By Dr. Jessica LaPrice
Dr. LaPrice teaches biology at Washington and Lee University in Lexington, Virginia. Recently I visited her at her university where she showed me a goat skeleton once used in a comparative anatomy class. It reminded me just how similar our skeletons are to Dave’s and Clancy’s, and even to that goat. So I asked Dr. LaPrice to write a guest blog on that topic. Here it is.
As a long-time reader of this blog, I was honored to be asked to write a guest post about comparative vertebrate anatomy. “Where shall I start?”, I thought. Then I remembered the little brown bats in the barn and realized that was my in.
As a child, and now as an adult, I’ve spent a not insubstantial amount of time on the land that is now Owl Acres, particularly in the loft of the big old dairy barn where we stored hay for our horses. One of the things that always fascinated me about the barn was the variety of tiny skeletons I would find scattered around in corners, whether wrapped up in the furry package of an owl pellet, or curled up intact when some larger predator hadn’t disturbed the final resting place of an elderly raccoon.
Even before I became an official biologist, I was fascinated by bones, the scaffold our bodies are built around. They are both flexible and inflexible – they know their shape but can be re-formed with exercise or after an injury. I’ve personally experienced this bone remodeling several times, usually from being catapulted off horses.
Our basic skeletal structure is familiar to even a young child. Remember “The Skeleton Dance” song, a takeoff from the “Dem Bones” spiritual–“The knee bone’s connected to the thigh bone…” But have you ever considered how similar our human skeletal structure is to those little brown bats hanging in their maternity roost in the barn on Owl Acres?
We humans are vertebrates – we belong in what is technically known as the subphylum Vertebrata, along with other mammals, reptiles, birds, amphibians, and fish. The basic vertebrate body plan is the same for all – a skull connected to a vertebral column (our backbone) with attached ribs protecting the vital organs (together known as the “axial skeleton”), with limb bones hung off of girdles that connect to the vertebral column (collectively known as the “appendicular skeleton”). Even snakes have an axial skeleton. They’ve just lost their arms and legs. The animals known as tetrapods (“tetra” = four, “pod” = foot), including amphibians, reptiles, and mammals like us, have more robust limbs capable of standing up against gravity.
Let’s compare a bat wing with our arm. Both are attached to the backbone by a collarbone (“clavicle”) in the front and a shoulder blade (“scapula”) in the back. These two bones together are called the “pectoral girdle”. Both bats and humans have a single large bone hanging off of the pectoral girdle (the “humerus”), which is connected to two thinner bones (the “radius” and “ulna”) that make up the forearm, and from there to the wrist bones (“carpals”), the hand bones (“metacarpals”), and the finger bones (“phalanges”). Count those finger bones in the photo above – bats have 5 long skinny finger bones supporting their wing membrane, the same number that we have. Same thing in the leg – both bats and humans have a pelvis with a single long thigh bone (“femur”) attached to two smaller bones (“tibia” and “fibula”) in the calf, and to a set of ankle bones (“tarsals”) and foot bones (“metatarsals”) with 5 toes. This is the basic tetrapod body plan, just shrunk down or elongated or strengthened or fused where needed to support the owner’s lifestyle.
Image attribution: https://commons.wikimedia.org/wiki/File:Comparative-view-of-skeletons-of-man-and-horse-C.jpg Alt text: A drawing of a skeleton of a horse and a human standing next to each other. The human arm and horse foreleg are shown with joints bent at approximately the same angle and shared joints are labeled with shared letters e.g. S for shoulder, E for elbow, etc. What we think of as the horse’s knee is actually its wrist.
So what if you need to run really fast to catch prey or escape predation? Animals born with random mutations for slightly longer legs or with some of their toes fused together were able to run a little faster than others of their kind. They lived to pass on those useful mutations to their offspring. Eventually we end up with something like a coyote with relatively long limbs, running tiptoe on a reduced number of 4 toes per foot that chases a white-tailed deer with long legs, running tiptoe on a reduced number of 2 toes per foot at 35 miles per hour. What if you need to move tons of earth and stones out of the way to dig a tunnel and find food without being eaten by something? A blunt flat skull and thick robust forearms make it possible for a mole to “swim” through the soil.
Vertebrates that fly (like bats and birds), have lighter limb bones that either elongate (in bats) or fuse (in birds) to reduce weight. Even a loss of limbs can sometimes be beneficial. The ancestors of whales had 4 limbs like a normal mammal, but over time after they moved back to the ocean, animals with mutations for reduced hind limbs had a more streamlined body shape and could swim faster. The front flipper of a whale has the same bones that we have in our arms and hands, just flattened and covered in skin for streamlining, but the hind limbs are lost entirely with just a few remnant pelvic bones left behind.
The foundational tetrapod body plan with its head, its backbone, and its four hanging limbs is the basic starting toolbox for all vertebrates. Each species, including humans and bats, over millions of years of evolution, have modified these basic skeletal structures in a way that suits the environment and meets the requirements of their owners.
Image attribution: https://commons.wikimedia.org/wiki/File:Mur_squelettes.jpg Alt text: A photo of a museum display showing skeletons of 24 vertebrate species including human against a black background. Each skeleton has a skull, a backbone, ribs, and limbs or fins modified for some form of propulsion.
Feature photo image attribution: https://commons.wikimedia.org/wiki/File:Antrozous_pallidus_2zz.jpg Alt text: A skeleton of a bat against a black background. The wings and legs are spread, showing the same set of bones that make up a human’s arms and legs with five fingers and toes at the end of each limb.
