Dr Michelle Dickinson: Nanotechnologist reveals why our shoulder and elbow joints are more unique than other animals

Our shoulders and our elbows – joints in our bodies that we usually take for granted unless they give us aches and pains. 

However these two joints are unique, and when it comes to primates we are the only subgroup to be able to throw small object fast and hard. While a chimpanzee might be twice as strong as a human, it can only throw an object 30 km/hour, which is nothing compared to human athletes who have been recorded to throw balls over 150 km/hour. 

New research published in the journal Royal Society Open Science has just concluded that our evolved shoulders and elbows were actually designed to prevent our ancestors from falling as they climbed down trees. 

The researchers started by watching videos they had made of wild chimpanzees and wild sooty mangabeys (which have shoulder and elbow joints more like a dog) and found that their styles were very different depending on whether they were climbing up or down a tree. They then used sports analysis software on the videos to measure the angle of the animals shoulders and elbows. 

They found that when climbing up, both monkeys and apes have similar shoulder and elbow angles.  However, on their way down the chimpanzees were able to flex their shoulders 21 degrees more and their elbows 33 degrees more than the mangabeys which allowed them to hold their bodies further away from the tree. 

This resulted in them being able to press their weight against the trunk of the tree with their feet preventing them from slipping down the trunk towards the ground – essentially allowing the chimps to use their feet as a brake. This function would have been important for our ancestors, who were bigger and heavier than a monkey making a fall out of a tree potentially deadly. 

It seems that this tree descending shoulder skill would later come in useful for help us to gather fruit, throw weapons and eventually throw other objects in sports. 

We still use this skill today when descending a ladder, as we hold on with our arms outstretched above our heads as we would have done with overhead tree branches while our feet helped to support our weight on the rungs below us. 

All of these advantages come with a drawback though, as our flexible joints mean that our shoulders are much more prone to dislocation than other animals. 

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