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