Bipedalism

Anatomical features associated with bipedalism (Harcourt-Smith 2010)

-Anteriorly positioned and horizontal foramen magnum: reflects vertical positioning of the spine

-S-shaped spine: transfer of weight from upright trunk to hip joint

-short, curved, and wide iliac blades: support of upright trunk

-wide sacrum: increased loading on pelvis

-large acetabulum, thick inferior neck of femur: increased loading through hip joint

-femur angles in medially from hip to knee: helps with balance

-relatively long and robust ankle region: increases efficiency of foot leverage

-arched foot: shock absorption

-fully abducted big toe: efficient weight transfer

Australopithecus remains (Renfrew and Bahn 2020)

-Lucy: 40% complete australopithecine fossil from 3.18 mya at Hadar, Ethiopia

-could prob walk but had hands and feet for grasping, others believe fully bipedal b/c had rigid arches in feet

-complete skull of A. africanus, 2.3 mya, “Mrs. Ples” from S. Africa had semi-circular inner ear canals with tilt suggesting upright walking (helps with balance) but greater forward-sloping angle than modern humans

-canal angle implies mixed bipedalism w/ tree climbing

Laetoli footprints (Renfrew and Bahn 2020)

-fossilized footprints at Laetoli, Tanzania

-discovered by Mary Leakey

-left by small hominins 3.75-3.6 mya

-raised arch, round heel, pronounced ball, forward pointing big toe, resembles upright-walking humans

-gait similar to modern humans

Ape vs. Hominin locomotive differences (Harcourt-Smith 2010)

-striding bipedalism extremely rare other than in humans, comes with special anatomical features

-loss of arboreal capacities like opposable big toe, curved toe bones, shoulder joint adapatations

-chimpanzees rarely biped, if so usually postural and not locomotive

-can be used to determine those closer to humans after LCA, significantly predated other modifications like increased brain size

Chimp-Human LCA (Harcourt-Smith 2010)

-5-7 mya based on genetic evidence

-some think knuckle walker but don’t have single miocene ape w/ knuckle walking features

-prob had a generalized locomotive strategy, capable of many locomotor behaviors

Savannah hypothesis refutation (Harcourt-Smith 2010)

-hominins older than 4 mya (up to 7 mya), bipedal before Savannah opened up

-however early hominins not obligate bipeds, modern human-like stride did coincide with opening up of grasslands

-prob multiple phases in development of bipedalism

Ardipithecus (Harcourt-Smith 2010)

-Ardipithecus discovered in Ethiopia, 4.4 mya, considered hominin b/c anteriorly placed foramen magnum, modified C/P3 complex, ulna morphology

-foramen magnum placement suggests bipedalism

-arm and hand bones suited for climbing, opposable big toe, no knuckle walking features

-shorter iliac blades than apes

-undisputably hominin, close to LCA

-other early hominins (sahelanthropus, orronin) could have been bipedal but limited fossil remains

Definition of bipedalism

-upright posture or movement on two legs

-locomotive bipedalism: walking on two legs

-postural bipedalism: standing on two legs

Locomotive theory of bipedalism

-since bipedalism is more efficient than quadrupedalism, it would have been selected for over time

-chimpanzee knuckle-walking has a higher energetic cost than human walking (Rodman and McHenry 1980)

Refutations of locomotive theory of bipedalism (Schmitt 2003)

-modern humans have stiff-legged gait, first bipeds prob had compliant gait instead

-Australopithecus had mix of apelike features and bipedal traits

-prob could walk upright but wouldn’t be capable of efficient modern human gait

Savannah hypothesis (Falk 1990)

-4 mya transition from forest to grassland

-bipedalism arose because of the need for persistence hunting in the savannah

-would have been impossible to do with knuckle-walking form

Heat exposure hypothesis (Wheeler 1984)

-related to savannah hypothesis

-standing upright would mean much less of the body would be directly under the sun

-evidenced by presence of hair on modern human heads to protect the head and shoulders from the sun, absence of hair elsewhere for evaporative cooling

-cooled and protected brain

Behavioral hypotheses (Hunt 1994)

-freed hands for tool use, not likely b/c chimpanzees tool users too

-provisioning hypothesis w/ arms free to carry food, prob would have found another way (in mouth, over shoulder like other animal) rather than develop entirely new morphology)

Squat-feeding hypothesis (Kingdon 2003)

-developed an upright posture before developing bipedalism, bipedal locomotion natural consequence of a vertical spine

-quadrupedal posture unsuited for searching ground for food since only one hand free at a time

-would have been more effective to squat, leading to morphological schanges like shortening of pelvis, strengthened lumbar region, etc leading to upright posture

-explains why early hominins had bipedal posture but lacked capability for efficient bipedal locomotion

-similar to Jolly’s (1970) seed eating hypothesis

Refutation to squat-feeding/seed eating theories (Hunt 1994)

-features associated with arboreality present until emergence of Homo erectus

-therefore prob didn’t have primarily terrestrial feeding habits

Arboreal hypothesis (Tuttle 1975)

-proto hominins arboreal and used their forelibs for suspension, climbing, etc

-therefore lost forelimb function necessary for quadrupedalism and eventually would have become exclusive bipeds

-morphology for climbing present in early bipedal hominins

-doesn’t explain why arboreal ancestors would become exclusively terrestrial

Tree Feeding hypothesis (Stanford 2005)

-observed Ruhija population of wild chimpanzees in Uganda

-counted instances of bipedal posture

-all instances of observed bipedalism occurred while chimpanzees foraging for fruit

-thus arboreal foraging behaviors may have contributed to adoption of bipedal posture

-research bias: only observed chimpanzees when in trees, not while on ground

Tree feeding hypothesis Hunt (1994)

-similar experiment recording bipedal posture in pop of chimpanzees in Tanzania

-80% of bipedal episodes associated with feeding, mostly terrestrial and from understory fruit trees

-concluded bipedalism may have arisen from fruit harvesting posture

Problems with tree feeding hypothesis

-we did not evolve from chimpanzees, both us and them have diverged since last common ancestor, can’t use them as model

-doesn’t explain transition to exclusive bipedalism (chimpanzees do it and are still quadrupedal)

My theory

-prob happened in multiple phases (see Harcourt-Smith 2010)

-could have first adopted it to feed, reach, or squat, then developed better locomotion later with emergence of the savannah

-thus no single theory enough