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Suppose we could copy a person, the way we can now Xerox a printed document. The consequences, if this could be done, would be mind boggling. I don't believe we are anywhere close to being able to do this; but I don't think it is impossible. It is interesting that real progress has been made in constructing an automaton, with arms and legs, able to perform physical tasks such as loading cartons onto a truck, or rescuing people from burning buildings. While not an easy problem, real progress has been made toward solving solving it .
Giving such an automaton the ability to think, is an enormously difficult problem, due largely to the great complexity of the brains of even relatively primitive animals such as mice. Over the past several decades, much has been learned about the nervous systems of animals, including humans. The operation of neurons, the basic components of brains and nervous systems, is now fairly well understood. Progress has been made in understanding the nervous systems of simple animals . One of the simplest of all animals is the very common roundworm, a parasite commonly found in the intestinal tract in humans, as well as in animals. There are about 300 neurons in a roundworm.How Complex is a Human?
Great advances have been made in integrated circuit technology. We have reached a point where it is possible to construct integrated circuits large enough to model animal nervous systems. It is estimated that a human body contains roughly one hundred billion (100,000,000,000) neurons. If we assume that it takes about 10 transistors to model a neuron, then about a trillion (1,000,000,000,000) transistors could model a human nervous system. An integrated circuit (IC) chip today can contain 20 billion transistors . So 50 IC chips, each containing 20 billion transistors, would have roughly the equivalent of the number of neurons in a human body, perhaps a trillion transistors.
An additional complicating factor is that the body of any animal contains a large number of micro-organisms, called microbiomes, that play an important role in protecting the animal against germs, breaking down food to release energy, and producing vitamins. Any effort to duplicate animals, including humans, must include "infecting" the animal with the appropriate microbiomes. This seems feasible.
We are a long way from being able to produce a duplicate of a particular human being. But there is no obvious reason to believe that this could never be done. Why would we want to do this? One reason might be to deal with problems caused by accidents or disease. Risky medical procedures might be carried out on a copy of an injured or ill person. If unsuccessful the copy would be destroyed, preferably without it ever becoming conscious. If successful, the original version of the person would be anesthetized and gently killed.Living Forever
A more dramatic goal would be to extend the life spans of humans indefinitely. We might start by learning how to build devices that behave like much simpler organisms, such as the roundworm, mentioned above. Then we might proceed to produce copies of increasingly complex animals. When we reach the point where we feel capable of synthesizing a human, the problem becomes much more difficult. We couldn't simply create a new adult human from scratch; such an entity, having no history, would have a blank memory and would be unable to function. In the case of non-human animals, we could simply copy the memory contents of some existing animal. (Copying a newborn human baby would be easier than copying an adult. But I find the idea distasteful, in that a baby obviously cannot volunteer for such a role.)
A solution to this problem might be to partition the specification of a person into 2 parts: one part dealing with the mechanical aspects of the human body such as the lungs, blood circulation system (including the heart), eyes, ears, muscles, nerves etc., and the other part dealing with higher level matters (thought). The first part would be maintained as it was at a young age--early adulthood. This part would be routinely treated to remain intact--perhaps replaced every decade by a refreshed and updated version. This would counter the detrimental effects of aging. The other, higher level, subsystem would be allowed to operate normally, but with the addition of new neurons to accommodate the growing need for memory.
There are some truly thorny issues here. If the death rate plunges, then, unless the birth rate also plunges, the already too large world population will soar. It is easy to imagine various controversies arising as the technology for extending the human life span is developed. All kinds of unpleasant scenarios are possible.
Fortunately, the problems would not arise suddenly. Even if a major effort were made to develop and implement the technology, it would probably take decades to produce significant results. There would be plenty of time to think about and discuss problems and possible solutions. Perhaps even more important, there would be time to discuss how humanity would be affected by such a drastic change, and whether the consequences are acceptable. For example, if the death rate were drastically reduced, the birth rate would have to be reduced by the same amount. Children would become scarce!
How would the first 100 people be chosen to have their lives extended? The first 10,000? The first million? It is easy to imagine some very ugly scenarios.References
, Wikipedia, "Humanoid robot"
 Wikipedia, "List of animals by number of neurons"
, "Transistor Count"