Different in degree or kind: we have less vertebrae (Introduction)

by David Turell , Monday, May 13, 2019, 18:44 (1809 days ago) @ David Turell

Ours total 24 while most mammals who give live birth have 26-27:

https://phys.org/news/2019-05-evolutionary-analysis-mammalian-vertebrae.html

"'The classic body plan of many mammals is built on a mobile back and this body plan is conserved regardless of running speed," explains New York University anthropologist Scott Williams, the paper's senior author. "More specifically, we find that a particular type of locomotor behavior—suspensory locomotion, which involves hanging below tree branches, rather than speed—is associated with increases in variation in numbers of vertebrae across mammals."

"The work centers on an effort to better understand why certain aspects of mammals remain consistent over time—a phenomenon known as evolutionary stasis.

"Despite the diversity evolution has yielded, there remain consistencies across a wide range of distantly related organisms. Of particular note is the number neck (cervical) and back (thoracic and lumbar) vertebrae of mammals.

"'Nearly all mammals have the same number of cervical vertebrae, no matter how long or short their necks are—humans, giraffes, mice, whales, and platypuses all have exactly seven cervical vertebrae," explains co-author Jeff Spear, an NYU doctoral student.

"In fact, the majority of mammals possess 19 or 20 thoracic and lumbar vertebrae, for a total of 26 or 27 "CTL" vertebrae (for "cervical, thoracic, and lumbar" vertebrae). There is little variation in these numbers, either within species or across different species—or even different species separated by over 160 million years of evolution. Humans, with 24 CTL, are one of the exceptions.

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"In their study, they counted the vertebrae of thousands of individuals for nearly 300 species of mammals. The researchers then compared variation in the number of CTL vertebrae to traits such as speed, habitat, locomotion, spine mobility, posture, and limb use.

"The analyses did not seem to show an association between vertebrae count and running speed. Rather, this trend was primarily driven by animals adapted to suspensory and other "antipronograde" behaviors, where limbs are held in tension during slow climbing, clambering, and suspension.

"This observation led the researchers to hypothesize that the classic body plan of certain mammals—therian mammals, which give birth to live young—is built on a mobile back and that this body plan is conserved regardless of running speed.

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"Changes in types of vertebrae are determined by Hox genes—the genes that organize animal bodies along the head-tail axis, ensuring that your eyes go on your face and your legs go at the base of your torso," explains Spear. "But changes in Hox gene expression sometimes creates vertebrae that are intermediate in type, which can impinge the mobility of the spine."

"For animals following the ancestral body plan, from possums to tigers, departures from the ancestral types of vertebrae in the back creates a risk of inefficient locomotion and are weeded out by natural selection, he adds.

"'Mammals that depart from this body plan, however, such as apes adapted for antipronograde behaviors, are more free to vary in their number of CTL vertebrae," says Williams. "Our own atypical number of CTL vertebrae, then, may be the consequence of our evolutionary history as antipronograde climbers.'"

Comment: Our running speed is due to pelvic changes, an arched lumbar spine and how our feet are built to propel us. Our long distance running is allowed by the fact that we sweat and lose heat while furred animals cannot do that. We are different in kind for many reasons, not just our giant brain.