Senie cilvēki attīstījās, lai staigātu pa divām kājām divos soļos

Walking on two legs is the single most distinctive characteristic that separates humans from other primates. Unlike apes, our pelvis is short, wide, and bowl-shaped, making it stable for upright walking while supporting internal organs and providing space for the delivery of large infants. For more than a century, scientists have known that bipedalism set our ancestors on a unique evolutionary path. However, the evolutionary origin of this transformation has been a mystery.

Traditional explanations, which focused on fossil anatomy and biomechanics, could not reveal the molecular and developmental processes at work. Moreover, unlike other skeletal features, the ilium, which is the broad upper bone of the pelvis, has no obvious counterparts in primates that could reveal signs of gradual, evolutionary stepwise change. So researchers from Germany, Ireland, Britain and the United States set out to uncover the hidden steps in embryonic development that shaped the pelvis into its human form. Their findings were published in Nature on August 28.

The team examined human embryonic pelvises at different critical weeks of development, using histology to map cartilage areas and micro-CT scans to track bone formation. They compared them with mouse, chimpanzee and gibbon embryos held in museum collections. At the molecular level, they used single-cell multi-comics and spatial transcriptomics to catalog which genes and pathways were active in different cell types.

The study revealed two major innovations. First, instead of extending vertically as in monkeys and mice, the growth plate of the iliac cartilage expanded horizontally, making the pelvis wider and shorter. Second, bone formation in humans began later, at the posterior edge of the ilium, and spread outward across the surface rather than through cartilage. This unusual timing and arrangement allowed the pelvis to continue to expand in width before solidifying into bone. These changes were linked to a network of developmental regulators, which in turn are shaped by changes in human DNA.

The results suggest that bipedalism arose through a two-step reprogramming of pelvic development: By redirecting cartilage growth and delaying bone formation, human embryos acquired a pelvis capable of supporting upright walking and accommodating childbirth. Understanding these pathways may shed light on the origins of pelvic malformations seen in skeletal disorders. In evolutionary terms, the authors said, this work may also shed light on why fossil hominins like Australopithecus were already short, broad-shouldered predators millions of years ago.