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Evolution of Human Locomotion
Evolution of human locomotion outline
Geologic timescale 4600 million years ago: formation of earth's crust - subsequent evolution of life
Why study evolution ? bipedal gait is unique to man
human locomotion
is a relatively recent phenomenon
Why study evolution? Recapitulation (Haekel's Law) 'during embryological development, and organism passes through stages which resemble the structural form of several ancestral types of the species as it evolved' Atavism the appearance in an organism of features similar to that of a distant evolutionary ancestor
Why study evolution ? 'Man's foot is all his own. It is unlike any other foot. It is a human specialisation, and whether he be proud of it or not, it is his hallmark. So long as man has been man, and so long as he remains man, it is by his feet that he will be known from all other members of the animal kingdom' (Buettner-Janusch, 1966) 'A major part of the podiatrist's contribution to foot health is to determine the most efficient structural posture of the supportive bony elements of the foot...a knowledge of the development of upright bipedal locomotion contributes greatly to such an understanding' (Tax, 1985)
Evolution defined: 'Evolution is a series of partial or complete and irreversible transformations of the genetic composition of populations, based principally upon altered interactions with their environment' (Dobzhansky et al, 1977). Example: changes in ratio of black to white moths following change in environment
Two models of evolution Gradualistic model: no such thing as a species; all organisms are a 'snap shot' in time of a gradually changing group of organisms problem is 'missing links' in the fossil record Punctuated model: production of new species as a result of short 'bursts' of evolutionary change each new species is an addition to the previous one, not a replacement explains 'missing links', as chances of an organism being fossilized from a short 'burst' of evolution are remote.
Primate Evolution includes monkeys, apes and man derived from a common ancestor appeared 70 million years ago
Primate development Dryopithecus tree ape that lived 12-20m years ago major 'missing link' in fossil record dryopithecus ' ramapithecus ' man dryopithecus ' d. africanus ' apes brachiators capable of upright posture but not locomotion opposable hallux necessary to wrap around a tree limb: still evident in man's thumb
Ramapithecus to modern man appeared 12-6 million years ago first ancestor to leave the trees and live in a terrestrial environment most important evolutionary changes occurred from this point through Australapithecus ('Lucy'), Homo Habilis, Homo erectus, Homo sapiens
Australapithecus afarensis appeared 3.5m years ago the first habitual biped 40% skeleton nicknamed 'Lucy' hips and knees built for upright walking footprints in Tanzania indistinguishable from modern man (slowly walking)
Homo habilis descended from Ramapithecus fossil foot bones found in the Olduvai Gorge in East Africa ankle joint articular surface extended anteriorly to enable greater dorsiflexion realignment of 1st tarso-metatarsal joint persistent bipedal walker
Homo erectus desceded from H. Habilis appeared 1-2m years ago completely upright walking man fossils found in Java in 1891 then South Africa, Algeria, China responsible for the spread of man from Africa to Europe and Asia
Neanderthal man 'spin-off' from the evolutionary chain of man appeared 100-40,000 years ago (?) became extinct by unknown mechanism foot bones found are very similar to modern man, but more robust
Bipedalism habitual bipedalism is unique to man (as is cerebral cortex development & vocal cord structure) energy efficiency, food gathering, free the hands, predator avoidance longer stride length rigidity to act as a lever for propulsion improved balance and shock absorption muscular efficiency
Bipedal adaptations vertical pelvis and spine increased femoral inclination (towards midline of body)
structurally longer legs full knee extension increased ankle dorsiflexion loss of hallux opposability decreased muscle bulk(esp. intrinsic) formation of medial longitudinal arch
Recapitulation 'during embryological development, an organism passes through stages which resemble the structural form of several ancestral types of the species as it evolved' (Haekel's Law) evident in the early stages of development of the human embryo and foetus possible explanation for the existence of structural anomalies in the lower limb
4 Weeks foetal development
6 weeks
8 weeks
9 weeks metatarsals adducted large IMA (opposable hallux) tibia and fibula form ankle mortise foot inverted, parallel to sagittal plane subtalar joint forms recapitulation: reptilian stage
12 weeks 56mm foot begins to dorsiflex nails begin to appear limbs begin to fold into thigh/leg and arm/forearm components recapitulation: mammalian
16 weeks - birth feet continue to evert bones begin to ossify, starting with the calcaneus at birth: genu varum low medial longitudinal arch ligamentous laxity external limb position recapitulation: primate
16 weeks - birth NB: changes which occur from birth to beginning walking are very similar to primate evolutionary changes
Atavism appearance in an individual of traits or characteristics more like those of an earlier ancestor commonly referred to as 'phylogenetic throwbacks' Examples: metatarsus primus adductus genu varum ('bow legs') syndactyly / polydactyly pes planus ligamentous laxity
Summary bipedalism is unique to man and relatively recent significant structural changes in the lower limb evolutionary development of locomotion has a number of parallels with embryological development (recapitulation)
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geologic timescale
