The Daily Omnivore

The recurrent [ri-kur-uhnt] laryngeal [luh-rin-jee-uhl] nerve is a branch of the vagus nerve (tenth cranial nerve) that supplies motor function and sensation to the larynx (voice box). It is referred to as ‘recurrent’ because the branches of the nerve innervate the laryngeal muscles in the neck through a rather circuitous route: it descends into the thorax before rising up between the trachea and esophagus to reach the neck.

The extreme detour of this nerve (about 15 feet in the case of giraffes)  is cited as evidence of evolution as opposed to intelligent design. The nerve’s route would have been direct in the fish-like ancestors of modern tetrapods, traveling from the brain, past the heart, to the gills (as it does in modern fish). Over the course of evolution, as the neck extended and the heart became lower in the body, the laryngeal nerve was caught on the wrong side of the heart. Natural selection gradually lengthened the nerve by tiny increments to accommodate, resulting in the circuitous route now observed.

Devolution [dev-uh-loo-shuh] is the notion that a species can change into a more ‘primitive’ form over time. Devolution presumes that there is a preferred hierarchy of structure and function, and that evolution must mean ‘progress’ to ‘more advanced’ organisms. This may include the idea that some modern species that have lost functions or complexity accordingly must be degenerate forms of their ancestors.

However, according to the definition of evolution, and particularly of the modern evolutionary synthesis in which natural selection leads to evolutionary adaptation, phenomena represented as instances of devolution are in every sense evolutionary. The idea of devolution is based at least partly on the presumption that ‘evolution’ requires some sort of purposeful direction towards ‘increasing complexity.’ Modern evolutionary theory poses no such presumption and the concept of evolutionary change is independent of either any increase in complexity of organisms sharing a gene pool, or any decrease, such as in vestigiality or in loss of genes.

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In mathematics, a Markov [mahr-kahv] chain, named after Russian mathematician Andrey Markov (1856 – 1922), is a discrete (finite or countable) random process with the Markov property (the memoryless property of a stochastic [random] process). A discrete random process means a system which can be in various states. The system also changes randomly in discrete steps. It can be helpful to think of the system as evolving through discrete steps in time, although strictly speaking the ‘step’ may have nothing to do with time.

A stochastic process has the Markov property if the conditional probability distribution of future states of the process depends only upon the present state, not on the sequence of events that preceded it. (Given two jointly distributed random variables X and Y, the conditional probability distribution of Y given X is the probability distribution of Y when X is known to be a particular value.) The term ‘Markov assumption’ is used to describe a model where the Markov property is assumed to hold, such as a hidden Markov model (in which the system being modeled is assumed to be a Markov process with unobserved [hidden] states).

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