Why hominids evolved upright walking is one of the biggest questions in human evolution. One school of thought suggests that bipedalism was the most energetically efficient way for our ancestors to travel as grasslands expanded and forests shrank across Africa some five million to seven million years ago. A new study in the Journal of Human Evolution challenges that claim, concluding that the efficiency of human walking and running is not so different from other mammals.
Physiologists Lewis Halsey of the University of Roehampton in England and Craig White of the University of Queensland in Australia compared the efficiency of human locomotion to that of 80 species of mammals, including monkeys, rodents, horses, bears and elephants. For each species, Halsey and White computed the “net cost of transport,” a figure that considers an animal’s metabolic rate (measured in oxygen consumption), given its speed, while traveling one meter. Next, they created an equation that predicts a mammal’s net cost of transport based on its body mass.
The researchers found that a typical mammal weighing 140 pounds (the average weight for humans) has a net cost of transport of 10.03 milliliters of oxygen per meter while running. Human running on average requires 12.77 milliliters of oxygen per meter—27 percent more than the researchers’ calculation. In contrast, human walking is 25 percent more efficient than the average, same-sized mammal’s walking. The team also estimated that the roughly three-million-year-old Australopithecus afarensis‘ walking was 26 to 37 percent more efficient than the average mammal’s, depending on the estimated weight of the chimp-sized hominid.
Although modern humans and A. afarensis are more efficient walkers than the average mammal, Halsey and White argue that neither species is exceptional. When looking at all of the data points, both hominids fall within the 95 percent prediction interval for mammals. Statistically speaking, that’s the range you’d expect 95 percent of predicted mammalian net transport costs to fall within on average. In other words, modern humans and A. afarensis fall within the normal realm of variation for mammals. There’s nothing special about the energetics of their walking, Halsey and White conclude.
To evaluate whether energy efficiency played a role in the evolution of upright walking, Halsey and White note that hominids should be compared to their closest relatives. For example, if human walking is more efficient than chimpanzee walking than you would expect based on chance alone, then it lends support to the energy-efficiency explanation. But that’s not what the researchers found. In fact, the energetic differences between humans and chimpanzees are smaller than the differences between very closely related species that share the same type of locomotion, such as red deer versus reindeer or African dogs versus Arctic foxes. In some cases, even different species within the same genus, such as different types of chipmunks, have greater variation in their walking efficiencies than humans and chimps do. The researchers speculate that factors like climate and habitat might explain why such similar animals have such different locomotor costs.
This one study is unlikely to be the last word on the matter. I’m curious how the estimated energy efficiency of A. afarensis compares to chimpanzees, or even to modern humans, something the researchers didn’t examine. It would also be interesting to calculate the net transport cost for the 4.4-million-year-old Ardipithecus, the oldest hominid for which anthropologists have a complete skeleton. That seems like the crucial test of whether energy efficiency played some kind of role in the evolution of bipedalism.