These legs were made for walking

It's official: long limbs are an advantage for modern humans. But it hasn't always been that way. Kate Ravilious investigates

Is it advantageous to have longer legs when going for a walk? For many years, scientists have been unable to agree over the answer to this question: some say that short legs have an energetic advantage, while others think that longer legs will be more efficient because the ground is covered in fewer steps. Now, two zoologists, Karen Steudel and Michael Tilken from the University of Wisconsin, have shown what many short-legged people already suspected: having long legs makes walking easier.

Is it advantageous to have longer legs when going for a walk? For many years, scientists have been unable to agree over the answer to this question: some say that short legs have an energetic advantage, while others think that longer legs will be more efficient because the ground is covered in fewer steps. Now, two zoologists, Karen Steudel and Michael Tilken from the University of Wisconsin, have shown what many short-legged people already suspected: having long legs makes walking easier.

In order to test the effect of leg length on walking efficiency, Steudel and Tilken measured the oxygen consumption of 21 volunteers (of varying leg lengths) while they pounded away on a treadmill. Each volunteer wore a mouthpiece to measure their respiratory gases, and did 12 minutes of walking at each of four speeds, from a gentle stroll to a brisk walk. As well as measuring the volunteer's leg length, the researchers recorded their height and weight and measured their percentage of body fat.

After taking all these factors into account, they found that the longer the legs a person has, the lower the amount of energy required to move each kilogram of body weight over the same distance. "A short-legged person uses about 10 per cent more calories than a long-legged person, to travel a given distance," says Steudel.

Evidently, long legs are an advantage for modern humans, but has it always been that way? Steudel and Tilken were also interested in how leg length has changed over time and what evolutionary pressures have driven these changes. To investigate this, they took measurements from fossil skeletons of some of our ancestors. They looked at two groups of ancient hominids (early humans) that have since become extinct: the Australopithecus and the Neanderthal.

Australopithecus was one of the first human ancestors to start walking upright on two legs. They lived in the wooded valleys of east and south Africa between about 4.5 and 2.5 million years ago. Neanderthals are more recent relatives of ours - they started to roam the earth around 500,000 years ago. They inhabited northern latitudes throughout Europe and lived by hunting. Around 35,000 years ago, they became extinct.

By measuring the length of the femur (upper leg bone) and tibia (lower leg bone) from a number of Australopithecus and Neanderthal fossil skeletons, Steudel and Tilken estimated that the average leg length for both types of prehistoric man. In addition, they used published estimates of the original body weights of each fossil skeleton.

Compared with modern man, Australopithecus was tiny. Their legs were about two-thirds of the length of a modern human leg, and an adult Australopithecus weighed only about 30kg. Neanderthals were more similar to modern man, but with a short, stocky build and a leg length that was about 10cm less than the average person today. Despite their shorter stature, Neanderthals were relatively heavy, weighing in at around 75kg.

When Steudel and Tilken plugged this data into their walking-efficiency equations, they discovered that modern man has by far the best body design for walking. "Due to the larger mass and shorter legs of Neanderthals, their energetic cost of walking would have been about 30 per cent greater than that of modern humans," says Steudel. It was more difficult to do a direct comparison between modern man and Australopithecus because they were so small that they fell outside of the scale of the human volunteers. None the less, by extrapolating the data, Steudel believes that Australopithecus suffered a similar amount of disadvantage to the Neanderthal.

This finding begs the question: if long legs are so good, why did the legs of the Australopithecus or the Neanderthal not become longer over time? "It seems surprising that ancient hominids never evolved to having long legs, and managed with short legs for hundreds of thousands of years or more. This strongly suggests that there must have been some considerable evolutionary pressure to keep short lower limbs," says Steudel.

Thinking about the lifestyles of the Australopithecus and the Neanderthal offered clues as to why these hominids maintained their stumpy stature. Australopithecus appears to have spent most of its time living in or around woodland. "There is some evidence to suggest that they may have spent large amounts of time in the trees. Short legs provide more power for climbing," explains Steudel. In addition, there is debate as to how good Australopithecus was at balancing on two legs. Some scientists believe that the orientation of their pelvis made them wobbly on their feet and that having shorter legs helped to keep the centre of gravity low and made balance a little easier.

The Neanderthal, on the other hand, lived in a relatively cold climate and relied on hunting and fishing for food. "Having shorter legs may have helped the Neanderthal keep warm in the cold weather," says Steudel. Having a smaller surface area helps to keep body heat in. What's more, their short and powerful legs could have been useful when thrusting a spear, making them better at hunting than a long-legged and lean man.

Another piece of research suggests that leg length is something that evolves very quickly. Sang-Hee Lee, from the University of California, and David Frayer, from the University of Kansas, have recently carried out a detailed study of European human leg length over the past 30,000 years. They measured the change in femur length and compared this with changes in climate across Europe during that period.

They found that femur length in humans was closely linked to the ups and downs in temperature. Between 30,000 and 18,000 years ago, Europe was in the grip of an ice age, becoming colder and colder as it headed towards its glacial maximum. From 18,000 years ago onwards, the ice began to retreat and by 10,000 years ago it was relatively balmy, enabling people to settle down and start trying to farm the land. Overall, there was a decrease in femur length during the entire period, but there was a short blip at the beginning, when femur length increased (and the climate cooled), before femur length settled into its downward trend.

Lee and Frayer are not yet sure how to explain the changes in femur length, but it is clear that leg length was responding to the climate in this case. One notion that they are interested in exploring further is whether changes in leg length are linked to changes in lifestyle, such as the introduction of agriculture. Compared with the hunter-gatherer lifestyle, farming is more sedentary and people have less need to be efficient at walking long distances. "It is likely that once people started farming, the population became more subject to malnutrition. They were putting all their eggs in one basket and a bad harvest could leave them with nothing to fall back on," says Lee. Possibly, evolving back towards a shorter stature would have made people better able to conserve energy and survive in times of hardship.

Interestingly, the femur length showed significant changes only in the male population during the past 30,000 years. This suggests that female leg length does not evolve as quickly as male leg length. Lee and Frayer think that men are more sensitive to these environmental changes. "Nature seems to experiment with the men," says Lee. Women appear to follow a more conservative pattern and only adopt evolutionary successes once the men have "tested" them out.

None the less, Steudel's and Tilken's study shows that women do catch up, and that leg length has been an important part of human development. From scrambling up trees to striding across the plains, and then settling down to become farmers, our legs have served us well. It would be intriguing to know how our leg length is responding to our current lazy lifestyle. If nature is adapting our legs to make us more efficient at sitting in front of televisions, computers and steering wheels, then our long-legged status may not last for much longer. Perhaps we had better enjoy walking while we can.

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