Do we owe our brains to our shoulders?

About 2 million years ago, human shoulders began evolving into a configuration ideal for throwing projectiles, such as rocks and
spears. Those hunting techniques may have improved our ancestors' diets, allowing them to evolve larger bodies and bigger brains.

Chimpanzees are perfectly suited for scrambling up trees and tearing things apart with their jaws and teeth. Their shoulders set the arms at an upward angle to the torso axis — great for swinging from branches but not the best for throwing.

Humans are relatively weak compared to chimps, but our ability to throw is
far superior. Our shoulders are low, aligning arms with the torso axis, which maximizes the transfer of energy during a throw.

Red lines highlight relative angles of body parts during a throw.

Internal rotation of upper arm.





An upper arm angled toward the head diminishes torque from the waist and builds only nominal elastic energy.

A short waist with minimal ability to twist contributes little if any torque to the throw.

To maximize inertia of the projectile, the chimp elbow must extend early in the throw, reducing its ability to add velocity at the end of the throw.

An upper arm perpendicular to the torso makes the most efficient use of torque from the waist.

When the hand is cocked back, arm tendons stretch, snapping back into place as the upper arm rotates internally.

A longer waist permits rotation of the torso, which builds torque and loads shoulder ligaments with elastic energy.

A bent elbow adds inertia to the upper arm, building elastic energy in the shoulder and adding velocity when the elbow extends at the end of the throw.

Unable to generate much rotation from its torso, a chimp's vertically oriented arm must toss an object overhead, using little elastic energy to propel the arm. The maximum velocity of its pitch is about 20 mph.

A human makes use of stretched tendons and ligaments to store elastic energy, which is released while compounding torque from the legs, torso, shoulder, upper arm, elbow and wrist to propel a ball in excess of 100 mph.

The “karate-chop” throw

The slingshot throw

SOURCES: Nature; Michael J. Rainbow, Harvard Medical School. GRAPHIC: The Washington Post. Published June 26, 2013.