Our ancestors were likely endurance runners. To clarify, I'm not talking about my great grandfather running marathons; I'm referring to the notion that natural selection acted on running performance in some early human populations, leading to the development of high endurance. Endurance capacity is thought to have been useful for persistence hunting, in which humans forced animals to run for long periods and at high enough speeds that they couldn't dissipate heat fast enough and would eventually succumb to hyperthermia (and their human hunters). This form of hunting is hypothesized to have worked best in dry, hot environments, where thermoregulating during heavy exercise is most challenging. David Raichlen and colleagues from the University of Arizona and Harvard University, who are interested in the role of running in human evolution, recently developed a way to test the potential importance of climate for the development of endurance running in Homo.
Running economy, or the mass-specific energy required to run at a given speed, has an anatomical basis. For example, previous work has shown that human running economy is inversely related to the moment arm of the Achilles tendon, an important site of elastic energy storage and return during running. Dr Raichlen and co-workers recognized that this connection between running economy and anatomy could be put to use in a paleontological framework. If they could demonstrate that the moment arm of the Achilles tendon is directly correlated to some measure of heel bone (calcaneus) size, then they could take measurements of fossil heels and make inferences about running economy in our long-gone ancestors. To address the importance of climate, they could make these measurements in Neandertals, who lived in colder environments and might not have developed high levels of endurance because persistence hunting wouldn't have worked as well.
To explore relationships between heel anatomy and running economy in modern humans, the researchers used eight trained endurance runners to quantify individual variation in mass-specific oxygen consumption rates during running at 16 km h–1. These same individuals subsequently underwent MRI scans of their ankles so that precise measurements of their heel bone (specifically, calcaneal tuber length CTL), could be taken. The measurements demonstrated a tight correlation between CTL and the moment arm of the Achilles tendon (r2=0.91) and, more importantly, between CTL and the mass-specific energy costs of running (r2=0.80).
The authors then took CTL values from the literature for seven Neandertals and 13 early Homo sapiens. Mean CTL was nearly 62 mm in Neandertals, just over 57 mm in the early humans and only 55 mm in the modern humans. Recall that running economy is inversely related to the size of the Achilles tendon moment arm, so longer calcanei imply less economical runners. In fact, the authors estimate that the metabolic costs of running in Neandertals and early humans would have been about 11.5% and 6.9% greater, respectively, than in modern humans. Such data must be interpreted cautiously, but they suggest that Neandertals were less economical runners and couldn't rely on endurance running for subsistence. Given the colder climates in which Neandertals lived, these data are also consistent with the idea that persistence hunting, running economy and climate were all linked in human evolution.