From the long necks of giraffes that help them reach treetops to the flexibility of a cheetah’s spine that helps it go from zero to sixty in three seconds, mammals not only have the figurative backbone to do amazing things, but, more importantly, they have the literal one. But they weren’t always that way.
A new study published in Nature Ecology and Evolution by Harvard researchers and collaborators at the Field Museum of Natural History sheds light on how and when changes in the spine happened in mammal evolution. Associate Professor of Organismic and Evolutionary Biology and curator of vertebrate paleontology Stephanie Pierce and postdoctoral researcher Katrina Jones discovered how a combination of developmental changes and adaptive pressures in the spines of synapsids, the extinct forerunners of mammals, laid the groundwork for the diversity of backbones seen in mammals today.
Using a novel combination of techniques including biomechanical experimentation on the cadavers of two modern animals, a cat and a lizard, and CT scans of synapsid fossils, the researchers were able to reject the traditional notion that the gradual development of different regions (or independent sections) of the spine alone accounted for its evolving complexity. Instead, their evidence suggests that regions (like the thorax and lower back) evolved long before new spinal functions (such as bending and twisting). The study points to the idea that the right selective pressures or animal behaviors combined with these already-existing regions played a significant role in the evolution of their unique functions.
The findings tap into the larger question of how mammals, including humans, evolved over millions of years. Mammals today, for instance, have developed compartmentalized spinal regions that take on a number of diverse shapes and functions without affecting other spinal regions. This has allowed the animals to adapt to different ways of life, said Jones, who was the study’s lead author.
In a previous study the authors showed that extinct premammalian land animals developed these small but distinct regions during evolution. “What we were able to show in 2018 was that even though all the vertebrae looked very similar in early mammal ancestors, they had subtle differences and those subtle differences created distinct developmental regions,” Pierce said. “What we’re showing with this new study is that those distinct regions were really important as they provided the raw material that facilitated functional differentiation to happen. Basically, if you don’t have these distinct developmental regions in place and you have a selective pressure, all the vertebrae are going to adapt in the same way.”
It’s long been thought that developing different spinal regions is one important step in evolving backbones with many functions, but Pierce and Jones show that this isn’t enough. An evolutionary trigger was also required, in this case the evolution of a highly active lifestyle that put new demands on the backbone.
