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Human Anatomy

The Evolution of Stomach Acidity and Its Relevance to the Human Microbiome - DeAnna E. Beasley , Amanda M. Koltz, Joanna E. Lambert, Noah Fierer, Rob R. Dunn Published: July 29, 2015

Gastric acidity is likely a key factor shaping the diversity and composition of microbial communities found in the vertebrate gut. The study conducted a systematic review to test the hypothesis that a key role of the vertebrate stomach is to maintain the gut microbial community by filtering out novel microbial taxa before they pass into the intestines. The study proposes that species feeding either on carrion or on organisms that are close phylogenetic relatives should require the most restrictive filter (measured as high stomach acidity) as protection from foreign microbes. Conversely, species feeding on a lower trophic level or on food that is distantly related to them (e.g. herbivores) should require the least restrictive filter, as the risk of pathogen exposure is lower. Comparisons of stomach acidity across trophic groups in mammal and bird taxa show that scavengers and carnivores have significantly higher stomach acidities compared to herbivores or carnivores feeding on phylogenetically distant prey such as insects or fish. In addition, the study found when stomach acidity varies within species either naturally (with age) or in treatments such as bariatric surgery, the effects on gut bacterial pathogens and communities are in line with our hypothesis that the stomach acts as an ecological filter. Together these results highlight the importance of including measurements of gastric pH when investigating gut microbial dynamics within and across species.

Comparison of the gastrointestinal anatomy, physiology, and biochemistry of humans and commonly used laboratory animals (ePDF)

The gastric acid and fluid secretion rates, gastric volume, and pH values for humans, beagle dogs, pigs, and Rhesus monkeys are given in Table 4. In dogs, the gastric acid secretion rate at the basal state is low. Therefore, the stomach pH of the dog can be as high as its duodenal contents in the unstimulated state.I9 Following stimulation (i.e., food, histamine), gastric acid secretion rates in dogs exceed those of the human and pig (Table 4). In humans, the stomach pH after food is initially higher due to the strong buffering action of food. However, the pH returns to a low value after about one hour (Table 4).

Brains and guts in human evolution: The Expensive Tissue Hypothesis - Leslie C Aiello - 1999– Full Article PDF

In 1995, anthropologists Leslie C. Aiello and Peter Wheeler published a paper on a theory they termed The Expensive Tissue Hypothesis (ETH). Expensive refers to human brain tissue, which is uniquely metabolically demanding compared to other primate brains (Aiello & Wheeler, 1995). However, human total metabolic rate is close to what would be predicted for a primate human size, so according to the ETH, humans compensated for the increased metabolic costs of the brain by evolving less metabolically expensive splanchnic organs, which include the gut and liver. Humans were able to fuel their large brains using only a relatively small gut because increased dietary quality reduced the need for gut mass. The hypothesis was that the main driver of this increased dietary quality was the increased use of animal products.

Energetics and the evolution of human brain size- 2011

The human brain stands out among mammals by being unusually large. The expensive-tissue hypothesis explains its evolution by proposing a trade-off between the size of the brain and that of the digestive tract, which is smaller than expected for a primate of our body size. Although this hypothesis is widely accepted, empirical support so far has been equivocal. Here we test it in a sample of 100 mammalian species, including 23 primates, by analysing brain size and organ mass data. We found that, controlling for fat-free body mass, brain size is not negatively correlated with the mass of the digestive tract or any other expensive organ, thus refuting the expensive-tissue hypothesis. Nonetheless, consistent with the existence of energy trade-offs with brain size, we find that the size of brains and adipose depots are negatively correlated in mammals, indicating that encephalization and fat storage are compensatory strategies to buffer against starvation. However, these two strategies can be combined if fat storage does not unduly hamper locomotor efficiency. We propose that human encephalization was made possible by a combination of stabilization of energy inputs and a redirection of energy from locomotion, growth and reproduction.

