1 11: 47–60, 2018 2 doi: 10.4033/iee.2018.11.6.n 3 © 2018 The Author 4 iee Received 5 June 2018; Accepted 1 October 2018 5 6 7 8 New Idea 9 10 11 The sexual selection of hominin bipedalism 12 13 14 Michael T. Dale 15 16 Michael T. Dale ([email protected]), Department of Philosophy, The University of Texas at Austin, 2210 17 Speedway, C3500, Austin, TX, USA 78712 18 19 20 Abstract Introduction 21 22 In this article, I advance a novel hypothesis on the Why did our hominin ancestors become bipedal? As it 23 evolution of hominin bipedalism. I begin by arguing is likely that bipedalism preceded the expansion of the 24 extensively for how the transition to bipedalism must brain, language, tool-use, loss of body hair, and many 25 have been problematic for hominins during the Neogene. other traits idiosyncratic to Homo sapiens (Tuttle 2014), 26 Due to this and the fact that no other primate has made bipedalism is perhaps the first step (!) in the development 27 the unusual switch to bipedalism, it seems likely that the of modern humanity. While many hypotheses have been 28 selection pressure towards bipedalism was unusually put forward, none have made a thoroughly convincing 29 strong. With this in mind, I briefly lay out some of the case, and thus this pivotal transition in our evolutionary 30 most promising hypotheses on the evolutionary origin of history remains a mystery. 31 hominin bipedalism and show how most, if not all, fail in In this article, I advance a novel hypothesis regarding 32 the face of the need for an unusually strong selection the evolution of hominin bipedalism. I will begin by 33 pressure. For example, some hypotheses maintain that discussing what we know about the timeline of the 34 hominins became bipedal so they could use their hands evolution of bipedalism. I will then argue extensively for 35 for carrying infants, food, or other valuable objects. But how the transition to bipedalism must have been 36 extant apes are able to carry objects in one of their front problematic for hominins during the Neogene. Due to this 37 limbs (while walking with the other three), and thus it and the fact that no other primate has made the unusual 38 does not seem plausible that our hominin ancestors went switch to bipedalism, it seems likely that the selection 39 through the troublesome transition to bipedalism just so pressure towards bipedalism was unusually strong. With 40 they could carry objects a little more efficiently. After I this in mind, I will briefly lay out some of the most 41 show that past hypotheses are wanting in the face of this promising hypotheses on the evolutionary origin of 42 challenge, I argue that there is only one selection pressure hominin bipedalism and show how most, if not all, fail in 43 powerful enough to instigate a strange and problematic the face of the need for an unusually strong selection 44 evolutionary adaptation like bipedalism, and that is pressure. Next, I will argue that there is only one selection 45 sexual selection. Specifically, from the fact that bipedal pressure powerful enough to instigate a strange and 46 locomotion is an important strategy for intimidating problematic evolutionary adaptation like bipedalism, and 47 others and ascending the dominance hierarchy in extant that is sexual selection. Specifically, I will contend that 48 apes, I argue that for no particular selective reason for no particular selective reason bipedal locomotion 49 bipedal locomotion became a signal for high fitness became a signal for high fitness (much as a large and 50 (much as a large and intricate tail became a signal for intricate tail became a signal for high fitness for peahens), 51 high fitness for peahens), and this led to the trait being and this led to the trait being continuously reinforced in 52 continuously reinforced in spite of all its deleterious spite of all its deleterious fitness consequences. Indeed, 53 fitness consequences. not only does this seem like the most plausible reason that 54 such a troublesome transition occurred, but it also fits 55 Keywords: bipedalism, evolution, hominin, quadru- well with the fossil record. I end the paper by discussing 56 pedalism, handicap, sexual selection, upright. iee 11 (2018) This work is licensed under a Creative Commons Attribution 3.0 License. 