Sustained Running Behavior
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Drastic anatomical and physiological difference appeared about 2 million years ago in the genus Homo, with a major feature being the development of striding bipedalism and long distance running. Humans are poor sprinters as compared to other apes and quadrupeds, but can out-run other mammals over time. Endurance running, defined as the ability to run more than 5 km using aerobic metabolism, is unique among primates in the genus Homo. Modern musculoskeletal and skeletal features might have developed as a result of long distance running. Endurance running might have been key to survival approximately 2.5 million years ago when competition with other carnivores for food was high.
Today endurance running is a form of exercise and recreation but its roots are ancient and shaped the human form. Around 2.5 million years ago sub Saharan Africa turned to savannah. Deep forest became grassland and the genus Homo had to adapt to the environment to survive. Around this time Australopithecine was bipedal, with the ability to both walk and climb trees. Then came Homo erectus who were completely bipedal and had the ability to run. Homo erectus is hypothesized to have been a passive scavenger, competing with other carnivores for carcasses, and consuming more meat than other primate species. Endurance running would have allowed persistence hunting to became a key advantage over other animals and acted as a selective advantage.
The genus Homo underwent drastic anatomical and physiological difference as compared to other Hominidae. Endurance running demands high-energy utilization and requires the body to store and release energy in an efficient way. During the aerial phase of running, kinetic and potential energy from the body is stored as elastic strain energy, which is released when elastic recoil happens. Energy turnover of 70 kg at 4.5 m/s is 100 J, in which 35 J are stored in the Achilles tendon and 17 J are stored as strain energy in muscle. Human specific changes in foot morphology act like a spring to help human energy efficiency in running. Examples include the Achilles tendon and plantar arch, which are enlarged as compared to chimpanzee and early hominid ancestor such as Australopithecines. Longer legs as compared to body size provided a larger stride while running and walking. This adaptation is found in humans and Homo erectus (1.8 million years old), with a 50% increase in relative leg length as compared to Australopithecines. An adaptation preventing rotation of the anterior Tarsals while running is observed in humans as well as fossils of Homo habilis. There are slow (type-1), intermediate (type-2a) and fast twitch (type-2b) muscle fibers that form motor units of the muscle. Slow twitch muscles are teeming with mitochondria, capillaries, and myoglobin that ensure constant oxygen supply and energy to run aerobically. Intermediate fibers use aerobic characteristic but can contract faster. Fast twitch muscle provides short burst of anaerobically fueled running for sprinting. Most human muscles are a mixture of slow and fast twitching muscle, but endurance running might have favored slow twitching muscle. Soleus muscles in human, which are vital for walking, standing and running, are made up of type-I muscle fiber. In upright posture, the soleus is responsible for pumping venous blood back into the heart from the periphery. Lastly modern humans have more fore tendons.
Endurance running was facilitated by several major modifications outside of the feet as well. The gluteus maximus is enlarged and has a large attachment to the spinal base as compared to other apes. Interestingly, the gleutus maxima fire only when running which is advantageous as it helps with coordination between the upper and lower halves of the body while running. The shoulder muscle Trapezius density is lower in humans allowing free motion of the shoulders. An anti-bobble head adaptation helps to hold the head still, as well as efficiently turn the head while running. Femur, neck, knee joint and hip joint size were expanded, reducing damage taken from mid-stride impact shock. Similarly the pelvis has undergone a size reduction that reduces shock to the head. Elongation of the thorax created a larger gap between the thorax and pelvis helped to coordinate the upper and lower body while running. Additional changes include shoulder movement opposite to the hip, an enlarged occipital cranium, and modification to the Nuchal ligament. Finally, the Inner ear (ear canal), which helps an organism balance, has two lobes in human compared to one in apes, improving balancing while running.
Apart from anatomical modifications allowing for better running, there were multiple adaptations that improved overall thermoregulation. Thermoregulation, imperative to long distance running, is a rate-limiting factor for other mammals. This may have allowed humans to chase other animals with poor thermoregulation in mid-day (39 degree Celsius) until exhaustion. Endurance running demands high energy and water usage and results in salt loss in humans. The Homo genus, beginning with Homo erectus, adapted to compensate for the loss and makes it an efficient process through more sweat glands and reduced body hair as compared to apes. Additionally, these features improved body cooling. Finally, increased scalp sweating and reorganized cranial circulation allowed for optimized cooling of the brain.
Humans expend 50% more energy while running, as compared to other quadrupeds, but have optimized energetics, strength, stabilization & thermoregulation allowing for the unique task of endurance running.
Endurance running is universal to all humans but certain ethnicities, such as East African, show increased endurance as compared to the standard.
While no one change is responsible for all differences, certain genetic changes have been correlated with running endurance. One example is a mutation in the ACTN3 gene (null variant), an actin binding protein expressed only in fast twitch muscle. Slow twitch muscle uses less oxygen to generate more energy and, as such, a higher proportion of slow twitch muscle leads to increased endurance. The ACTN3 null mutant may influence long distance running ability. The mutation is more prevalent in Asians and least abundant in African populations.
Environmental factors, such as sub Saharan Africa turning to savannah, and persistence hunting could both have acted as selective forces.
Endurance running appears to be independent evolution that happened in the Homo genus and a few domesticated quadrupeds, such as horses.
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