Comparative support for the expensive tissue hypothesis: Big brains are correlated with smaller gut and greater parental investment in Lake Tanganyika cichlids –Masahito Tsuboi Jan 2015

The brain is one of the most energetically expensive organs in the vertebrate body. Consequently, the energetic requirements of encephalization are suggested to impose considerable constraints on brain size evolution. Three main hypotheses concerning how energetic constraints might affect brain evolution predict covariation between brain investment and (1) investment into other costly tissues, (2) overall metabolic rate, and (3) reproductive investment. To date, these hypotheses have mainly been tested in homeothermic animals and the existing data are inconclusive. However, there are good reasons to believe that energetic limitations might play a role in large-scale patterns of brain size evolution also in ectothermic vertebrates. Here, we test these hypotheses in a group of ectothermic vertebrates, the Lake Tanganyika cichlid fishes. After controlling for the effect of shared ancestry and confounding ecological variables, we find a negative association between brain size and gut size. Furthermore, we find that the evolution of a larger brain is accompanied by increased reproductive investment into egg size and parental care. Our results indicate that the energetic costs of encephalization may be an important general factor involved in the evolution of brain size also in ectothermic vertebrates.

Metabolic costs of brain size evolution

In the ongoing discussion about brain evolution in vertebrates, the main interest has shifted from theories focusing on energy balance to theories proposing social or ecological benefits of enhanced intellect. With the availability of a wealth of new data on basal metabolic rate (BMR) and brain size and with the aid of reliable techniques of comparative analysis, we are able to show that in fact energetics is an issue in the maintenance of a relatively large brain, and that brain size is positively correlated with the BMR in mammals, controlling for body size effects. We conclude that attempts to explain brain size variation in different taxa must consider the ability to sustain the energy costs alongside cognitive benefits.

What Organs Can You Live Without?

Colon: People may have their colon removed as a way to treat colon cancer or Crohn’s disease, or in some cases, to prevent colon cancer. People can live without a colon, but may need to wear a bag outside their body to collect stool. However, a surgical procedure can be performed to create a pouch in the small intestine that takes the place of the colon, and in this case, wearing a bag is not necessary, according to the Mayo Clinic.

Appendix: The appendix is a small, tube-shaped organ that juts out from the first part of the large intestine. It’s unclear what its function is, but it can be removed if it becomes inflamed or ruptures.

  • Persistence hunting You Tube Video

  • Does Persistence Hunting Really Work?

  • Persistence Hunting by Modern Hunter‐Gatherers – Louis Liebenberg Current Anthropology December 2006Sci-Hub

    Endurance running may be a derived capability of the genus Homo and may have been instrumental in the evolution of the human body form. Two hypotheses have been presented to explain why early Homo would have needed to run long distances: scavenging and persistence hunting. Persistence hunting takes place during the hottest time of the day and involves chasing an animal until it is run to exhaustion. A critical factor is the fact that humans can keep their bodies cool by sweating while running. Another critical factor is the ability to track down an animal. Endurance running may have had adaptive value not only in scavenging but also in persistence hunting. Before the domestication of dogs, persistence hunting may have been one of the most efficient forms of hunting and may therefore have been crucial in the evolution of humans. >Xo and Gwi hunters at Lone Tree maintain that they concentrate on different species at different times of the year. They say that steenbok, duiker, and gemsbok can be run down in the rainy season because the wet sand forces open their hoofs and stiffens the joints. This is consistent with what Schapera (1930) reported. Kudu, eland, and red hartebeest can be run down in the dry season because they tire more easily in loose sand. Kudu bulls tire faster than cows because of their heavy horns. Kudu cows are run down only if they are pregnant or wounded. Animals weakened by injury, illness, or hunger and thirst are also run down. When there is a full moon, animals are active all night, and by daybreak they are tired and easier to run to exhaustion. The best time for the persistence hunt is at the end of the dry season (October/November), when animals are poorly nourished. When running down a herd of kudu, trackers say that they look to either side of the trail to see if one of the animals has broken away from the rest of the herd and then follow that animal. The weakest animal usually breaks away from the herd to hide in the bush when it starts to tire, while the others continue to flee. Since a predator will probably follow the scent of the herd, the stronger animals have a better chance of outrunning it, while the weaker animal has a chance to escape unnoticed.

  • Evidence for Persistence Hunting in early Homo

  • Endurance Running and Persistence Hunting

    Although humans are relatively slow runners, compared to other mammals we are exceptionally good at running long distances. The evolutionary significance of this ability becomes apparent when it is remembered that our closest living relatives, the great apes, exhibit relatively poor running performance; being specialized for climbing. We have suggested that our bipedal hominin ancestors evolved to be exceptional distance runners as a result of selection to run prey animals to exhaustion in the heat of the day (Carrier, 1984), commonly known as persistence hunting.

  • Muscle mechanical advantage of human walking and running: implications for energy cost. 2004

  • Steudel-Numbers, K. L. (2003). The energetic cost of locomotion: humans and primates compared to generalized endotherms. Journal of human evolution 44, 255-262.

