bipedalism and traits of early hominins

reduced masticatory muscles

apes have larger chewing muscles than these, sagittal crest, prognathism.

seed eating hypothesis

bipedalism hypothesis based on baboons, who stand up to gather grass seeds. would be efficient for humans to walk on two feed for gathering seeds. also allows them to see predators more easily. however, the wear on the teeth of early hominins doesn't necessarily match what would be expected with a regular diet of seeds.

radiator hypothesis

bipedalism hypothesis based on thermoradiation. by Dean Falk. says bipedalism developed as a way to limit the absorption of solar radiation in hot climates. also allows for more efficient cooling, which would be especially helpful w/ the expanding brain.

patchy forest hypothesis

bipedalism hypothesis having to do w/ subsistence and food resources. Peter Rodman and Henry McHenry came up with it. about the time that hominins appeared in the fossil record, there was a drying trend. African forests became fragmented. bipedalism may have begun in trees, and hominins walked on two feet between patches of food resources (like gibbons). efficient because can see predators more easily. enhanced reproductive success.

chimpanzee hypothesis

bipedalism hypothesis made by Adrienne Zihlman. said early hominins did not form pair bonds. thought Lovejoy's explanation was ridiculous and sexist. we are closest to chimpanzees.

male provisioning hypothesis

bipedalism hypothesis made by Owen Lovejoy. early hominins formed monogamous pair bonds, males assisted with child rearing, needed to be bipedals to carry food and provide for defense. this bipedalism led to the loss of body hair and loss of fast development for children. male sees the offspring as his and wants it to grow up well. humans have concealed ovulation, males wanted t ensure reproductive success, based on observations of human anatomy.

Rodman and McHenry

came up with patchy forest hypothesis.

Owen Lovejoy

came up with the male provisioning hypothesis

Clifford Jolly

came up with the seed eating hypothesis.

in line hallux

derived trait for bipedalism. big toe is in line with other digits. for chimps, toe is opposable, curved outward from rest of toes, walk by shuffling.

quadriceps and handsprings

derived trait for bipedalism. different structure than chimps, longer, stronger, more spring like. elastic quality, allows our bipedalism to be efficient and fast. humans are capable of walking long distances.

femur

derived trait for bipedalism. femur angles in like a V-shape, makes humans stand closer together

foramen magnum

derived trait for bipedalism. spinal cord comes down from center of brain.

knees

derived trait for bipedalism. tendons and joints help squeeze knee, locking it in place, helps us to use less energy. chimps have to use more muscle force to do this, can't stand for very long. center of gravity passes straight from neck to pelvis and hips and down to heels; for chimps, balance is less equal.

feet

derived trait for bipedalism. we put our foot forward, fall on the heel, arch helps absorb weight, push off again with hallux. very different from other bipeds.

pelvis

derived trait for bipedalism. wide and short ichium. bowl-shaped. constricts birth canal, lead to historically high mortality among first time mothers. most quadrupeds do not have such a long birthing period. harder for women to walk than men. one of the most useful features for determining sex of an individual.

hunting hypothesis

first proposed by Charles Darwin. popularized and worked on by Raymond Dart. humans took on bipedalism to help them become efficient hunters. early hominins were going out and dragging animals back to caves. no recognizable tools; were probably using unmodified teeth, bones, and horns. however, what was probably actually happening was that leopards were eating hominins in trees and dropping them into sinkholes.

convergent evolution

given a similar environment, plants and animals will assume similar characteristics/traits. (ex: sharks, ichthyosaurs, and dolphins and different classes, yet look very similar.) trend toward convergence because some mutations work better than others.

habitual bipedalism

how humans get around. bipedalism is the main form of the locomotion. (ex: birds, macropods/kangeroos, some dinosaurs). homo sapiens' form of bipedalism is unique.

cranial capacity

humans have an enlarged brain and more wrinkling in the neocortex. orangutan: 340-440 cc. gorilla: 455-530 cc. chimp: 320-480 cc. humans: 1250-1500 cc.

reduced canines

humans have smaller canines than other primates, result of sexual selection. also non-honing, do not shred food during chewing, do not sharpen or rub against each other.

born-to-run hypothesis

hypothesis that humans are designed for long-distance running (takes evidence from several Native American tribes and physiology/anatomy). Christopher McDougall wrote about this. also, idea that it is natural for us to be barefoot. this would have been a way of hunting animals with groups, take turn running after prey to tire it out. flaws are that this doesn't consider really young, old, or pregnant people. more likely that we are designed for long distance walking. regularly doing long distance marathons can lead to larger heart, thinner cardiovascular walls, overtaxed muscles and skeleton.

gluteus muscles

medius, minimus, maximus. derived trait for bipedalism. big muscles. helps keep human body straight while walking.

enamel thickness

most other primates have thinner enamel. we are equipped to eat more in general, harder diets.

Raymond Dart (1893-1988)

popularized the hunting hypothesis for bipedalism.

primitive traits

traits found in ancestors of animals and multiple groups (ex: opposable thumbs)

derived traits

traits specific to a certain class/species of animals (ex: bipedalism). for bipedalism, includes centered position of foramen magnum, wide and short pelvis, arched foot, in line hallux, longer legs and shorter arms, different musculature, gluteus muscles to help pull us straight, and muscle in back of calf to help lock the knee.

parallel evolution

two groups of animals (esp. mammals) split. in one area, they continue to develop a certain way. in another area, they developed in a different way. parallel development in types of animals. (ex: gray wolf vs tasmanian wolf.)

osteodontokeratic technology

using unmodified tools, like teeth, bones, and horns.

bipedalism

walking on two feet. when looking for fossils most similar to humans, look at derived traits of humanity, and this is the most important one (ex: speech, language, large brains).

facultative bipedalism

when an organism is a bipedal on the ground, but that isn't their main locomotion style. some primates do this (ex: gorillas, gibbons, chimps, orangutans). seems more like a gymnastic trick; anatomy is not adapted to it.