We can survive well enough on only one lung and one kidney. Why does the human body only have duplicates of two organs, and why isn’t one of those the heart?
At a guess you’d get into trouble with two hearts connected in parallel if they got out of synch - they’d partly work against each other. Would need backflow valves which could fail. If the two hearts were coupled more loosely (i.e. effectively two separate circulation systems) you’d lose the benefit of redundancy.
Would the size be an issue as well? A heart is pretty big. Plus, it takes up a lot of energy. It is beating all the time and all.
Classically, too, I suspect heart disease was not a huge issue. So really, it was only if you had a genetic heart illness or you got stabbed in the heart. In the former, both hearts would be affected, and in the latter, there is s good chance you’d get killed by the fluid loss.
We also have duplicate arms and (arguably) legs, and ears and eyes. But in each of those cases, you’ll be somewhat impaired with only one. The testicles, on the other hand, are completely redundant: A one-balled man is just as fertile as the next guy. Just a little reminder, there, of evolution’s priorities.
I doubt there is much benefit to two hearts anyway even in a redundant system (and as already mentioned that’d be pretty complex to work efficiently). If you sustained sufficient injury to stop one heart chances are you will die regardless of having a second heart. There might be a few instances you could posit that a second heart would become useful in but not enough to prod mother nature into trying for one.
In short there are more downsides than upsides.
FYI, the human heart consists of four chambers, two atriums, two ventricles.
The brain can also be considered as two pieces, as well as the nose.
Heart: heart trouble doesn’t prevent the human species from reaching sexual maturity and reproducing. Therefore, heart related trouble which shortens life has virtually no impact on the ability of the species to survive.
Survive is defined as reproducing.
- Originally posted by* ** Nanoda**
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FYI, the human heart consists of four chambers, two atriums, two ventricles.
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All of which must function together to provide adequate circulation.
A second heart would be beneficial only if either heart by itself was capable of sustaining activity needed to procreate and rear progeny efficiently. The amount of energy needed to build, repair, and run a second heart would be high, so the benefit would have to be very great, in terms of increasing survivability, since the increase in energy needs would be detrimental.
Heart disease among adults over fifty is a net gain in the survivability of the species. That’s why it never got selected out during human evolution. The young are the fittest, since the old will no longer produce progeny. Individual deaths are a part of survival, if you are looking at the species as a whole.
Tris.
Yes, but not for redundancy.
One atrium to receive blood from systemic circulation and feeds into a ventricle which then pumps unoxygenated blood into the lungs.
Another atrium to receive newly oxygenated blood from the lungs and feeds into the other ventricle which then pumps this blood into systemic circulation.
Remember this old joke?
*Three engineers (mechanical, electrical and civil) were discussing what the best type of engineer was.
The mechanical engineer said, “Well, God must have been a mechanial engineer. Surely if you look at the human body, with all the joints and pounding the body takes, it’s clear He was a mechanical engineer.”
The electrical engineer spoke up, “But if you look at the nervous system, with all its intricasies, one must admit that God had to be an electrical engineer”.
The civil engineer finally spoke, “God must have been a civil engineer. Who else would built a waste disposal system right in the middle of a recreation area?” *
Even if we can determine that a pair of hearts would provide a survival benefit, there would need to have been a genetic mutation somewhere in our decent giving rise to such a modification. So to answer the OP, the reason why we don’t have two hearts is because we haven’t evolved them and the reason why we have pairs of some other organs is because we have evolved them.
We can imagine numerous alterations to our bodies’ design (such as not having dual function genitalia or having two hearts), but not all designs are necessaily possible to achieve by shuffling the pack of genetic cards.