  • Bramble, D. M. and Lieberman, D. E. (2004). Endurance running and the evolution of Homo. Nature 432, 345-352.

  • Biewener, A. A., Farley, C. T., Roberts, T. J. and Temaner, M. (2004). Muscle mechanical advantage of human walking and running: implications for energy cost. Journal of Applied Physiology 97, 2266-2274.

  • Liebenberg, L. (2006). Persistence hunting by modern hunter-gatherers. Current Anthropology 47, 1017-1026.

  • Steudel-Numbers, K. L. (2006). Energetics in Homo erectus and other early hominins: the consequences of increased lower-limb length. Journal of Human Evolution, 51(5), 445-453.

  • Lieberman, D. E. and Bramble, D. M. (2007). The evolution of marathon running. Sports Medicine 37, 288-290.

  • Pontzer, H. (2007). Predicting the energy cost of terrestrial locomotion: a test of the LiMb model in humans and quadrupeds. Journal of Experimental Biology 210, 484-494.

  • Sockol, M. D., Raichlen, D. A. and Pontzer, H. (2007). Chimpanzee locomotor energetics and the origin of human bipedalism. Proceedings of the National Academy of Sciences 104, 12265-12269.

  • Steudel-Numbers, K. L., Weaver, T. D. and Wall-Scheffler, C. M. (2007). The evolution of human running: effects of changes in lower-limb length on locomotor economy. Journal of Human Evolution 53, 191-196.

  • d’Août, K. and Aerts, P. (2008). The evolutionary history of the human foot.In Advances in plantar pressure measurements in clinical and scientific research. Maastricht: Shaker Publishing. p, 44-68.

  • Liebenberg, L. (2008). The relevance of persistence hunting to human evolution. Journal of Human Evolution 55, 1156-1159.

  • Lieberman, D. E., Bramble, D. M., Raichlen, D. A. and Shea, J. J. (2009). Brains, brawn, and the evolution of human endurance running capabilities. In The First Humans–Origin and Early Evolution of the Genus Homo (pp. 77-92). Springer Netherlands.

  • Pontzer, H., Raichlen, D. A. and Sockol, M. D. (2009). The metabolic cost of walking in humans, chimpanzees, and early hominins. Journal of Human Evolution 56, 43-54.

  • Steudel-Numbers, K. L. and Wall-Scheffler, C. M. (2009). Optimal running speed and the evolution of hominin hunting strategies. Journal of Human Evolution 56, 355-360.

  • Lieberman, D. E., Venkadesan, M., Werbel, W. A., Daoud, A. I., D’Andrea, S., Davis, I. S., …and Pitsiladis, Y. (2010). Foot strike patterns and collision forces in habitually barefoot versus shod runners. Nature 463, 531-535.

  • Noakes, T. (2010). Thermoregulation and Hydrating Strategies in Human Evolution. In The Anthropology of Sport and Human Movement: A Biocultural Perspective, 103.

  • Rolian, C., Lieberman, D. E. and Hallgrímsson, B. (2010). The coevolution of human hands and feet. Evolution 64, 1558-1568.

  • Archer, E. and Blair, S. N. (2011). Physical activity and the prevention of cardiovascular disease: from evolution to epidemiology. Progress in cardiovascular diseases 53, 387-396.

  • Raichlen, D. A. and Gordon, A. D. (2011). Relationship between exercise capacity and brain size in mammals. PloS one 6, e20601-e20601.

  • Raichlen, D. A., Armstrong, H. and Lieberman, D. E. (2011). Calcaneus length determines running economy: implications for endurance running performance in modern humans and Neandertals. Journal of Human Evolution 60, 299-308.

  • Ruxton, G. D. and Wilkinson, D. M. (2011). Thermoregulation and endurance running in extinct hominins: Wheeler’s models revisited. Journal of human evolution 61, 169-175.

  • Pontzer, Herman, David A. Raichlen, Brian M. Wood, Audax ZP Mabulla, Susan B. Racette, and Frank W. Marlowe. (2012). Hunter-gatherer energetics and human obesity. Plos ONE e40503.

  • Pontzer, H. (2012). Ecological energetics in early Homo. Current Anthropology 53, S346-S358.

  • Raichlen, D. A., Foster, A. D., Gerdeman, G. L., Seillier, A. and Giuffrida, A. (2012). Wired to run: exercise-induced endocannabinoid signaling in humans and cursorial mammals with implications for the ‘runner’s high’. The Journal of experimental biology 215, 1331-1336.

