Rib Cage Flaring

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Human Uniqueness Compared to "Great Apes": 
Absolute Difference
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Humans and apes share in having a transversely broad and dorsoventrally flattened rib cage, which contrasts with the transversely narrow and dorsoventrally deep thorax of monkeys. Hominoids are also characterized by a more ventral position of the vertebral column (such that it lies more within the thorax), which again contrasts with the condition seen in monkeys (in which the vertebral column is on the dorsal aspect of the thorax). However, humans and other great apes differ in the relative mediolateral dimensions of the upper and lower ribs, which leads to a difference in the overall contour of the rib cage (barrel-shaped in humans, conical or funnel-shaped in the great apes, as seen in anterior view). These differences in thoracic shape are largely a function of differences in the size of the superior-most and inferior-most ribs in the series, but also come about from differences in the curvature and torsion of the bodies of all of the ribs. The overall shape of the hominoid thorax may be an adaptation that serves to better dissipate tensile forces in the body during under-branch suspensory behaviors.


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

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

Schmid, 1983. Eine Rekonstruktion des Skelettes von A.L. 288-1 (Hadar) und Deren Konsequenzen. Folia Primat 40:283-306. Aiello & Dean, 1990. An introduction to human evolutionary anatomy. London: Academic Press. Hunt, 1991. Mechanical implications of chimpanzee positional behavior. Am J Phys Anthropol 86:521-536.  

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Striding Bipedalism Speculative


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