How often do you think of your bones? Most people don't think of their bones unless they experience a medical condition such as a broken bone or osteoporosis, but our bones carry us through every movement and each breath. Our bones are at the core of everyday living, allowing us to move through the world as we resist the gravity of our planet. Our bones are our internal support structure, acting as levers for our muscles to pull on so that we can change direction and speed as we explore Earth. As we use our bones, they constantly change, adapting to our lives. Every vertebrate animal contains an internal support structure that is unique to their locomotion, respiration, metabolism, and diet. Bones reflect & record the lives of humans, dogs, cats, crocodiles, dinosaurs, frogs, and fish. We are all the bearers of the endoskeleton and share a common ancestor, the first vertebrate.
When we were growing our bones lengthen, and even when growth stops our bones continue to change throughout our lives. Bone is absorbed and then it is replaced by new bone, and a cycle of ever-changing internal support that reflects our diet, internal chemistry (hormones), and how we move through the world. Our bones bear the marks of our lives, which will end at some point, and for those who choose to be buried our bones will be the last evidence of our physical lives, depending on the conditions in which we are buried. Most buried bones will weaken in the Earth overtime, but they are by far the strongest parts of our body and the most capable of leaving behind a long-term natural record of our existence. Overtime some humans have been buried in perfect conditions for fossilization, a true rarity. Even more rarely living humans have found those bones, carefully removed them from their earthed graves, and brought them to museums and labs where those bones are studied and their stories are in part revealed. An incomplete record of life is preserved within bones.
The bones of many creatures have been fossilized, turned to stone. There are bones that have been found that are remarkably similar to ours, those of mammals. As archaeologists and paleontologists have unearthed bones from across the ages, some of which have been buried for millions of years, a pattern of skeletal commonalities has emerged. The commonalities and differences are analyzed by comparative anatomists, revealing a family tree of vertebrates. All of the animals that have an endoskeleton, and at its core a spine, have a common ancestor, one from which we inherited our internal structure. Our oldest common ancestor dates back to the Cambrian Period, the dawn of the Phanerozoic Eon, the eon of visible life that began 542 million years ago. The first were the fish. They evolved a spine made of multiple vertebral bones, a skull & ribs, the features that allowed them to move in water. Spines, aka backbones, protect our spinal cord but also allow for leverage of muscles to maneuver through the environment. Tetrapods are a branch of vertebrates that have limbs that push them across land, and through water & air. Fish and amphibians differentiated and diversified into two seemingly separate branches of life. Fish stayed bound to the water while amphibians crawled on to land and breathed air, but are forever dependent on returning to watery habitats. Amphibians and reptiles diversified as the reptilian branch of life evolved adaptations to living on land away from water. Mammals branched off from this family tree, and the earliest mammals have very reptilian features. Birds are a branch of the reptilian family tree as well, a branch of dinosaur descendants that took to the skies. We are lucky when fossils record diversification, and preserve familial relationships that are otherwise hard to recognize in modern vertebrates, the descendants of the Cambrian common ancestor.
The fossil record provides snapshots of the ongoing evolution of the skeleton through the ages as organisms adapted to shifting environments. The surface of the Earth changes because it is in motion due to plate tectonics and the rock cycle, though the rate of change varies. The fossil record is a climate record, because animals adapt to survive the climate, and their bones record physical and chemical adaptations. Yet fossilization is rare, so our understanding of the evolution of the skeleton will always be incomplete. Despite this rarity there are millions of fossils, and through comparative anatomy & chemical analysis we can look at the physical differences between the bones and reveal how they moved, what they ate, where they lived, body temperature fluctuations, and the environment(s) they lived in. The bones are the record of life, environment, and climate. And your bones are reforming within your body right now reflecting changes in your environment and lifestyle.
I think of my bones every day. I recognize how they move and how that allows me to explore the world. I recognize that my movement and diet affects how my bones are changing and that sometimes we don't have control over that as much as we wish. I think of how my shoulder blade moves as I reached for a glass on the shelf, and recognize that most vertebrates that have lived in the last ~500 million years never had that option, their scapula, aka shoulder blade, could not perform that seemingly simple action. We inherited it from the primate branch of the mammalian tree. When I sit on a chair and I feel my sit bones supporting me, I think of how differently shaped our pelvis is than that of the dinosaurs, and that the dinosaurs are divided into two groups based on the shape of their pelvis’. And despite the difference in bone shape the sit bones of dinosaurs and humans serve the same basic purpose, they connect to hamstring muscles that allow us to stand and walk forward. The shape of our pelvis has evolved to serve the functions of our primate family tree, bipedalism and live birth. Our bipedal locomotion creates an oxygen demand that requires a good lung capacity and fast metabolism. When I breathe, I think of how my rib cage is expanding and contracting to create room for my lungs to expand, draw in air and process the chemistry of the air that fuels so many systems of my body. I think of the dozens of muscles that connect to my rib cage that allow for the expansion during inhalation in the contraction during exhalation, and I recognize that birds cannot do this, their breathing system is so different from ours and yet we all have ribs. We also all have skulls that allow us to see, smell, taste, and hear the world we are navigating.
When I look at the beauty of our planet and appreciate nature in all its forms, I recognize that the orientation of my skull determines what I see and that the skull is so specialized that there are orbits for our eyes to sit within to give us visual sense that is routed directly to our brain. That visual sense is a huge part of how I experienced this world, but so is the sense of smell, and my nasals, the holes in my skull below my orbits, allow me to pull air in and smell pasta cooking, cookies baking, and the perfume of flowers. One of the most fascinating parts of the skull is the jaw, hinged on two joints. The bottom part of my jaw is muscularly attached to the upper part of my skull with huge long muscles that allow me to open and close my mouth to speak and eat, and my teeth within my jaw are so uniquely human. Many organisms of the past are identified only by their teeth, and when it comes to specialization of teeth mammals steal the show. When we trace the evolution of the mammalian jaw back in time, we see fascinating connections between the sense of hearing and the bones of the jaw. We recognize that some of the jaw bones of early vertebrates eventually evolved into the inner ear bones of mammals, and it is the movement of our teeny tiny inner ear bones that allows us to hear the world around us, the approach of footsteps, the sound of music, and the tone of individual voices.
Each of our skeletons is so similar within the vertebrate family tree, and yet we each move and experience the world uniquely. You and I are uniquely human & mammalian, belonging to the tetrapods and vertebrates. We have more in common with amphibians than we do with ants because we inherited an endoskeleton. We all belong to one expansive branch of the animal family tree, within which there are commonalities and differences that reveal how all life on this planet is interconnected and diversity is what makes life on earth so incredibly fascinating. So may you give your bones daily consideration and appreciate the history they bear and the mobility they offer.
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