born to run: running and human evolution william rose sources: bramble & lieberman (2004) nature...
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Born to Run: Running and Human Evolution
William Rose
Sources: Bramble & Lieberman (2004) Nature 432: 345-352; Wilford, New York Times, Nov. 18, 2004.
Austr. afarensis
Homo sapiens
Homo erectus
Present10 Mya 5 Mya 1.8 Mya3.5 Mya
Gorillas
Chimp-anzees
Human Evolutionary Timeline
Background
Australopithecines walked habitually > 4 Mya
H. erectus a better walking design than Australopith.: walking / swinging tradeoff
Was human running selected for? Did running influence human evolution?
Most have said probably not. Humans not very good sprinters. Horses, antelopes, greyhounds can run faster longer.
Sources: Bramble & Lieberman (2004) Nature 432: 345-352; Wilford, New York Times, Nov. 18, 2004.
Run vs. walkWalk
Inverted pendulum, KE – PE tradeoffC.o.m. vaults over extended leg in stanceU-shaped cost-of-transport (COT) curveOptimum speed a function of leg length
RunMass-spring mechanism, KE – PE tradeoffTendons, muscles, ligaments store PELimbs flex more in run to store energy
Walk-to-run transition occurs where COT curves intersect – as one might expectSources: Bramble & Lieberman (2004) Nature 432: 345-352; Wilford, New York Times, Nov. 18, 2004.
Running gaitHuman running like trotting
Bipeds can’t gallopForelimbs move with opp. hindlimbsHuman running, trotting both bouncy
RunMass-spring mechanism, KE – PE tradeoffTendons, ligaments store PELimbs flex more in run to store energy
Walk-to-run where COT curves intersect
Sources: Bramble & Lieberman (2004) Nature 432: 345-352; Wilford, New York Times, Nov. 18, 2004.
Endurance Running (ER)
ER: many kilometers, aerobically, 3-6.5 m/sHumans: only primates that do ERBetter than most mammalsHumans can run faster than most trotting animals trot, esp. when consider body sizeDistance: >10% Americans run kms/dayDistance: Thousands/yr run 42 kmUnknown in other primates; unusual in other mammals
Sources: Bramble & Lieberman (2004) Nature 432: 345-352; Wilford, New York Times, Nov. 18, 2004.
Running Adaptations
What adaptations make ER possible?
When do they appear in fossil record?
Four areas of adaptation required for ER
• Energetics
• Strength
• Stabilization
• Thermoregulation
EnergeticsLong tendons, short muscles
Chimps: short calcaneal tendon
Australopithicus: Calcaneal tendon insertion site is chimplike
Plantar arch: another energy storage site in humans
Chimps: flat feet, weight bearing, large medial tuberosity on navicular.
Austr. like chimps, but early Homo lack large medial tuberosity on navicular
Bramble & Lieberman (2004) Nature 432: 345-352.
Energetics: Stride lengthHumans have longer stride than expected for animal their size
Humans increase speed mostly by increasing stride length
Long (relative to body size) legs in humans, H. erectus. Chimps short. Austral ??
Oscillating long legs is costly unless minimize moment of inertia, hence small human feet
Human feet small compared to chimps & pithecines (9% v 14% leg mass, hmn v chmp)
Bramble & Lieberman (2004) Nature 432: 345-352.
Skeletal strengthRunning: large skeletal stresses
Force at heel strike = 3-4X body wt
Force travels up skeleton
AdaptationsLarger lower limb joint surfaces in human v chimp, even after adjust for weight: knee, hip, sacroiliac, lumbar centra
Reduced femoral neck length & inter-acetabular distance reduces bending moments on femoral neck, sacrum, lower back – compare Homo to chimps, Australopithicus
Bramble & Lieberman (2004) Nature 432: 345-352.
StabilizationGluteus max: its “increased size is among the most distinctive of all human features”
Enlarged sacral transverse process
Enlarged area for erector spinae attachment on sacrum, PSIS – allows the forward pitch of trunk during running
Decoupled head & shoulder (longer neck, fewer/smaller muscles) Homo vs Pan, Austr
StabilizationReduced forearm mass in Homo (50% smaller than Pan when adjust for body weight) reduces effort to keep arm flexed
Decoupled head & shoulder (longer neck, fewer/smaller muscles) Homo vs chimp, Austr
Wide shoulders of Homo enhance counter-balancing effect of arm-swinging in running
Head StabilizationOccipital projection behind condyles improves balance, reduces pitch-forward tendency at footstrike
Larger relative diam of posterior semicircular canal increaes sensitivity to sagital plane accelerations of head
Large nucchal ligament seen in humans, cursors, & large-headed mammals (elephant) but not chimps; Australopith lack nucchal line on occipital bone
ThermoregulationDissipate waste heat of running
Humans: Larger & more eccrine sweat glands for evaporative cooling
Lack of body hair
Larger near-surface cranial venous circulation
Mouth breathing (also lowers work of breathing)
Summary of some human adaptations for running