I Wouldn't Start From Here.

Abstract

An oft-told Irish joke concerns a lost tourist who asks a passing local for directions to some destination. After considering the matter for a moment, the local answers: ‘‘Well, if I were trying to get there, I wouldn’t start from here.’’ The joke’s not very good, but the punchline can be adapted to contradict the notion that the human body (or any other biological entity) is perfectly designed for its functional life. If a deliberate designer wanted to make the perfect dominant mammal to occupy, preside over and manage our planet, she could do better than the human bodies we inhabit. Of course, human beings are not ‘‘designed.’’ We are, instead, the accidental residue of aeons of evolution, mutation and adaptation. Natural selection has certainly favored some of those mutations, but many have been random, causing changes which have no meaningful impact on whether an individual survives and reproduces or not. And some of the phenotypic outcomes of these mutations are, frankly, mind-bogglingly inefficient. Many aspects of human anatomy & physiology are not exactly how one might design an organism to fulfill certain functions. For example, the retinal photocells in human (and all vertebrate) eyes are positioned ‘‘behind’’ the nerves that connect them to the brain, with the nerves emerging anteriorly (toward the light), before converging and passing backward through the retinal ‘‘blind spot’’ to the brain. This is not an ideal physical arrangement; having the nerves pass directly from the back of the photocells to the brain would make more sense, if our eyes were deliberately designed. But by the time this arrangement became disadvantageous, the vertebrate eye had evolved sufficiently far that reversing and re-engineering the process could not have happened by natural selection. ‘‘The vertebrate retina faces the way it does because of the way it develops in the embryo, and this certainly goes back to its ancient ancestors.’’ It’s only by understanding embryology, how some of our bodily features are molded by inherited past forms from which we have evolved, that we can understand many bodily functions, disease processes and anomalies. The weird becomes explicable when we know our embryology, which in part relates to our evolutionary development. The semicircular canals of our inner ears share an evolutionary origin with the neuromasts of fish, sensory organs on their surfaces which allow them hold their position in flowing water by sensing the currents flowing past. Both are gel-filled structures containing hair-like projections. When we drink too much alcohol, the alcohol diffuses into the gel of our semicircular canals, reducing its density. This stimulates movement of the hair cells within the canals, making our brains believe we’re moving. The next time the room starts to spin when you drink too much, blame our evolutionary connection with a trout. Gut rotation in the embryo and crossed-fused renal ectopic are two common anomalies that become easily understandable when one considers the embryology of the development of the alimentary tract and the kidney. The relatively-narrow human birth canal, with its potential consequences of obstructed, difficult labor, is an evolutionary trade-off for bipedalism. ‘‘Intelligent design’’ clearly is neither. When studying embryology as medical students, many of us considered it a subject to be learnt (perhaps with passage of a required exam), and then filed away somewhere deep in our brains, often forgotten, with little relevance for daily practice. It’s only later in practice that we realized that understanding human embryological development is key to understanding many disease processes, anatomical anomalies, and quirks of how the human body functions and malfunctions. Knowledge of and familiarity with the embryology relevant to one’s daily work is not just for those in specialized academic practice. It can inform and enhance practitioners in all environments, from the family practitioner explaining the origin and significance of a horseshoe kidney or an undescended testis to her patient, to the university hospital cardiac surgeon explaining the need for revascularization of an asymptomatic malignant coronary artery anomaly. In this month’s journal, Qazi et al outline the embryology and resulting anatomical variants of lower limb arteries. They explain the embryological processes of vasculogenesis, angiogenesis and remodeling, in which many factors (‘‘sprouting of new vessels, intussusceptive growth, regulation by hemodynamic factors, growth and inhibitory factors, and tissue environment’’) interact. The authors outline the unlikely manner in which continuous arteries running from the aorta to the toes achieve their final form, involving development, regression, connection etc. of a series of primitive arteries and precursors, leading to the continuous network with which most of us are fortunately endowed. They present splendid illustrations of