Prenatal development of the foot and evolution

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Prenatal development of the foot and evolution:
'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' .

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 fossilised from a short 'burst' of evolution are remote.

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

Primitive lifeforms:
• developed in an aquatic environment; eg amoeba, jellyfish, amphioxus; paired fins parallel to the long axis if the body are characteristic as the primitive life form evolved; a primitive form of a femur,tibia, fibula and digits developed
• the paired fin in these primitive life forms is analogous to the appearance of the limb buds in the 5 week human embryo

Amphibious lifeforms:
• five toed (pentadactylous) foot type appears; limbs become perpendicular to the body as a mechanism to push the body forward, however they are poor weightbearing structures
• the foot structure at this stage contained more forefoot bones than the human foot today (may be analogous to the polydactly deformities seen in human foot)
• analogous to the 7 week old human embryo – the extremities are projected at right angles to the body. The foot is very inverted, forefoot adducted and in extreme equinus

Reptiles:
• developed in a terrestrial environment rather than aqueous environment; the limbs underwent rotation to be fully weightbearing structures; a ‘subtalar joint’ develops; long axis of foot becomes parallel to the body; the number of tarsal bones reduces
• in the human embryo at 7-8 weeks the limbs undergo a similar rotation

Mammals:
• appeared 265 million years ago; the foot was digitigrade - bear weight on ends of metatarsals (eg. cat or dog) or unguligrade - walk on tips of toes (eg. horse) – the human foot is plantigrade

Primate Evolution:
• eg monkeys, apes and man; assumed to be derived from a common ancestor; appeared about 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

Bipedalism
Bipedalism does not require:
grasping ability of hallux
fine dexterity of digits
bulky intrinsic musculature
Hand vs. foot function

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

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