You might have to rethink that one. "There's good evidence to show that somewhere between 2 million and 2.5 million years ago, a suite of adaptations specific to running long distances did occur," says professor of anthropology Daniel Lieberman, who, together with Dennis Bramble of the University of Utah, is researching the idea that evolution has indeed left us well suited for the annual Hopkinton-to-Boston run.
This period marks the beginning of the genus Homo. The earlier line of hominids, the australopithecines, were bipedal, but in a much different way. Their short, blunt legs, wide hips, and curved toes left them better suited to climbing trees than lining up for a 10-kilometer race. With the advent of Homo, however, the adaptations began. The evidence in the fossil record suggests to Lieberman and Bramble that Homo evolved to go the distance.
Consider the lower body. Legs increased in length, allowing longer strides, and became more tapered, enabling them to swing more efficiently, like pendulums. Pelvises narrowed, decreasing the lateral sway of the body's center of gravity while moving forward. Long, thin Achilles tendons developed at the back of the heel storing energy, then using it to spring off the ground. The plantar arch evolved: the mid-tarsal joint locks the foot on impact and then acts as a spring on the next step. According to Lieberman, each of these adaptations provided little in the way of energy savings when walking or even sprinting. It's only in distance running that their cumulative effects would have added up. In addition, the size of the hip, knee, and ankle joints all increased, reducing the strain running placed on them by distributing the stress over larger surface areas.
At the other end of the body, the skull's shape also changed, causing its center of gravity to move back, balancing the head on the neck and allowing it to be held steady easily. The nuchal ligament, which also assists in holding the head still, developed in the back of the neck. An external nose developed, providing a chamber in which dry air could be humidified by mucous membranes, helping to stave off dehydration during long bouts of exertion. Again, none of these adaptations seem beneficial while walking or sprinting.
Imagine the advantages such adaptations might have provided early hominids: the ability to cover long distances in search of food, or to hunt and wear down otherwise swifter animals. It's easy to imagine our ancestors staring out at a flock of vultures circling in the distance, knowing that there was food on the ground and that the strongest runners, human or animal, would reach it first.
It's still too early to say if the Marathon Man hypothesis will attract support. "Up until now, all of the focus and research has been on understanding bipedalism and walking," says Lieberman. "And once people start working on something, they tend to just keep doing it. But there really isn't any animosity to this idea in the scientific community. It just hasn't been thought about a great deal yet."
~Jerry Shine
Daniel Lieberman e-mail address: dlieberm@fas.harvard.edu
