Charles Darwin unveiled his theory of evolution in 1859, in “On the Origin of Species.” But it took him another 12 years to work up the courage to declare that humans evolved, too.
In “The Descent of Man,” published in 1871, Darwin argued that humans arose from apes. And one of the most profound changes they underwent was turning into upright walkers.
“Man alone has become a biped,” Darwin wrote. Bipedalism, he declared, was one of humanity’s “most conspicuous characters.”
Scientists have now discovered some of the crucial molecular steps that led to that conspicuous character millions of years ago. A study published in the journal Nature on Wednesday suggests that our early ancestors became bipeds, as old genes started doing new things. Some genes became active in novel places in the human embryo, while others turned on and off at different times.
Scientists have long recognized that a key feature for walking upright is a bone called the ilium. It’s the biggest bone in the pelvis; when you put your hand on your hip, that’s the ilium you feel.
The left and right ilium are both fused to the base of the spine. Each ilium sweeps around the waist to the front of the belly, creating a bowllike shape. Many of the leg muscles we use in walking are anchored to the ilium. The bone also supports the pelvic floor, a network of muscles that acts like a basket for our inner organs when we stand up.
As vital as the ilium is to everyday life, the bone can also be a source of suffering. The ilium can flare up with arthritis, grow brittle in old age, especially in women, and fracture from a fall. Genetic disorders can deform it, making walking difficult. The ilium also forms much of the birth canal — where babies can sometimes get stuck, endangering the mother’s life.
And yet, as important as the ilium is to us, its development has long been a mystery. “It’s remarkable to me,” said Terence Capellini, a developmental geneticist at Harvard. “The ilium is essential to how we walk and how we give birth, and yet very little is known about it.”
Dr. Capellini and his colleagues embarked on an intensive study of the bone. As part of the research, Gayani Senevirathne, a postdoctoral researcher at Harvard, examined human fetal tissue from a University of Washington repository. Dr. Senevirathne created three-dimensional models of the human ilium as it developed in embryos. She also analyzed the different types of cells that combine to form the bone, as well as the genes that switch on and off inside those cells.
She then did similar experiments on mice, dissecting their embryos and analyzing the cells in the developing ilium. Comparing the two species, she gathered some clues about how our own ilium evolved.
But there were limits to what mice could tell her, since they are only distantly related to humans. To get a better sense of what sort of ilium early humans inherited, Dr. Senevirathne needed to look at primates.
She reached out to museums across the United States and Europe to see if they had any primate specimens. She tracked down embryos of chimpanzees, gibbons and other species preserved in jars, and arranged for museum curators to scan them for her.
One day on her quest for material, she left Boston before dawn and drove to the American Museum of Natural History in New York. There, she loaded the car with crates of 100-year-old glass slides, each preserving a slice of a lemur embryo. Then she drove right back home.
“I was worried that we’d get pulled over by the police,” Dr. Senevirathne said. “But it was definitely worth it. We actually needed that material to complete our story.”
All told, the researchers studied 18 different species of primates. “The fact that they assembled so many embryonic samples was really impressive,” said Camille Berthelot, an evolutionary geneticist at the Pasteur Institute in Paris who was not involved in the study.
Dr. Senevirathne and her colleagues found that primates develop the ilium in much the same way mice do. Two tiny rods of cartilage take shape on either side of the spine and parallel to it. The rods grow and fuse to the spine, and bone cells replace the cartilage.
Dr. Senevirathne and her colleagues figured that the human ilium had evolved from this ancient blueprint. They expected that in a human embryo each ilium would start as a rods of cartilage parallel to the spine; eventually t would stop growing in that direction and expand forward.
“Lo and behold, that’s not the case,” Dr. Capellini said. “It’s not a stepwise process. It’s actually a complete flip.”
The human ilium, the scientists were surprised to discover, starts as a rod perpendicular to the spine; one end points forward toward the belly, and the other points toward the back. The cartilage rod retains this orientation as it grows into the final shape of the ilium.
“That was really striking to us,” Dr. Capellini said. “Nowhere in the human body do you find a place where humans have just changed the way we grow altogether.”
Just as strikingly, Dr. Capellini and his colleagues found, our ilium employs the same network of genes that are active in ilium cells in mice; they just work differently.
In human embryos, ilium cells turn the genes on and off in a new pattern in response to molecules released by neighboring cells. The result is a rod of cartilage forming in a new direction.
Dr. Berthelot said that this hypothesis made sense. Other researchers have discovered that the evolution of other parts of the skeleton were driven by similar changes to existing genes. “There are not so many ways that you can change the shape of a bone,” she said.
Dr. Capellini and his colleagues argue that this flip was crucial to the evolution of bipedalism. It allowed early human ancestors to grow a new kind of pelvis that supported muscles strong enough for walking upright.
But the new study also suggests that the ilium underwent a second major change millions of years later, when humans evolved big brains. The scientists discovered that the ilium is slow to switch from cartilage to bone, lagging about 15 weeks behind the rest of the skeleton. “It’s a unique, radical shift,” Dr. Capellini said.
Dr. Capellini suspects that this shift occurred as the brains of early humans expanded about a million years ago. While a big brain likely boosted our ancestors’ mental powers, it also created new risks. The large heads of babies could get stuck in the birth canal. Natural selection favored new curves on the ilium that gave human mothers a rounder birth canal, which made deliveries easier.
Dr. Capellini’s team is eager to continue their research, to better understand the history of the pelvis and to learn how its evolution in humans made us vulnerable to certain disorders. But in May the Trump administration terminated billions of dollars of funding to Harvard, including the grant that supported the pelvis research. At the time, Dr. Capellini and his colleagues were only two years into a five-year project.
“We are all wondering what would have come next had we not lost this funding,” Dr. Capellini said.
Carl Zimmer covers news about science for The Times and writes the Origins column.
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