life?” of prehuman and even preprimate mammals. In his title, he began the chapter with an interrogation mark and he concluded his reasoning with the careful notion: “Such, at the first glance, so improbable hypotheses are, of course, just meant to be stimulations, in which directions research might be done, like a detective, who pursues also the most improbable indication alongside others, which seem to be more promising.” Later, the Aquatic Ancestor Hypothesis (AAH) was renamed as the so-called Aquatic Ape Theory or AAT (Morgan 1990).To the author's conviction, the Aquatic Ape Theory, as described by Morgan (1990) did neither fulfil the criteria of a hypothesis nor of a theory. And, in fact, by that time, its author did not intend to postulate a hypothesis of her own: Instead, she listed analogies of features of savannah type mammals on the one hand and of aquatic mammals and man on the other, asking the scientific community for explanations other than a common aquatic ancestor of extant man. Nevertheless, it was a rewarding enterprise by Roede and colleagues to compile a volume of not less than 22 chapters, partly by renowned scientists, on the relationship of human ancestors to water (Roede et al. 1991). As an example for that volume, Preuschoft and Preuschoft (1991) presented a biomechanical study, which showed that humans are far too bad swimmers ever to have been derived from a swimming ape ancestor. More recently, Morgan (1997) has stressed more the littoral aspects of her ideas, approaching, to some extent, Niemitz's conclusions (2000, 2004). And indeed, in a weaker version, there are positive votes to be noted (e.g., Hrdy 1999). Groves and Cameron (2004) wrote: “... although the authors shy away from more speculative reconstructions in favor of phylogenetic scenarios, we insist that the AAH take its place in the battery of possible functional scenarios for hominin divergence”.

The Thermoregulation Hypothesis Like in other hypotheses on the evolution of the erect posture, some of its basic ideas were thought and published a long time before the hypothesis itself was formulated. In 1967, Ward and Underwood had calculated the advantage of an upright stance with respect to the exposure to equatorial solar radiation. It lasted more than 15 years, until Wheeler (1984) wrote: “... it is proposed that hominid bipedality could have evolved as an adaptation to alleviate what is probably the single most stressing problem of open equatorial environments: heat gain from direct solar radiation”. Hence, that author derives the evolution of human bipedality from only one single assumed cause.Moreover, in all of his main articles (1984, 1985, 1990, 1991a, b), Wheeler expressly related this evolutionary development to a savannah scenario, which has been shown to refer to a later stage in already bipedal hominins. Also, applying simulation calculations, Chaplin et al. (1994) contradicted Wheeler concluding “that thermoregulatory considerations were not sufficient to overcome the difficult morphological transition from quadruped to committed biped”. The reply of Wheeler (1994), although justifying some of his own results successfully, was not suitable to abolish the main difficulties to accept his hypothesis as a whole. Nevertheless, I consider his findings to be an important contribution to the understanding of later human evolution as a walker and runner.

The energetic and anatomical threshold from quadrupedalism to bipedalism, as described above, can be crossed best by postulating a partly arboreal, partly terrestrial quadruped ancestor wading bipedally in the water along the gallery forests and on the shores of the African lakes. This is consistent with energetic calculations, biomechanical findings, and with the present fossil documentation. Certainly, this wading phase has to be regarded mainly as a Miocene process, until the first definitive bipeds had evolved between 7 and 6 Ma bp. Further, fossils besides O. tugenensis will be unearthed and permit to establish a more precise time scale. Whether or not extensive shore dwelling continued further on for some time in fossil representatives of the human clade is poorly known, although shore use was demonstrated continuously up to modern Homo sapiens (Niemitz 2004).