From quadrupedalism into bipedalism: towards a new biology of human balance John Skoyles C oMPLEX University College London 8th Feb 2006

“to see the familiar as unfamiliar, and the unfamiliar as familiar” Francis Bacon “Man is a flightless biped” Aristotle Two quotations

Human bipedality is unprecedented in biology Human bipedalism has an unique dependence upon balance (1) The risk of falls drove the evolution of balance mechanisms (2) Human balance depends upon an exquisite ability to make forward feed body adjustments Both (1) and (2) need mathematical analysis Towards a new biology off human balance

≠ What human bipedalism is not

≠ Human is axial but still unique

≠ Human bipedalism is obligate

≠ Human bipedalism is evolved

What makes human bipedalism biologically unique? Vertically axial Obligate and Habitual Evolved

Human bipedalism combines with upper body activities

Human bipedalism provides a platform for other balance challenging activities

Human bipedalism goes far beyond any other biological locomotion Upper limbs are never used for locomotion but employed in many activities A platform upon which diverse secondary motor actions are superimposed

foramen magnum Human bipedalism is biologically unique due to balance: anatomy ChimpHuman

Posterior fat deposits aid balance during pregnancy and lactation Pawlowski B Current Anthropology

Human bipedalism is biologically unique due to balance: “brainware” Totally dependent upon skeletomuscular adjustment Walking and running takes seven years to become adult-like

Simple bipedalism, however is easy, the problem is not falling and breaking bones This individual has no cerebellum and yet can walk – in a manner cerebellar agensis Titomanlio et al 2005 Neurology, 64, E21

Dysequilibrium syndrome is due to a lack of this biped balance “brainware”

Dysequilibrium syndrome suffer an inability to reliable stand and walk linked to balance Some do learn to walk but at great delay like “tin solders” They have problems in falling when they attempt to walk bipedally

A bipid that breaks a bone is unlikely to survive if they can no longer walk

When did this balance faculty arise? A clue lies in a link between injuries when tripping and height The risk of bone injury links to height The speed with which a large organism hits the ground is proportional to the square root of its height. Since mass is proportional to the cube of its length, momentum is proportional to its length raised to the power 3.5, and kinetic energy to the fourth power

Height and Australopithecine and Homo species Australopithecus afarensis AL (Lucy) was 1.18 m tall H. erectus KNM-WT (Nariokotome Boy) would have been as an adult 1.86 m

Momentum on impact increasing with height to the power of 3.5, and released kinetic energy, to the power of 4 The height increase between Australopithecus and Homo required the evolution of new balance mechanisms Lucy fell with six-fold more kinetic energy than Nariokotome Boy

Proper models of the risk Australopithecus and Homo to injury falls are needed. Back of the envelop calculations: Vogel S. Comparative biomechanics. Princeton University Press, Effects of body type – skinner people suffer worse injury than those with fat The length of hands compared to legs – chimp hands are 110% longer than legs, humans only 72% their length -- Austropithecus in between. The risks of tripping up when running compared to walking.

There was a biological shift from australipithecine to homo species 2 millions ago. Enlarged brains Enlarged vestibular organs Running Increase in height Migration from Africa into Asia Common to them is improved balance abilities

Expanded brain size could link to improved balance abilities Cannot talk and walk phenomena Balance impaired when people do spatial tasks and vice versa Expansion of the cerebellum closely associated with expansion of the cerebral cortex Enchanced balance depends upon internal motor models that enable adjustment of the body

Human balance depends upon internal models that map actions into balance adjustments To keep balance every action must be synchronized with a skeletonmuscular adjustment to counteract its effect upon the centre of mass. Thus the brain must be able to map changes in posture to know the counteracting changes needed to keep balance

Internal motor models are of two kinds Inverse models: provide the motor command necessary to achieve a desired state Forward models: predicts the next stage of a movement given the current state and motor command

Inverse balance models keep us upright For every action – step, hand movement etc the brain must create an inverse balance model so it can counteract its effect on the projection of the center of mass over support This is not at present a problem being modelled in biomechanics

Human balance presents key problems for mathamatics Human balance underlies the origins of our species Human balance underlies the nature of everyday lives Studying the mathematics of falls could provide insights into the origin of Homo Studying the inverse balance problem is needed to understand how the brain keeps us upright

Huysein suffers from dysequilibrium syndrome. He has compensated for lack of bipedality by using all fours.

One of the sisters

In spite of being unable to coordinate and balance for walking, Huysein is still coordinated nonbipedality

Huysein can stand but lacks the ability to walk without falling

All the family initially walked with a bear-crawl. The mother shows also signs of doing this The quadrupeds have perfected such a gait into an unique form of adult locomotion