Ribs evolved for movement first, then co-opted for breathing

Date:

Share post:

When early tetrapods transitioned from water to land the way they breathed air underwent an evolutionary revolution. Fish use muscles in their head to pump water over their gills. The first land animals utilized a similar technique–modern frogs still use their head and throat to force air into their lungs. Then another major transformation in vertebrate evolution took place that shifted breathing from the head to the torso. In reptiles and mammals, the ribs expand to create a space in the chest that draws in breath. But what caused the shift?
Ribs evolved for movement first, then co-opted for breathing
Researchers captured the 3-D motion of lizard ribs and vertebrae using XROMM, a combination of CT scans
and X-ray videos. They recorded three savannah monitor lizards and three Argentine black and
white tegus walking slowly on a treadmill [Credit: Cieri et al., 2020]




A new study published in Scientific Reports posits a new hypothesis–the intermediate step was driven by locomotion.

When lizards walk, they bend side-to-side in a sprawling gait. The ribs and vertebrae are crucial to this movement, but it was unclear how until now. Researchers captured the 3-D motion of lizard ribs and vertebrae using XROMM, a combination of CT scans and X-ray videos. They recorded three savannah monitor lizards and three Argentine black and white tegus walking slowly on a treadmill. 
The resulting images revealed that while the spine is bending, every rib in both species rotated substantially around its vertebral joint, twisting forward on one side of the body and backward on the other side alternatively with each stride. The mechanics follow nearly the same pattern as when the reptiles inhale and exhale.
Ribs evolved for movement first, then co-opted for breathing
Credit: University of Utah




“It’s really exciting because we didn’t previously have plausible hypotheses for how rib-breathing evolved,” said first author Robert Cieri, a postdoc at the University of the Sunshine Coast and who conducted the research while at the University of Utah, where he is still affiliated. “We’re proposing that these rib movements first started to facilitate locomotion, then were co-opted for breathing.”

The work also included John Capano and Elizabeth Brainerd at Brown University, and undergraduate Samuel Hatch at the U.
Why did the lizard cross the road? To catch its breath.
Ribs evolved for movement first, then co-opted for breathing
An XROMM animation showing how the savannah monitor lizard’s gait has two phases: propulsion – when the foot
pushes the ground; and swing–when the foot is in the air. On the forelimb’s propulsion side, the ribs rotate
 towards the head while on the swing side, the ribs rotate towards the tail. This is the movement that
facilitates inhalation and exhalation when the ribs move bilaterally [Credit: Cieri et al., 2020]




Reptiles, birds and mammals, all considered amniotes, use costal aspiration where the ribs and vertebrae control breathing by expanding and contracting. The motion is facilitated by the ribs rotating around the vertebrocostal joints–where the ribs and vertebrae meet. It’s possible that an early amniote ancestor first used these rib movements to enhance its lateral undulation, the side-to-side trunk motion that helps propel lizards forward. 

During locomotion, each limb has two phases: First is propulsion, which is when the foot is pushing against the ground. Second is swing, which is when the foot is in the air. The new research found that during the arm’s propulsion phase, the ribs on the same side of the body rotate towards the head, and while it’s in the swing phase, they rotate towards the tail. This mimics what the scientists previously found the ribs doing inhalation and exhalation–but this time simultaneously on both sides.
Early tetrapods used sprawling, undulatory locomotion, but likely still had a head-driven breathing system. The authors propose that rib movements and increased costal joint mobility evolved during locomotion to increase stride length, hold the thorax steady, or as the passive result of the ribs moving to prevent being bunched together as the animals bended. 
Ribs evolved for movement first, then co-opted for breathing
A simulation showing how ribs might crowd together if they didn’t rotate relative to the vertebrae 
when the spine bends during locomotion [Credit: Cieri et al., 2020]

At some point, early amniotes evolved the ability to express these rib movements on both sides simultaneously, which allowed for the expansion and contraction of the trunk that support inhalation and exhalation.
“Aspiration breathing was one of the key innovations that allowed amniotes to diversify on land. We were surprised and thrilled to find clues to how it evolved from a study on locomotion,” co-author Brainerd said.

ADVERTISEMENT

spot_img

Related articles

Ancient origins of viruses discovered

Research published in Nature has found that many of the viruses infecting us today have ancient evolutionary histories...

Hidden genetic variations power evolutionary leaps

Laboratory populations that quietly amass 'cryptic' genetic variants are capable of surprising evolutionary leaps, according to a paper...

Global genome search sparks new insights into microbes that breathe rocks

Microbes may be miniscule, but they have a massive impact on Earth and its habitability. They are uniquely...

Scientists identify rare evolutionary intermediates to understand the origin of eukaryotes

A new study by Yale scientists provides a key insight into a milestone event in the early evolution...

Darwin’s finches may face extinction

Mathematical simulations at the University of Utah show parasitic flies may spell extinction for Darwin's finches in the...

Scientists discover new chemistry that may help explain the origins of cellular life

Before life began on Earth, the environment likely contained a massive number of chemicals that reacted with each...

Natural affinities – unrecognized until now – may have set stage for life to ignite

The chemical components crucial to the start of life on Earth may have primed and protected each other...

Study supports distant relationship between Archaea and Bacteria in tree of life

Scientists have found further evidence to support the idea that the primary two domains of life, the Archaea...