Evolution Of The Stress Response

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The concept of natural selection was first introduced by Charles Darwin and has had an immense impact in all aspects of biology. The idea is that genes that confer a fitness advantage increase in the population while ones that decrease fitness become less common. Natural selection can help us gain insight to important physiological concepts such as stress and its response. Understanding the evolutionary history of the stress response can guide us into knowing how stress hormones such as adrenaline and cortisol evolved and why it prevailed in ancestral environments and continues to have such important physiological functions.

Adrenaline and cortisol both respond to stress, but in different ways and at different times. Adrenaline is known as the fight-or-flight hormone and is more immediate, whereas cortisol –known as the stress hormone – is produced by the adrenal glands and comes into play after adrenaline’s effects. The evolutionary bases of these hormones can be understood by analyzing a number of scientific papers; one of which being: “Evolutionary origins and functions of the stress response” (Nesse and Young, 2016).

This paper explains that for the traits of these hormones to have evolved, they must have had an adaptive function. In ancestral environments, there were stress presenting situations everywhere. From the difficulty to acquire food to fleeing away from a predator to rampant infections, stress could be present any time. Nesse and Young explain that for an organism to have survived, the traits they evolved must have conferred a fitness advantage. Adrenaline evolved to allow the body to reach its peak performance level in situations where needed. Its hormonal effects result in much sharper senses and an extreme alertness when the organism needs to fight off danger or flee from it. Another article called the “Evolution of the fight or flight response” by Heather Scoville analyzes the importance of the fight or flight response through geological time. The most ancient organisms were known to have this response, even when they lacked the complex brains that many species have today. This response was obviously key to the survival of many organisms because it helped them gain an advantage in being more physiologically equipped to deal with stress which would have had a fitness advantage. Pertaining to the microevolution of the gene responsible for either of these hormones, the ScienceDaily journal published a paper (https://www. sciencedaily. com/releases/2013/06/130624152617. htm) explaining that researchers dug into some “molecular time traveling” and found two mutations that apparently triggered an evolutionary leap 500 million years ago.

They stated that changes in two letters of the genetic code in our evolutionary past caused a massive shift of one protein and set in motion the evolution of our present-day hormones responsible for stress responses. Knowing this, we can now see how evolutionary effects on the molecular level like gene duplication can contribute to the success of one organism and demise of others. In the first paper, Nesse and Young also explained that another way we could understand the evolutionary basis behind these hormones is to make a cross species comparison to reconstruct the phylogeny of the stress response. In doing this, it was found that signaling hormones like adrenocorticotrophic hormone (ACTH) was found across vertebrate species. The relevance of this hormone is that it is associated with signaling molecules such as epinephrine (aka adrenaline) and cortisol. This confirms that the evolution of these stress hormones was in fact derived from a single lineage.

In conclusion, natural selection allows fixation of beneficial genes and loss of maladaptive ones. We know that for organisms to survive the savage environment in which they resided in, they had to find efficient stress coping mechanisms to promote their survival – hence why hormones like adrenaline and cortisol evolved in stress response in animals.

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