The Evolution of Circulatory Systems in Vertebrates (and brief discussion of respiratory systems)
Evolution is the reason for all biodiversity in the world. It illustrates how life takes the antiquated tools and body structures passed down by a species’ ancestors and transforms them into something that suits the current world, so that the species may do the same for their children. A beautiful demonstration of evolution presents itself in how circulatory systems evolved from vertebrate to vertebrate. Fish first came into fruition approximately 530 million years ago. (source) Amphibians evolved between 390 and 360 million years ago when the first ancient fish–most likely Eusthenopteron or Ichthyostega– crawled on land. (source) (source) Reptiles were the next to evolve, evolving a mere 75-45 million years after amphibians–approx. 315 million years ago. (source) Reptiles became dinosaurs and with the help of therapsids, became mammals at least 178 million years ago. (source) Birds evolved from dinosaurs about 160 million years ago, thus completing the four animals that will be discussed today. (source) This paper will start with looking at fish, and end with birds and mammals. Circulatory systems dictate how quickly cells can receive the materials needed for homeostasis, and therefore how quickly they are able to produce energy. Fish did not need to evolve complex circulatory systems, as they were cold-blooded and could keep metabolic rates fairly slow. They had only one atrium and one ventricle, as well as single-loop circulation, meaning there is little separation between deoxygenated and oxygenated blood. When said little separation occurs, it takes more time for the circulatory system to function and to provide nutrients and oxygen to cells. (source)
Despite needing to live near water and in moist environments, amphibians still had to evolve more complex circulatory systems. They did not have the time to wait for the inefficient fish system to sort out the blood. They evolved double-loop circulation, meaning that the oxygenated and deoxygenated blood followed separate routes back to and away from the heart. On top of this, amphibians also evolved a second atrium to allow for more separation of oxygenated and deoxygenated blood.
Reptiles did not deviate much from amphibians. They evolved a ‘half-septum’ in their ventricle (see attached image), which allows d for more separation of oxygenated and deoxygenated blood than not having a septum, but not as much as a full septum would. Alligators are an exception to this, as they had to evolve an entire septum–giving them two ventricles.
Fish, amphibians, and reptiles are all cold-blooded. They do not need to have the most efficient circulatory system because they do not need to generate their own body heat. Not generating their own body heat means that they do not need as efficient a body as warm-blooded animals do. They also do not need to eat as often as warm-blooded animals do for the same reasons. Mammals and birds–both warm-blooded animals–have the same type of circulatory system, meaning that it works and is the most efficient for the current situation. Mammals and birds both have double-loop circulation, two atria, and ventricles. The evolution of circulatory systems is a feat over 530 million years in the making.
Body systems need to evolve together. The circulatory system cannot evolve light separation of oxygenated and deoxygenated blood if the respiratory system cannot reoxygenate the blood fast enough. Alongside the change from gills to lungs (and breathing through the skin and lungs as amphibians do), the respiratory system also had to keep up with metabolic changes.
Evolution is both an internal and external process. Every single organism can be traced back to another organism completely randomly There is no logical rhyme or reason for how evolution unfolds it simply relies on who survives. The way that life finds a way to change drastically in order to keep flowing is something truly whimsical.
Works Cited
https://www.birmingham.ac.uk/facilities/lapworth-museum/collections/palaeontology/vertebrate.aspx
https://evolution-outreach.biomedcentral.com/articles/10.1007/s12052-009-0119-2
https://www.nature.com/articles/d41586-019-03170-7
https://www.nhm.ac.uk/discover/how-dinosaurs-evolved-into-birds.html
https://www2.nau.edu/lrm22/lessons/heart_evolution/heart_evolution.html (Diagram/heart image)