From four to two

How evolution tinkers to achieve major changes

As you probably figured, I am interested in the origin of language, probably the most obvious characteristic of our species. But we often forget that language is not the only unique feature of humans. Humans are also the only mammals that permanently walk on two legs. Birds do it too, but for completely different evolutionary reasons. Our ancestors’ transition from walking on four legs to using only two for locomotion started a long time ago¾around 6 to 7 million years ago when our ancestors split from the ancestors we shared with chimpanzees. That ‘decision’, to move out of the trees and commit to a life on the ground, was monumental as it was the beginning of us. Monumental as the decision may have been, the process that led a bipedal lifestyle turns out to be much more mundane as a recent paper showed. A little tinkering here and there did the trick.

Let me explain.

In a paper published in the journal Nature in 2025, Senevirathne and colleagues, explore the ways in which the development of our pelvis differs from that of other primates. Instead of some massive overhaul of part of human development, they discovered that the changes of the human pelvis needed to be able to walk upright are the result of small, spatially and temporally targeted changes in the gene network that orchestrates bone formation. That gene network regulates the so-called conserved endochondral programme, a highly regulated and evolutionarily preserved biological process by which a temporary cartilage template is systematically replaced by bone during development and growth. This process is fundamental to the formation and growth of most bones in the vertebrate skeleton, particularly long bones.

A typical human pelvis is a robust, basin-shaped ring of bones that serves as the foundation of the trunk, connecting the spine to the legs. Its primary functions are weight-bearing, providing muscle attachment for the strong muscles required for walking, and protecting the internal organs. The pelvis consists of four primary bones that are joined by strong ligaments: two hip bones, the sacrum and the coccycx, or tailbone. The hip bones form the sides and front of the pelvis and are in turn comprised of three bones that fuse during puberty. Because they are not set in stone, so to speak, until relatively late in development, the pelvis adapts as a child moves from crawling to walking. The most important part of the hip bones with respect to evolutionary changes is the ilium, the large, blade-shaped upper part of the pelvis that forms the width of the hips.

Changes to the way genes are expressed that regulate the endochondral programme, result in a change in the main direction of growth of the iliac cartilage coupled with a change in the pattern that determines where and when cartilage is transformed into bone. The change in the main direction of growth results in a short, wide ilium blade typical of humans. The delay in ossification¾the transformation of cartilage into bone¾allows the ilium blade to continue to grow for longer so forming the ‘flare’ of the human pelvis. The pelvis so provides a large surface for the attachment of major muscles of the core, hips and thighs, including the gluteal muscles, hip flexors and hamstrings. These muscles generate the force required to move the legs.

Major changes due to minor tinkering. By tuning the developmental gene-regulatory network rather than inventing new skeletal elements, evolution provided a mechanism to transform a quadrupedal primate pelvis into a short, wide, bowl-shaped pelvis suitable for obligate bipedalism and childbirth of large-brained infants. Because of the malleability of the underlying gene-regulatory network, our ancestors’ pelvis gradually adapted to a bipedal lifestyle as they spent more time on two legs.

I just love the way in which evolution ‘plays’ with little changes here and there to huge effect. By adapting the way genes are expressed instead of inventing completely new ways of doing things, evolution allows rapid changes. It may well be that our earliest ancestors were forced out of the trees due to changes to the climate and thus their food supply, but by being a tinkerer and not a wizard, evolution made it possible for them to make the best out of a bad job.