The human heart will not beat, and therefore will not force blood throughout the body, without the the brain “telling” it to. The brain cannot survive without blood that the heart pumps throughout the body.
Explain, please, how this extraordinary state of affairs came to be.
Simplistic Answer:
A heart will continue to beat without the brain “telling” it two. If you grow heart cells in culture, they will reach a point where they “beat” in synchronicity.
The brain can be kept alive mechanically (i.e. without a heart), as long as it is supplied with blood.
The current state of affairs occured because of evolution.
Even in humans the heart will beat quite happily without the brain telling it what to do. The brain is needed to some degree to regulate heart rate, but without any signals from the brain whatsoever the heart will beat at a constant steady pace. Death from brain damage usually involves literal brain death: the brain stops carrying out all sorts of functions. This can stop the heart because the brain centres override normal heart function or causes to much stress to be put on the heart from other malfunctioning systems… Or death can result from a cessation of breathing, which does require input form the brain.
But if you carefully severe all nervous connections to the heart, and keep the organ supplied with nutrients, it will beat for some time after the owner has expired.
In ‘lower’ animals the connection between the heart and brain is even less vital, and in animals like earthworms and insects it essentially ceases to exist. Remove a worm or fly’s brain and the body will continue to live until it dies of thirst or starvation. The heart doesn’t skip a beat.
So that basically explains how the system evolved. In ancestral organisms the heart beat at a constant, self-regulated rate. The brain had no control over this. Animals had a greater survival chance if the heart rate could be modified in response to stress, and as a result those animals with hearts that responded to chemical and then later nervous input from the brain survived longer had more offspring etc.
No mystery. The brain, like any other organ in the body, requires the oxygen and nutrients supplied by the blood to live. As for the heart, heart tissue has a natural tendency to beat and to self-synchronize. People who have had heart transplants have no intact nerve connection to their new hearts, which still respond to chemical signals in the blood.
The human body was not made from scratch. It is a modification of older models, inheriting the linkages between circulatory and nervous systems which allowed those older creatures to survive long enough to reproduce.
Oh, and heart cells seperated from the organ will “beat” in a nutrient solution, and will synchronize with neighbouring cells if they come into contact with them. The first part of your scenario is incorrect; the heart does not need a brain to “tell” it to beat.