  • Wall-Scheffler, C. M. (2012). Energetics, locomotion, and female reproduction: implications for human evolution. Annual Review of Anthropology 41, 71-85.

  • Ruxton, G. D., & Wilkinson, D. M. (2013). Endurance running and its relevance to scavenging by early hominins. Evolution, 67(3), 861-867.

  • Raichlen, D. A., Foster, A. D., Seillier, A., Giuffrida, A. and Gerdeman, G. L. (2013). Exercise-induced endocannabinoid signaling is modulated by intensity. European journal of applied physiology 113, 869-875.

  • Bartlett, J. L., Sumner, B., Ellis, R. G. and Kram, R. (2014). Activity and functions of the human gluteal muscles in walking, running, sprinting, and climbing. American journal of physical anthropology 153, 124-131.

  • Pontzer, H., Suchman, K., Raichlen, D. A., Wood, B. M., Mabulla, A. Z. and Marlowe, F. W. (2014). Foot strike patterns and hind limb joint angles during running in Hadza hunter-gatherers. Journal of Sport and Health Science 3, 95-101.

  • Lieberman, D. E. (2015). Human locomotion and heat loss: an evolutionary perspective. Comprehensive Physiology 5, 99-117.

  • Pontzer, H. (2015). Energy Expenditure in Humans and Other Primates: A New Synthesis. Annual Review of Anthropology 44, 169-187 .

  • Longman D, Wells JCK and Stock JT (2015) Can Persistence Hunting Signal Male Quality? A Test Considering Digit Ratio in Endurance Athletes. PLoS ONE 10(4): e0121560. doi: 10.1371/journal.pone.0121560

Evolution of High-Speed Throwing


Humans are the only species that can throw objects both incredibly fast and with great accuracy. This unique throwing ability may have been critical to the survival and success of our hominin ancestors, helping them to hunt and protect themselves. Our research asks: How are humans able to throw so well? When did this behavior evolve? Was throwing important in our evolutionary past?

We found that humans are able to throw projectiles at incredible speeds by storing and releasing energy in the tendons and ligaments crossing the shoulder. This energy is used to catapult the arm forward, creating the fastest motion the human body can produce, and resulting in very rapid throws. We show that this ability to store energy in the shoulder is made possible by three critical changes in our upper bodies that occurred during human evolution: 1. the expansion of the waist, 2. a lower positioning of the shoulders on the torso, and 3. the twisting of the humerus (bone in the upper arm). All of these key evolutionary changes first appear together nearly 2 million years ago in the species Homo erectus.

We propose that this ability to produce powerful throws was crucial to the intensification of hunting that we see in the archaeological record at this time. Success at hunting allowed our ancestors to become part-time carnivores, eating more calorie-rich meat and fat and dramatically improving the quality of their diet. This dietary change led to seismic shifts in our ancestors’ biology, allowing them to grow larger bodies, larger brains, and to have more children.

Armed with nothing but sharpened wooden spears, the ability to throw fast and accurately would have made our ancestors formidable hunters and provided critical distance between themselves and dangerous prey.


  • Humans are remarkable throwers, and the only species that can throw objects fast and accurately.

  • Chimpanzees, our closest relatives, throw very poorly, despite being incredibly strong and athletic.

  • We have shown that humans produce high-speed throws by storing elastic energy in the tendons, ligaments, and muscles crossing the shoulder.

  • When this energy is released, it powers the rapid acceleration of the arm and the projectile, including the fastest motion the human body produces.

  • Three changes to the anatomy of the torso, shoulder, and arm that occurred during human evolution make this elastic energy storage possible.

  • These morphological changes are first seen together 2 million years ago in Homo erectus.

  • Concurrent with these changes, archaeological evidence of more intensified hunting behavior suggests that throwing may have played a vital role in early hunting.

  • Hunting had profound effects on our biology. For example, by improving diet quality our ancestors were able to grow larger brains leading to cognitive changes such as the origins of language.

  • Today, most throwing athletes throw much more frequently than our hominin ancestors did and, accordingly, frequently suffer from overuse injuries.


Roach, N.T., Venkadesan, M., Rainbow, M.J., Lieberman, D.E. 2013. Elastic energy storage in the shoulder and the evolution of high-speed throwing in Homo. Nature. 498. 483-486.

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