What makes a mammal a mammal?According to Wikipedia, vertebrates within the class Mammalia are distinguished from reptiles and birds by the possession of a neocortex (region of the brain), hair, three middle ear bones, and mammary glands.But new research, led by Harvard University’s Stephanie Pierce and Katrina Jones, suggests there’s a simpler answer: our spine.“The spine is basically like a series of beads on a string, with each bead representing a single bone—a vertebra,” Pierce, curator of vertebrate paleontology at Harvard, said in a statement. “In most four-legged animals, like lizards, the vertebrae all look and function the same.Phylogenetic tree showing mammal backbone evolution: Edaphosaurus, Thrinaxodon, today’s mouse (via Harvard University Museum of Comparative Zoology)“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,” she continued. “They function separately and so can adapt to different ways of life, like running, flying, digging, and climbing.”In an effort to better understand the transition from a “lizard-like” ancestor to a mammal, scientists must examine the fossil record for our extinct forerunners, the non-mammalian synapsids.Fossils aren’t exactly a dime a dozen, though; finding long-gone animals with all 25-plus vertebrae in place “is incredibly rare,” lead study author Jones said.With some monetary help from the National Science Foundation (NSF), the researchers combed museum collections around the world, looking for the best-preserved fossils of creatures that lived some 320 million years ago.A fossil specimen of Thrinaxodon, on display in the Harvard Museum of Natural History (via Harvard University Museum of Comparative Zoology)“Looking into the ancient past, an early change in mammals’ spinal columns was an important first step in their evolution,” Dena Smith, a program director in NSF’s Division of Earth Sciences. “Changes in the spine over time allowed mammals to develop into the myriad species we know today.”Pierce and Jones, along with co-author Ken Angielczyk of the Field Museum in Chicago, examined dozens of fossils, as well as more than 1,000 vertebrae of living animals, to determine whether mammal vertebral regions were as ancient as previously thought.It turns out mammals “were doing something unique.”A modern-day opossum shows the addition of a fifth region in the backbone, the lumbar region (via Harvard University Museum of Comparative Zoology)When the team compared the positioning and shape of the vertebrae, they found the spine had gained new regions during mammal evolution: Some 250 million years ago, a new region evolved near the shoulders and front legs, later, a region emerged near the pelvis. Changes also appeared in the forelimbs of non-mammalian therapsids.These developments, scientists believe, likely occurred in conjunction with changes in how critters walked and ran.“We’ve been able to make connections among changes in the skeletons of extinct animals and ideas in modern developmental biology and genetics,” Jones said. “This combined approach is helping us understand what makes a mammal a mammal.”A paper describing the research was published this week in the journal Science.Despite centuries of animal evolution—allowing lemurs to smell weakness in injured mates and songbirds to use brute force to kill larger prey—human interference is affecting wildlife in lasting ways. Read more about animals here. Stay on target Watch: Dolphin Leaps Feet Away From Unsuspecting SurferWatch: Deep-Sea Octopus ‘Billows Like a Circus Tent’ Let us know what you like about Geek by taking our survey.