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

September 20, 2018, National Science Foundation
Illustration showing an early mammal relative, Thrinaxodon, which was part of the first group to have an extra fourth section of their backbones. Credit: April Neander

Mammals are unique in many ways. We're warm-blooded and agile in comparison with our reptilian relatives.

But a new study, funded by the National Science Foundation (NSF) and led by Harvard University researchers Stephanie Pierce and Katrina Jones, suggests we're unique in one more way—the makeup of our spines. The researchers describe their finding in a paper published this week in the journal Science.

"The is basically like a series of beads on a string, with each bead representing a single bone—a vertebra," said Pierce, curator of vertebrate paleontology at Harvard. "In most four-legged , 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 .

Edaphosaurus, an early mammal relative that lived around 300 million years ago, which had a more primitive backbone with just three different regions. Credit: Field Museum
"There are no animals alive today that record the transition from a 'lizard-like' ancestor to a mammal," said Jones, lead author of the study. "To do that, we have to dive into the fossil record and look at the extinct forerunners of mammals, the non-mammalian synapsids."

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

But studying fossils isn't easy. "Fossils are scarce and finding extinct animals with all 25-plus vertebrae in place is incredibly rare," Jones said.

To tackle this problem, the researchers combed museum collections around the world to study the best-preserved fossils of animals that lived some 320 million years ago.

"Looking into the ancient past, an early change in mammals' spinal columns was an important first step in their evolution," said Dena Smith, a program director in NSF's Division of Earth Sciences, which funded the research. "Changes in the spine over time allowed mammals to develop into the myriad species we know today."

Skeletons of a modern dog and cat -- note the regions with different shapes of bones that make up the spine. Credit: Field Museum

Pierce and Jones, along with co-author Ken Angielczyk of the Field Museum in Chicago, examined dozens of fossil spines, as well as more than 1,000 vertebrae of living animals, including mice, alligators, lizards and amphibians.

They wanted to find out whether mammal vertebral regions were as ancient as previously thought, or if mammals were doing something unique.

"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," said Pierce.

Shows the primitive number of regions (3) for synapsids (mammals and their relatives). The fossil, Edaphosaurus, belongs to a group of mammal ancestors known as 'pelycosaurs.' Edaphosaurus lived during the late Carboniferous to early Permian (300-280 million years ago) of North America and Europe. Credit: Stephanie E. Pierce, Museum of Comparative Zoology, Harvard University

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," said Jones.

About 250 million years ago, a new 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.

The three stages of mammal backbone evolution on a phylogenetic tree. Bottom right: Edaphosaurus; middle left: Thrinaxodon; top: a modern day mouse. Credit: Stephanie E. Pierce, Museum of Comparative Zoology, Harvard University
"There appears to be some sort of cross-talk during development between the tissues that form the and the shoulder blade," Pierce said. "We think this interaction resulted in the addition of a region near the shoulder as the forelimbs of our ancestors evolved to take on new shapes and functions."

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," said Pierce.

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 and ideas in modern developmental biology and genetics," Jones said. "This combined approach is helping us understand what makes a mammal a ."

Explore further: Tiny fossils reveal how shrinking was essential for successful evolution

More information: K.E. Jones el al., "Fossils reveal the complex evolutionary history of the mammalian regionalized spine," Science (2018). science.sciencemag.org/cgi/doi … 1126/science.aar3126

Related Stories

Tiny fossil horses put their back into it

April 27, 2016

Modern horses are expert runners. They reach top speeds using a special running gait in which they hold their back stiff as they move. A new study published today reveals that tiny fossil ancestors of modern horses may have ...

Recommended for you


Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (4) Sep 21, 2018
I ONLY have human relatives.

These scientists have reptiles and chimps and frogs and pigs and wolves and hydra as relatives....what kind of monsters are they?
1 / 5 (4) Sep 21, 2018
"Looking into the ancient past, an early change in mammals' spinal columns was an important first step in their evolution," said Dena Smith

There, right there - looking into the past - lies the most ubiquitous problem facing the evolutionist.
No one can look back into the past and see what happened there. One can only look at the present and using some basic assumptions, begin to make inferences about the past.

The whole evolutionary model is founded on some terribly unjustifiable assumptions.
The most glaring and basic one is that life can arise from non-living (i.e. DEAD) material all by itself via random chemical and physical processes with no intelligent and powerful help from outside. So far this assumption is pure non-science - it has not been observed, cannot be verified and definitely cannot be repeated. Yet without abiogenesis, darwinian evolution is dead in the water. If you cannot get the first ancestor going by itself you've got NOTHING.
5 / 5 (2) Sep 21, 2018
Well, to paraphrase Samuel Clemens...

fj along with all the corrupt evangelists, altright fairytails, pseudo-conservatives, fake libertarians and false patriots...

"Are lizards pretending they are descended from dinosaurs."

Mr. Clemens urged us to pity them...


No, the quislings and copperheads have exhausted what little sympathy I ever had for their mass stupidity.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.