The Heart: Back to the Beginning

In this project, I endeavored to explain the evolution of the human (mammalian) heart from fish to mammal and fetus to live human. The three course objectives I explored are analyzing the evolutionary history of the heart, knowing the path of blood through the heart and circulatory system, and explaining the structure and function the heart. To do so, an understanding of how the human heart functions, and why it is so successful are required.

The mammalian heart evolved from the heart of a fish, in a very complex and not direct process. A distant relative member of the Sarcopterygii (lobe finned fish) family, emerged onto land and their heart changed. (Shubin 2009). This was a long process that did not actually occur in one generation, with one fish. After this transition, however, different lines branched off of to create the amphibian, reptile, bird, and mammal hearts.

To better be able to follow the following information, I included the diagram from Stephenson et al. (2017) which I referred to as I attempted to understand how the heart evolved from fish to human (Figure 3). In the simplest terms, to describe evolutionary relationships, the fish that humans descend from had two chambered hearts. They exchanged oxygenated and de-oxygenated blood. This enables them to oxygenate their blood and be very tolerant of hypoxia (Stephenson et al., 2017, p. 790). A trait such as this would have been very beneficial for the transition from the water to the land.

Amphibians, a species which frequently moves from the land to the water, can have lungs and no lungs. In those with no lungs, they have a partially separated atria that results in frequent mixing of oxygenated and de-oxygenated blood. In frogs and toads, amphibians with lungs, their atria have completely separated to create a heart with three atria. Reptiles can also have three atria, but crocodiles evolved to have four (Stephenson et al., 2017, p. 791). All of these various adaptations allowed the body to ensure cardiac output in different environments and with different needs.

The human (mammalian) heart, is a four chambered heart which is possible due to a complete intraventricular septum (Stephenson et al., 2017, p. 794). It shares many of the same structures of the hearts mentioned above but one in particular demonstrates the relationships here best, the Sinus Venosus. It is present in lobe finned fish, in many reptiles and amphibians, and in humans in utero. This enables the filling of the atria to maintain proper cardiac output in different environments (Jensen et al., 2014, p. 18). The high heart rate of a human heart doesn’t require the use of the sinus venosus, but a fetus does use that structure (Jensen et al., 2014, p. 19).

Much like semiaquatic species, a fetus and eventual infant will also move from a pseudo-aquatic environment to one where they are required to breathe in air. It is possible that the presence of the sinus venosus may aid the low cardiac output present in fetal development to help the fetus along (Jensen et al., 2014, p. 20). Humans, reptiles, and amphibians look quite similar in early fetal development, especially in their hearts. This reflects their common ancestry and evolutionary history.