Tuesday 26th April 1:37 pm
Humans evolved to run marathons
Tuesday 3rd June 2014 11:26 am
For our size, we humans have the biggest brains in the animal kingdom, but physically we appear pretty mediocre.
Surprisingly, it turns out that your average fit human can outrun a deer. In fact, a theory claims that we humans evolved the ability to be good endurance runners, so we could chase animals for hours, run them to a standstill, and kill them. The theory continues that their high-density protein and energy helped our brains evolve bigger and bigger.
Back in 1978, Michael Baughman wrote an article in Sports Illustrated about how he outran a deer. The deer is more of a sprinter, than a marathon runner.
On a warmish day (around 27°C) it took him about four hours to run down the deer over a distance of 24 kilometres, across the open range lands and orchards near his home. He got to within a few metres of the exhausted animal, talking quietly and soothingly, and then touched the deer’s sweaty flank. Unlike our primeval ancestors, he let the deer escape.
Michael could run down the deer just like his Indian Mohawk ancestors, not just because he could run a marathon in under three hours. There was also some basic biomechanical physiology involved.
Let me introduce you to COT — the metabolic cost of transport. It’s how much oxygen you have to breathe so you can shift a kilogram of body weight a distance of one kilometre. For most animals, it’s a U-shaped curve. The bottom of the U is where you use the least amount of energy.
A horse has three of these U-shaped overlapping curves. For walking, the most efficient speed is around 4.3 kilometres per hour; for trotting it’s around 11.5 kilometres per hour; and galloping is most efficient at around 22.7 kilometres per hour. The horse will transition from walking to trotting to galloping at the points where these curves cross over.
For humans, the most efficient walking speed is around 5.4 kilometres per hours. For speeds both slower and faster than 5.4 kilometres per hour we burn up more oxygen and energy to shift our weight over a given distance.
But as our walking speed gets to around 8.3 kilometres per hour, we reach the point where the costs of transport curves for “walking” and “running” cross over.
Above 8.3 kilometres per hour we actually burn less energy and oxygen by running instead of walking. The act of running uses what the physicists call a “mass-spring mechanism”. Various tendons and ligaments in our legs act as springs to capture and store the elastic strain energy as our legs hit the ground. They then release this energy through recoil, when we push off onto the next step.
How did we get to be such efficient long distance runners?
It seems that the long process of evolution began with our ancient primate ancestors. We have found strong hints of these evolutionary changes in Homo habilis some 2.6 million years ago and in Homo erectus 1.8 million years ago.
Endurance running posed four major demands: energetics, strength, stabilisation and thermoregulation.
One issue in energetics is the “mass-spring mechanism”. The main springs are the Achilles tendon and the longitudinal arch of the foot. In fact, the arch of the foot just by itself returns about 17 per cent of the energy captured in each step, when we run.
The second factor needed for endurance running is a skeleton strengthened in a few critical areas. These changes include a shorter neck of the femur so that it doesn’t bend so much when our foot slams into the ground.
As compared to a four-legged animal, the act of running for a two legged animal is inherently unstable. The leg that is in contact with the ground generates enormous twisting forces as it pushes off. So we evolved features to enhance the stabilisation of our trunk, such as greatly enlarged muscles on the buttocks and on the spine. There are also changes in the neck and skull to enhance stabilisation of the head, while we run.
Finally, we humans have a heat advantage over all the other primates, and most of the quadrupeds. We have a superior ability to get rid of the excess metabolic heat generated by running. We have less body hair, vastly increased numbers of sweat glands, and a narrow long body to radiate away the heat. Our very efficient mouth breathing gives us both higher airflow rates with lower muscular effort, as well as another way to dump excess heat.
The theory is that we evolved endurance running to push our prey to exhaustion, and then kill and eat them.
These days the furthest we have to chase our prey is the few steps to the fridge. If we had to resort to wearing out a deer to get our evening meal its likely most of us would run into trouble.
This blog first appeared on Dr Karl's Great Moments in Science
© 2017 Karl S. Kruszelnicki Pty Ltd