Muscle Fiber Evolution and Energetic Trade-offs

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The genetic & developmental changes behind bipedalism & human anatomy.

Wide release date: October 15, 2025.

Episode Summary: Dr. Terence Capellini talks about the evolution of bipedalism in humans, exploring when and why it emerged, the anatomical changes required, and the genetic mechanisms behind these adaptations. They discuss how environmental shifts, like shrinking forests, drove the need for upright walking, the gradual skeletal changes in the pelvis and limbs, and how these changes may have facilitated larger brain sizes. Capellini highlights the complexity of evolutionary processes, emphasizing the role of multiple genetic changes in regulatory regions rather than single genes.

About the guest: Terence Capellini, PhD is a professor and chair of the Department of Human Evolutionary Biology at Harvard University. His research focuses on developmental genetics and human evolution.

Discussion Points:

• Bipedalism likely became common ~3.5 million years ago with Australopithecus afarensis, with earlier hominins like Ardipithecus showing mosaic traits.
• Environmental changes, such as shrinking forests and expanding grasslands, created selective pressures favoring bipedal locomotion.
• The human pelvis evolved to be shorter, wider, and curved, with muscles like the gluteus medius shifting to stabilize upright walking.
• Genetic changes in non-coding regulatory regions, not protein-coding genes, drive the developmental shifts in pelvic growth, with hundreds of small-effect changes involved.
• Bipedalism may have widened the birth canal, potentially enabling the evolution of larger brains in later hominins like Homo erectus.
• Humans have more slow-twitch muscle fibers than chimpanzees, supporting endurance activities like long-distance running, possibly linked to energetic trade-offs with brain growth.
• Shoulder and arm adaptations for throwing and tool use evolved more gradually, becoming prominent ~2 million years ago with Homo erectus.

Reference paper:

• Study: The evolution of hominin bipedalism in two steps

Related content:

• M&M 171: Comparative Brain Evolution: Mammals, Primates & Humans | Robert Barton

*Not medical advice.

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