Living things are wondrous for many reasons, but two stand out. First, all living organisms are related to one another. Comparative anatomy, embryology, DNA, the fossil record, the structure of molecules from which life is built and the metabolic processes organisms undergo tell the same story.Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.
Second, evidence demonstrates that organisms are cobbled together. The structure and function of all living things have been adapted from what previously existed. This has been achieved through gradual change to meet the necessities of survival in new or changing environments.
If living things are both related and cobbled together there should be evidence of both continuity and divergence. Organism more closely related should share larger numbers of common traits. They should have structures or metabolisms showing a common history.
For organisms more distantly related, one expects some structures and functions modified to the unique circumstances of that organism. This is exactly what is seen.
Nature can't start from scratch every time an organism must adapt to environmental changes that threatening its survival. Rather, gradual modifications of what already exists must be made. When change is too slow, extinction threatens.
DNA, physical structures and metabolic paths in existing organisms are a wealth of raw material for transforming organisms. The value of these entities is well established from having been honed through long lineages. Often, a modest tweak is all that is needed. More substantial change is typically brought about through a series of small modifications.
Tweaks arise from mutations of genes. Mutations occur randomly and without foresight as to their utility. Most are harmful, leading to less well adapted organisms. However, mutations occur often enough, and in sufficient variety, that a small fraction of the changes they bring about enhance an organism's chance of thriving.
Well over 99 percent of all organisms that have ever existed are now extinct. This is evidence of lineages transforming themselves in response to environmental change, or succumbing to better adapted competitors of another lineage.
Sometimes, rather than tweaking a structure for a slightly different but similar task, whole new paths are taken. There are many examples where something that worked for one function got modified for a completely new task. This process is called exaptation.
The result of this cobbling hasn't always been pretty. A change may improve an organism's ability to survive and leave off-spring. At the same time, the change can leave a less than optimal legacy in some less critical function.
For example, the continuity of mammal lineages (humans included) to ancestral fish is well established. All have similar body architectures, having body segments arranged around a linear spine. Comparison of the anatomy of embryos makes the parallels starkly evident.
Fish develop with two-chambered hearts. A ventral aortic artery emerges from the heart in the direction of the head. In adjacent and symmetrical segments, six pairs of gills are fed by blood vessels coming off the aorta. Blood returning from the gills is routed into six pairs of blood vessels that merge to form the dorsal aorta. Nerves to each gill follow the route of blood vessels.
At about five weeks, human embryos develop six adjacent pharyngeal arches. These are anatomic analogs of the six vascular segments in fish embryos. Each arch has cartilage material, an aortic arch, and a cranial nerve. As the embryo develops the segmented and symmetrical arrangement of vessels becomes elongated and reshaped to accommodate a neck and thorax.
To move about on land, animals weren't in need of gills and were better suited to having a thorax with shoulders and a neck. During embryonic development in mammals, tissue of some pharyngeal arches is redirected to become thyroid and parathyroid glands, and parts of the larynx (voice box).
The same blood supply and nerves that would serve gills in fish have come to nourish and enervate these mammalian structures in the neck and upper thorax. Much could be written about the genetic orchestration of tissue transformations as new and ancestral lineages diverge. However, some real oddities arise from this cobbling.
The routes of nerves for these structures are bundled with their blood vessels. Living and extinct mammals come in quite a variety, some having very long necks. New wiring and plumbing layouts of nerves and blood vessels are not possible for each. Rather, they have been adapted from what existed in ancestral lineages. This has meant they have become stretched and contorted as needed.
The route of the recurrent laryngeal nerve illustrates one nonsensical outcome from such adaptations. In humans this nerve, serving our voice box, must detour several inches down into the chest, around the dorsal artery, and back to the neck. In an adult giraffe this route is approximately 15 feet!
This is hardly an optimal, economic way to route nerves to the larynx. Typically, changes in tissue development such as those outlined above arise from mutations of genes and/or changes in gene expression. At the same time many genes are preserved within a lineage. Such is the origin of continuity and the cobbling that gives rise to divergence.