A significant study published in the journal *Nature* has detailed two crucial evolutionary transformations that allowed human ancestors to walk upright. Researchers from Harvard University, working with an international team, analyzed embryonic tissue from humans and primates. They identified key developmental shifts in the pelvis, the structure central to bipedal locomotion.
The first major adaptation involved a significant reorientation of pelvic growth plates during embryonic development. In humans, the ilium, or upper part of the hipbone, rotated 90 degrees. This change resulted in a shorter, wider, and more curved pelvis compared to the tall, narrow ilium seen in other primates. This anatomical shift, which began between 5 and 8 million years ago when hominins diverged from African apes, is estimated to have provided enhanced balance and efficiency for upright walking and running.
The second critical innovation concerned the timing of pelvic ossification, the process where cartilage turns into bone. In humans, this ossification was delayed, particularly in the interior of the pelvis. This delay, likely occurring within the last 2 million years, allowed the pelvis to maintain its newly acquired shape as it grew. It also helped accommodate the increasing brain size of early human ancestors, addressing the "obstetrical dilemma"—the challenge of balancing a pelvis optimized for locomotion with one wide enough for childbirth.
These two evolutionary changes, driven by complex genetic networks involving over 300 genes, represent a fundamental shift in pelvic development with no known parallel in other primates. The research employed advanced techniques such as histology and CT scans on both contemporary and historical specimens, demonstrating how minor alterations in embryonic development can lead to substantial anatomical differences. Mutations in genes like SOX9 and PTH1R, involved in these processes, have been linked to disorders with abnormally narrow hipbones, highlighting the genetic basis of these adaptations.
The findings offer new insights into human origins, the genetic basis of pelvic and spinal conditions, and the importance of interdisciplinary research combining genetics, developmental biology, and paleontology. Lead author Gayani Senevirathne, a postdoctoral fellow at Harvard, stated that these changes resulted from a complex interplay of DNA elements regulating gene behavior during early growth, rather than a single gene. The fossil record, with early hominin pelvises such as those of *Ardipithecus ramidus* (4.4 million years ago) and *Australopithecus afarensis* (Lucy, 3.2 million years ago), provides evidence of these transformations, showing increasingly human-like pelvic features over time. This research clarifies the evolutionary path to our upright gait, which freed our hands for tool use, and underscores the intricate relationship between genetic programming and developmental processes in shaping significant life adaptations.