Charles Darwin considered getting about on two legs as one of the most important evolutionary steps toward becoming human, and the fossil record supports his view.

Walking and running on two legs, known as bipedalism, freed up the arms and especially the hands for other tasks such as tool-making, creating art and music, all of which was enabled and fostered by increasingly larger brains.

The last common ancestor leading on one hand to chimpanzees and bonobos and on the other hand eventually to modern humans lived six to seven million years ago. While most of the last common ancestor’s time was spent in the trees searching for food, rest and sleep, and keeping a wary eye out for predators, early adaptations for a bipedal existence on the ground gradually evolved.

The alternative to bipedal walking was knuckle-walking — an awkward way to get about on the ground, especially if you had to move quickly, because the feet weren’t centered beneath the center of gravity, making for awkward walking and near impossible running.

To center successive footprints beneath the centre of gravity required re-engineering the pelvis to angle the hip, knee and foot joins more toward the midline, while the pelvis which would need to be refashioned to make birthing newborns as safe as possible.

Which brings up this question: how did natural selection reshape the pelvis and realign the hips, knees, ankles and feet for walking and running with a pelvis of the right shape and size for childbirth?

Those interrelated questions were tackled by an interdisciplinary group at Harvard and elsewhere led by Gayani Senevirathne, a post-doc student who with her colleagues, studied the anatomical changes and genes responsible for reshaping the pelvis in various primates past and present, mice and modern humans to make bipedalism efficient.

When you place your hands on your hips, your hands rest on the upper outermost rim of the largest bone in the pelvis: the ilium.

The latter creates a large bowl-like structure anchored by the sacroiliac joint to the lumbosacral spine medially and which supports the pelvic floor muscles and much of the contents of the abdomen and pelvis.

The ilium also forms the boney sockets for the femur’s bulbous heads to rotate in, and anchors several major muscles involved in walking.

Changes to the ilium played crucial roles in the evolution of bipedalism in species such as the australopiths, some variants of which such as A. afarensis (the famous Lucy) were bipedal most of the time while retaining some capacity for scampering up trees as needed. Then, beginning more than two million years ago, all homo species were fully capable of walking and running bipedally.

That capacity required remodeling the shape of key bones such as the ilium, femur, tibia and fibula and all the ankle and foot bones — as it turns out, without changing the genes responsible.

What changed was the timing with which the responsible genes were activated or silenced. That made all the difference in the shape of the forming cartilage templates that were later ossified to form the bones with which we’re familiar.

Some of changes were dramatic. For example the cartilage template for humans forms horizontally in the anterior-posterior axis, whereas in primates who are not bipedal and mice, the cartilage template for the ilium forms in the top to bottom axis.

Once the cartilage templates are formed, osteocytes form calcified bone on the surface of the cartilage, thereby taking the form of the underlying cartilage template.

The process governing the formation of the ilium is probably universal. Tweak the timing and selection of existing genes to change the shape of cartilage first, then the bones to reshape other bones such as those that make up the skull, jaw, spine and their associated joints. That’s the lesson Gayani Senevirathne and her colleagues taught us.

Their study also reminds us that the genome is far more complex than the many thousands of genes that code for specific protein. Genes sometimes works in teams and other genes, together other molecules called transcription factors, influence which genes are active, which are silent and the sequence of timing of both.

The genome might be likened to a deck of cards. What happens depends on what cards are in play — even as the whole deck of cards remains the same. Or, to piano keys: the music produced depends on the order and selection of the keys, but the keys remain the same.

Unfortunately, funding for this project came to an end last year as part of the federal governments cutbacks in the U.S.

P.S. If you want a readable book on how walking and running evolved, turn to Jeremy DeSilva’s 2021 book, “First Steps.”

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the InfoHealth series at the Niagara-on-the-Lake Public Library.