Great apes and humans evolved from a long-backed ancestor

Area of Science:

• Paleoanthropology
• Comparative Anatomy
• Evolutionary Biology

Background:

• The lumbar column's structure in the last common ancestor (LCA) of Homo and Pan is debated: short and stiff (great apes) or long and flexible (Miocene hominoids).
• Lumbar stiffening involves transitional vertebra (TV) position, spinous process orientation, and vertebral entrapment by iliac blades.

Purpose of the Study:

• To investigate whether lumbar stiffening features in great apes are homologous (inherited from a short-backed LCA) or convergent (evolved independently from a long-backed LCA).
• To compare vertebral morphology and counts across humans, apes, and monkeys to infer LCA characteristics.

Main Methods:

• Quantitative and qualitative analysis of human, ape, and monkey thoracic and lumbar vertebrae using 3D surface scanning.
• Osteological measurements of spinous process morphology and sacral depth.
• Assessment of transitional vertebra (TV) position and lumbosacral boundary variation using hominoid vertebral counts.

Main Results:

• Humans and orangutans share a TV position at the 19th postcranial segment, differing from other apes (20th segment).
• Human spinous process morphology resembles that of longer-backed gibbons, monkeys, and Miocene hominoids, not great apes.
• Chimpanzees exhibit unique sacral depth for lumbar entrapment; African apes show distinct lumbosacral boundary variation.

Conclusions:

• Lumbar stiffening is convergent among great apes, indicating independent evolution of these traits.
• Human bipedalism likely evolved from a more generalized, long-backed ancestor, aligning with evidence from Australopithecus.
• The LCA possessed a more generalized vertebral column, with specialized stiffening evolving later in different lineages.