What makes a mammal a mammal? Our spine, say scientists

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.

Phylogenetic tree showing mammal backbone evolution: Edaphosaurus, Thrinaxodon, today's mouse. Image credit: Harvard University Museum of Comparative Zoology

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.

These ancient ancestors hold the key to understanding the origin of mammal-specific characteristics, including the spine.

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.

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.

About 250 million years ago, a new region evolved near the shoulders and front legs. Dramatic changes also began to appear in the forelimbs of animals known as non-mammalian therapsids.

These simultaneous developments, the scientists believe, likely occurred in conjunction with changes in how creatures walked and ran.

Later, a region emerged near the pelvis. It is this last region, the ribless 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. (National Science Foundation)

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