How mammal spines changed in evolution decoded

Researchers have revealed for the first time how and when changes in the spine happened during the evolution of mammals, a finding which sheds more light on how different forms of backbones came into existence in several modern terrestrial animals.

The study, published in the journal Nature Ecology and Evolution, found the developmental changes in the extinct ancestors of mammals which laid the groundwork for the diverse forms of backbones seen today. In the study, the researchers, including those from Harvard University in the US, compared the bone and muscle movement patterns of two modern animals, cat and lizard, and the X-ray scans of fossils belonging to the mammal ancestor, the synapsid, which may have gone extinct around 252 million years ago.

They assessed how each animal's spinal joints bent in different directions to measure how the form of the backbone reflects their function. According to the scientists, while some spinal regions can function differently, others do not. Citing an example, they said, the lizard's backbone comprised several distinct regions, but they all acted in the same way. The findings, according to the researchers, overturned the traditional notion that the gradual accumulation of different regions of the spine alone account for its evolving complexity.

Instead, the study suggested that regions like the lower back evolved long before new spinal functions, such as bending and twisting. Modern mammals have developed compartmentalised spinal regions which take on a number of diverse shapes and functions without affecting other spinal regions, the study noted. The researchers said this has allowed the animals to adapt to different ways of life.

"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," said study co-author Stephanie Pierce from Harvard University. "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," Pierce said. Without these distinct developmental regions in place, she said all the backbones may have adapted in the same way under a selective pressure. Developing different spinal regions was long believed to be an important step in evolving backbones with many functions, the researchers said.

However, Pierce and her team showed that an evolutionary trigger was also required -- in this case the evolution of a highly active lifestyle that put new demands on the backbone. "We're trying to get at something that's quite a fundamental evolutionary question which is: How does a relatively simple structure evolve into a complex one that can do lots of different things? Is that determined by the limitations of development or natural selection related to the behaviour of the animal," said study co-author Katrina Jones from Harvard University.