進化：人間の二足歩行への二つの小さなステップ

The upper part of the human pelvis, known as the ilium, underwent two major structural innovations during evolution that enabled humans to walk on two legs, a Nature paper reveals. The study lays the developmental and genetic groundwork for the human-defining trait of bipedalism.

The ilium is the big, flared part of the pelvis that anchors the important gluteal muscles that humans use to stay upright. Differences in the illum between humans and other apes are a defining evolutionary feature. However, the developmental processes that led to the unique human ilium shape remain unknown.

Terence Capellini and colleagues use histological, anatomical, and functional genomic approaches to reveal how the structure acquired its unique shape. One key change involved cartilage formation. A shift in the orientation of the cartilage growth plate enabled the ilium to sit perpendicular to the direction seen in other primate ilia. A second key change involved the process of bone formation. The authors identify timing and spatial differences in the way that bone cells are laid down over the cartilage in the human ilium, compared with non-human primate ilia and human long bones.

Both innovations are interconnected at tissue and molecular levels. Hundreds of regulatory sequences were identified that are active during the development of the human ilium. These show evidence of human evolutionary change, suggesting that key complex, interacting sequences were selected for through time to endow the human pelvis with its unique shape.