bipedalism

when did bipedalism start

7 million years ago

tool use

manufacturing tools using stone tools and making a cleaver

functonional morphology of bipedalism

valgus knee joint, hip musculature, large adducted hallux

valgus knee joint

• closer together than hip joints

• shaft of femur approaches joint at an angle (less than 90 degrees)

• can bear all weight on one leg and balance bc femora is inclined to joint

• femora inclined to joint so you only have to shift your body weight a little bit

hip musculature- gorilla

• ilium is tall and paddle-like with flat part facing forward

• hip bones form this wall behind the lower back and pelvis that faces forwards

• gluteal muscles arise off packside of pelvis and insert behind hip joint

hip musculature- human

• ilium faces the side where outside of the bowl faces

• extensor changes to abductor

• undergo abduction and go out to side when muscles contract

• walk level because of this abductor contraction

• ilium oriented sagital and changes function of lesser gluteal muscles (making them abductors)

hip musculature- general

• size and hip orientation of their hip bones

• gluteus medius

• gluteus minimus

large adducted hallux

unlike thumb, on the same plane as rest of toes; 80% of weight on big toe from putting weight on heel first and shifting next foot forward

theories on why bipedalism exists

tool use, carrying, thermoregulation, feeding posture, locomotor efficiency, vertical climbing hypothesis

tool use in relation to bipedalism

• in order to use hands for tools, you have to remove them from the ground thus making you stand up right → favors evolution of precision grip

• earliest hominins pre-date earliest tools by 4.5 million years denying Darwin’s hypothesis in The Descent of Man 1871

carrying in relation to bipedalism

• difficult to test

• a mom needs her hands and forelimbs not on the ground in order to carry her child

thermoregulation in relation to bipedalism

• likely false

• ground hets hot, when on all fours there’s more surface area exposed to sun and less exposure to wind

• when on feet only, less surface area directly from sun and you can feel more effects of wind

feeding posture in relation to bipedalism

• some aspect of this promotes standing upright

• have to reach for fruits on trees because fruits are less abundant

• bipedalism could’ve evolved for fruit eating and then animals adapted to it

locomotor efficiency in relation to bipedalism

• travling between food trees

• standing upright on 2 legs promoted efficiency in environments where fruit trees are greatly separated → upright standing conserves more energy

• bipedalism in earliest hominis would’ve been inefficient

vertical climbing hypothesis in relation to bipedalism

• likely false

• hip joint muscles in a particular way and knee joints are fully extended when we walk- legs are straight

• ancestors were pre-adapted to walk because of forests disappearing and less vertical tree trunks being available

• knuckle walking in chimps and gorillas have joints that can support this weight

• wrists have boney special process

why is the vertical climbing hypothesis likely untrue

hominins are descended from knuckle walkers

which hypothesis behind bipedalism is most likely

carrying and feeding posture