Small/Large Intestine Length Ratio

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In humans, the small intestine is about 6 meters or 20 feet long and the large intestine is about 1.5 meters or 5 feet long. The gastrointestinal tracts of the Chimpanzee, Orangutan, and adult human and a human fetus were studied and compared by Stevens and Hume in 1995. The chimpanzee intestines showed a longer appendix,  a much lower number of loops in the small intestine, and the large intestine had increased haustrations, as compared to humans. It is thought that reduction of the gut is a function of the higher-quality, easier to digest (i.e., less fibrous) diet of humans realtive to other apes, and that an evolutionary trend for gut reduction began when early members of the genus Homo began to incorporate a greater amount of animal tissues (marrow fat, brain matter, and muscle) about 2.5 million years ago.  Since gut tissue is metabolically expensive, the reduction of the gut may have allowed early members of our genus to devote greater metabolic energy to brain growth and maintenance, thus relaxing a constraint on the evolutionary increase in brain size ( an idea known as the "expensive tissue hypothesis": Aiello & Wheeler, 1995). It has also been suggested that cooking, which breaks down plants fiber (cellulose and lignin) and connective tissue in meat (collagen) allowed early humans to extract more nutrients from foodstuff with less digestive effort, thus also contributing to an evolutionary reduction in (shortening of) energetically expensive gut tissue.

Timing

Timing of appearance of the difference in the Hominin Lineage as a defined date or a lineage separation event. The point in time associated with lineage separation events may change in the future as the scientific community agrees upon better time estimates. Lineage separation events are defined in 2017 as:

• the Last Common Ancestor (LCA) of humans and old world monkeys was 25,000 - 30,000 thousand (25 - 30 million) years ago
• the Last Common Ancestor (LCA) of humans and chimpanzees was 6,000 - 8,000 thousand (6 - 8 million) years ago
• the emergence of the genus Homo was 2,000 thousand (2 million) years ago
• the Last Common Ancestor (LCA) of humans and neanderthals was 500 thousand years ago
• the common ancestor of modern humans was 100 - 300 thousand years ago

Possible Appearance: 2,000 thousand years ago Background Information:

Aiello & Wheeler, 1995. The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution. Curr Anthropol 36:199-221. Wrangham, 2009.  Catching Fire: How Cooking Made Us Human. New York: Basic Books.

References

1. Comparative physiology of the vertebrate digestive system, Stevens, Charles E., and Hume Ian D. , (1995)
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