47 some of the interesting consequences this hypothesis has (Brunet et al. 2002), but many claim that the evidence is for the evolutionary history of Homo sapiens. unreliable (Pickford 2005, Wolpoff et al. 2002, 2006, Schwartz 2004, Senut 2012). Interestingly, Senut et al. Evolutionary Timeline (2001) firmly argue that the femora of Orrorin tugenensis (6.1–5.7 Ma) indicate that the species was terrestrially The earliest uncontroversial evidence of obligate bipedal while the phalanges and humerus imply that it bipedal1 Homininae is the 3.66 Ma Laetoli (site G) was also arboreal. But again, it is difficult to make such footprints uncovered 45 km south of Olduvai Gorge. claims convincingly, as only a few bones of O. tugenensis These preserved footprint trails are the result of three have been recovered. Moreover, just because there are bipedal hominins walking through volcanic ash (Tuttle signs of bipedality doesn’t mean that the species was a 2014: 143). The hominins that made them are considered terrestrial biped. Perhaps O. tugenensis was a bipedal obligately bipedal because they walked through 27 branch walker, similar to Pongo. meters of open habitat without leaving a single handprint. Overall, as the previous paragraphs have illustrated, Moreover, even though there is some contention over hominoid fossils from the Late Miocene are few and far whether it was the same creature that left the Laetoli between. Notwithstanding the few bones from the species footprints, Australopithecus afarensis (Lucy), a hominin just discussed (which really don’t tell us much about that lived between 3.9 and 2.9 Ma, was most likely locomotion specifics) and some miscellaneous teeth and bipedal (Lovejoy 1988). Interestingly, though, A. mandibles (e.g. Samburupithecus kiptalami, Chorora- afarensis also had adaptations for arboreality, such as pithecus abyssinicus, and Nakalipithecus nakayamai), curved phalanges, and ape-like scapula, a long pisiform this time period represents a significant gap in the fossil bone, and a laterally flared ilium (Tuttle 1981, Green and record, which is unfortunate as it was likely a pivotal Alemseged 2012). A plausible interpretation of the period in hominoid evolution. evidence is that A. afarensis was terrestrially bipedal but However, we do know that we most likely evolved still engaged in significant amounts of arboreal activity. from quadrupedal primates that existed during the Middle However, this is ultimately just speculation. All we can Miocene. While some middle Miocene primate fossils really state is that it is very likely that a terrestrially show evidence of semi-terrestriality (e.g. Kenyapithecus bipedal hominin existed in east Africa around 3 to 4 Ma. and Equatorrius), all of them were quadrupedal animals If we look back further than the mid-Pliocene, the that spent a significant amount of time climbing and evidence becomes murkier. While there is a compelling maneuvering in trees (Senut 2012). Thus, up until 10 Ma., case that Ardipithecus ramidus (4.4 Ma) was a woodland our ancestors were primarily arboreal, quadrupedal dwelling hominin capable of bipedalism (Lovejoy 2009, primates. Then, sometime between 10 Ma. and 4 Ma., Lovejoy et al. 2009), it is unclear how much bipedal there was a major transition to bipedalism. Unfortunately, activity Ar. ramidus engaged in. For one, its opposable this is about all we can say for sure about the timeline of toes (Lovejoy et al. 2009), which seem to be much more the evolution of hominin bipedalism, but we can put adapted to grasping branches, would have made any type forward one important (yet tentative) conclusion. bipedal gait primitive and awkward. Indeed, even Because many of the hominins that existed between 10 Lovejoy (2009) (one of the prominent advocates of a and 4 Ma. show adaptations for arboreality, terrestriality, bipedal Ar. ramidus) admits that upright walking would and bipedalism, it is likely that the transition to bipedal- not have been an energy efficient option for the species. ism was a very slow, complicated process. Indeed, this Moreover, its upper pelvis and large femoral shaft also makes sense because becoming bipedal is no straight- indicate that Ar. ramidus spent a significant amount of forward matter. As the next section will show in more time in the trees (White et al. 2009, Senut 2012). With all detail, not only must the physiology of a transitioning this said, however, it is important to understand that many biped undergo significant change, but the animal’s pivotal aspects of the skeleton of Ar. ramidus have yet to behavior, diet, and social life must revolutionize, as well. be discovered and thus it is difficult to put forward a truly Thus, the transition to bipedality was most likely a convincing argument either way (Senut 2012, Tuttle gradual, tinkered process that occurred over many 2014: 246). millennia. Going back even further, not enough fossils of Ardipithecus kadabba (5.6–5.2 Ma) have been found to The Unwelcome Consequences of Bipedalism make a compelling argument about the locomotion of the species (Stern and Susman 1983, Susman et al. 1984, Homo sapiens is the only bipedal primate, and there is Stern 2000). Some maintain that that the cranium of good reason why all other primates are not bipedal. Sahelanthropus tchadensis (6–7 Ma) hints at bipedality Indeed, the physiology and skeleton of primates are so 1 An obligately bipedal animal is one that uses bipedalism a primary means of locomotion.