What makes a mammal a mammal?

Mammals are unique in many ways. We're warm-blooded and agile in comparison with our reptilian relatives. But a new study suggests we're unique in one more way -- the makeup of our spines. 

The spine is basically like a series of beads on a string, with each bead representing a single bone -- a vertebra. In most four-legged animals, like lizards, the vertebrae all look and function the same.

But mammal backbones are different. The different sections or regions of the spine -- like the neck, thorax and lower back -- take on very different shapes. They function separately and so can adapt to different ways of life, like running, flying, digging and climbing. While mammal backbones are specialized, the regions that underlie them were believed to be ancient, dating back to the earliest land animals. Mammals made the most of the existing anatomical blueprint, or so scientists believed. However, the new study is challenging this idea by looking into the fossil record.

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.

If vertebral regions had remained unchanged through evolution, as hypothesized, we would expect to see the same regions in the non-mammalian synapsids that we see in mammals today.

But that doesn't seem to be the case. When the researchers compared the positioning and shape of the vertebrae, they found something surprising. The spine had gained new regions during mammal evolution. The earliest non-mammalian synapsids had fewer regions than living mammals.

"There appears to be some sort of cross-talk during development between the tissues that form the vertebrae and the shoulder blade, Later, a region emerged near the pelvis. "It is this last region, the rib less lumbar region, that appears to be able to adapt the most to different environments.

The final step in building the mammal backbone may be linked with changes in Hox genes, important to spine regions early in their development.We've been able to make connections among changes in the skeletons of extinct animals and ideas in modern developmental biology and genetics