When mammal ancestors evolved flexible shoulders, their backbones changed too

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Credit: Field Museum

Shrug your shoulders. If you're not in public, shimmy a little. You can do that because you, as a mammal, have flexible shoulders. And scientists have just discovered that those flexible shoulders are the key to how mammals' complex backbones evolved.

"If you look at animals today, mammals stand out because that they have backbones where you can clearly see different regions; the vertebrae in each region look different from those in other regions," says Ken Angielczyk, a Field Museum paleontologist and co-author of a new study in Science. "And now that we have a better understanding of how mammals' backbones evolved, it looks like those changes were closely tied to changes that were happening in the shoulders."

Mammal spines are made up of differently-shaped backbones (or vertebrae) in different sections of the spine–we have thin, flexible neck bones, thoracic vertebrae in our torsos connected to our ribs, and thick, rib-less lumbar vertebrae down by our hips. Reptiles' spines appear more uniform because the bones in the different sections are all pretty similar. If mammal spines are an elegant five-course meal, reptile spines are more like a hotdog-eating contest–more of the same.

For years, it was a mystery why mammals' spines were so much more complex than reptiles'. In the Science study, though, Angielczyk and his colleagues, including lead author Katrina Jones from Harvard University, looked for clues in the backbones of mice, alligators, lizards, amphibians, and fossil mammal relatives that lived between about 300 and 190 million years ago. In examining these animals' spines, the team found a pattern.

Mammals' ancient relatives began to evolve smaller, more flexible shoulders about 270 million years ago. At about the same time, the animals' upper backs underwent changes too–a move toward the many-sectioned mammal spines we see today. Both the shoulders and the spine play an important role in locomotion, and these simultaneous evolutionary modifications likely reflect changes in how ancient mammal relatives walked and ran.

These complex backbones opened the floodgates for mammal diversification. "It's a little like that drawing game where you fold up a piece of paper and everyone draws a different part of a person–someone draws the head, someone else draws the shoulders, someone else draws the torso," says Angielczyk. "The more sub-sections you have, the weirder-looking the result." With different regions of the spine evolving separately, mammals were able to branch into wildly different body plans. "We use our backbones in very distinctive ways that are closely tied to our different modes of life," says Angielczyk. "Cheetahs have flexible spines that let them conserve energy and take bigger strides, humans have unusual vertebral columns that let us walk upright, and whales have stiffer backbones with a few sections that are highly mobile so that they're better swimmers."

Compared to our fellow four-limbed animals, mammals' different ways of moving are unusual. Lizards all crawl in the same general fashion, and birds, from penguins to falcons, move their wings more or less the same way. Mammals, as they alternately gallop, swim, fly, and stroll, break the rules.

The paper's authors note that the study is important because it illuminates how mammals, including humans, evolved into the myriad forms we see today. "This is part of our history, why we're bipedal. Without these changes 260 million years ago, the form that humans have wouldn't have evolved," says Angielczyk.

"This study shows that changes to the blueprints important for building the mammalian back likely occurred many millions of years before the origin of mammals," says lead author Katrina Jones. "This demonstrates that evolutionary transitions can occur in surprising ways, which we would not expect from looking only at living animals. Often, fossils are the only direct evidence we have of the origin of animal groups and can change our understanding of the history of life."

"Mammals can be found on continents and in oceans around the world," says Dena Smith, a program director in the National Science Foundation's Division of Earth Sciences, which funded the research. "Looking into the ancient past, an early change in mammals' spinal columns was an important first step in their evolution. Changes in the spine over time allowed mammals to develop into the myriad species we know today."

Angielczyk also notes that the study shows the importance of paleontology. "You wouldn't have discovered this without fossils. A lot of the specimens we looked at were collected in the 1890s to the 1950s, when the techniques we used didn't exist. Specimens collected for one reason are useful later on for a totally different reason. It's a great plug for why natural history museums matter."

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