The Evolution of Human Physical Activity

All animals need to be physically active, but the human lineage clearly underwent selection to be considerably more physically active than our relatively inactive ape ancestors and cousins including chimpanzees and gorillas. This selection likely occurred in at least two stages, first involving the most fundamental type of moderate physical activity, walking, and then the most fundamental type of vigorous physical activity, running. In terms of walking, the earliest hominins were likely selected to become bipedal in order to help them efficiently walk longer distances to forage. Bipedal walking brought many benefits and led to further selection for making and using tools, but also rendered hominins slow and awkward, hence vulnerable to predation. With the origins of hunting and gathering in the genus Homo, however, there was additional selection for endurance running which helped hominins become scavengers and hunters. Until recently, hunter-gatherers engaged in several hours a day of moderate physical activity from walking 9-15 km per day, as well as regular vigorous physical activity from occasional long-distance running. Industrial and post-industrial innovations, however, have changed how much and the ways we walk and run, and they enable humans to no longer have to engage in much physical activity. Today we face a growing epidemic of physical inactivity that increases people’s vulnerability to a wide range of chronic non-infectious diseases as well as infectious diseases including Covid-19.

Humans are unusual among primates in being capable of sustained long-distance running, a key phenotype that emerged in genus Homo about 2 million years ago (Mya). The underlying genetic changes that defined this exercise phenotype are not well understood. About 2-3 mya, an exon deletion in the CMP-Neu5Ac hydroxylase (CMAH) gene became fixed in our ancestral lineage, completely eliminating the hydroxylase activity required to add an oxygen atom to the sialic acid Neu5Ac to form Neu5Gc. Thus unlike the case in other old world primates, human cells do not express Neu5Gc, and instead have an excess of Neu5Ac on cell surfaces and secreted glycoconjugates. Modeling Cmah loss in mice allows us to explore potential mechanisms involved in the evolution of this hominin endurance phenotype. Untrained Cmah-/- mice demonstrate a remarkable increase in endurance during treadmill running exercise and voluntarily run further and faster when housed with a running wheel. Cmah-/- mice also exhibit skeletal muscle adaptations that are known to occur in exercise training, including more capillaries, enhanced mitochondrial respiration and greater fatigue resistance, despite remaining sedentary. Metabolic pathway analysis of sedentary and exercise trained Cmah-/- mice reveal increased and decreased patterns in Cmah-/- mice that augment those that occur with exercise training. In particular, elevations in amino acid and pentosphosphate pathways were identified. Taking a closer look at the final steps of the oxygen transport in peripheral skeletal muscle, our data suggest that loss of Camh-/- may allow for more efficient transfer of oxygen from Hb in red blood cells to deoxy-Mb, increased endothelial cell permeability and, enhanced mitochondrial activation in Cmah-/- myofibers at very low, near limiting extracellular oxygen levels for mitochondrial respiration. Taken together, these data suggest that CMAH loss may have contributed to multiple adaptations in genus Homo that improved skeletal muscle capacity for oxygen delivery and/or utilization, and perhaps provided a selective advantage during the transition towards persistence hunting and other features of Homo, such as increased foraging range and resource exploration.

Most primates live and move in the trees, but humans have evolved to move bipedally on the ground. Primates’ arboreal life-style has long been thought to have allowed the evolution of human beings’ unusual form of movement. We know much about how horizontal movement on branches (or simulated branches) differs in primates relative to most other mammals. But only recently have we begun to learn about how vertical movement (i.e. climbing) is accomplished by non-human primates, and how such movement may have permitted early human ancestors to move upright. However, climbing is a physically challenging activity, and not all primates (including humans) do it the same way. Body size plays a role in how muscles are able to accomplish overcoming gravity. But muscles consume energy to do this. What are the biomechanical and energetics costs of climbing? Key findings regarding the biomechanics of climbing, and what these data may mean for understanding human movement and exercise, are discussed.

Our thesis, that humans are at some level anatomically specialized for physical aggression, is based on two premises. First, although humans are arguably the most empathic and cooperative species on the planet, we also have a real problem with violence. Second, to the extent we can find ways to reduce aggression, intolerance, and violence in the future we should do so. As scientists, we know that the best solutions to our most important problems stem from understanding. Evidence that human aggression is partially tied to male contest competition (i.e., mating competition through the use of force or threat of force) comes from a growing number of subfields within evolutionary anthropology: primatology, archaeology, population genetics, sexual selection theory, and evolutionary psychology. I believe the field of comparative physiology can also contribute to a better understanding of the evolutionary basis of human violence by (1) determining if the anatomical and physiological characters that are known to distinguish hominins from the other great apes actually improve fighting performance, and (2) assessing the extent to which these characters are expressed differently in males and females, i.e., are sexually dimorphic. In this presentation, I will present results from two experiments we have done to test the controversial hypothesis that hominins are anatomically specialized for fighting by punching with a clenched fist. Our results are consistent with the suggestion that selection on male contest competition played a role in the evolution of the shape of the human hand and the pronounced difference in upper body strength observed between human males and females. Thus, our results add to a growing body of evidence suggesting that the evolutionary roots of much of the aggression, intolerance, and violence that plagues modern societies ultimately lies in the selection that shaped our mating system. Acknowledging and understanding the legacy of male interpersonal and group aggression can help guide policy directed at reducing violence in the future.

It is generally assumed that humans need physical activity to stay healthy, but health recommendations are rarely different for women and men. However, the effects of physical activity on female physiology and health are much more complex. Just like in men, activity is beneficial for many aspects of health, but physically active women also face important physiological trade-offs, or, in other words, quid pro quos.

In this talk, I will discuss beneficial aspects of exercise for women, especially in prevention of breast cancer, but I will also point to suppressive effects of activity on reproductive function. Physical activity influences levels of hormones that are crucial for female health – estrogens and progesterone. But how much and in what way these hormones are affected depends not only on the type and intensity of physical activity, but also on other factors, such as the quality of environment that women experienced during their own fetal development and childhood.