We all develop from a single cell known as a zygote. This zygote divides and becomes two cells, then four, eight and so on. At first, most of the cells are alike, but as this division goes on something wondrous occurs: the cells begin to commit themselves to adopting different fates as eyes or ears, or livers or kidneys, or brains and blood cells. Eventually they produce a body of immense and unimaginable complexity, making things like supercomputers and space shuttles look like Lego toys. No one knows how they do it. No one is there to tell the cells how to behave, there is no homunculus directing cellular traffic, and no template to work to. It just happens.
If scientists could figure out how cells enact this miracle of development they could produce phenotypes—the outward form of our bodies—at will and from scratch, or at least from a zygote. This, or something close to it, will happen in our lifetimes. When we perfect it—and we are well on the way—we will be able to recreate ourselves, even redefine the nature of our lives.
The problem is that development isn't just a matter of finding a cell and getting it to grow and divide. As our cells differentiate into our various body parts they lose what is known as their 'potency', they forget how to go back to their earlier states where, like the zygote, all fates are possible. When we cut ourselves the skin nearby knows how to grow back, erasing all or most of the damage. But we can only do this on a very local scale. If you cut off your arm it does not grow back. What scientists are learning bit by bit to do is how to reverse cells back to their earlier potent states, how to re-program them so they could replace a limb.
Every year brings new discoveries and new successes. Cloning is one of the more visible. At the moment most cloning is a bit of a cheat, achieved by taking special cells from an adult animal's body that still retain some of their potency. But this will change as cell re-programming becomes possible, and the consequences could be alarming. Someone might be able to clone you by collecting a bit of your hair or other cells left behind when you touch something or sit somewhere. Why someone would want to do this—and wait for you to grow up—might limit this in practice but it could happen. You could become your own "father" or at least a very grown up twin.
More in the realm of the everyday and of real consequence is that once we can re-program cells, whole areas of science and medicine, including aging, injury and disease will vanish or become unimportant. All of the contentious work on 'embryonic stem cells' that regularly features in debates about whether it is moral to use embryos in research exists solely because scientists want a source of 'totipotent' cells, cells that haven't committed themselves to a fate. Embryos are full of them. Scientists aren't interested in embryonic stem cells per se, they simply want totipotent cells. Once scientists acquire the ability to return cells to their totipotent state, or even what is known as a 'multi-potent' state—a cell that is not quite yet fully committed—all this stem cell research will become unnecessary. This could happen within a decade.
School children learn that some lizards and crabs can re-grow limbs. What they are not taught is that this is because their cells retain multi- or even toti-potency. Because ours don't, this makes car crashes, ski accidents, gun shot wounds and growing old a nuisance. But once we unlock the door of development, we will be able to re-grow our limbs, heal our wounds and much more. Scientists will for once make the science-fiction writers look dull. The limbs (and organs, nerves, body parts, etc) that we re-grow will be real, making those bionic things like Anakin Skywalker gets fitted with after a light-sabre accident seem primitive. This will make transplants obsolete or just temporary, and things like heart disease will be treatable by growing new hearts. Nerve damage and paralysis will be reversible and some brain diseases will become treatable. Some of these things are already happening as scientists inch-by-inch figure out how to re-program cells.
If these developments are not life changing enough, they will, in the longer-term usher in a new era in which our minds, the thing that we think of as "us", can become separated from our body, or nearly separated anyway. I don't suggest we will be able to transplant our mind to another body, but we will be able to introduce new body parts into existing bodies with a resident mind. With enough such replacements, we will become potentially immortal: like ancient buildings that exist only because over the centuries each of their many stones has been replaced. An intriguing aspect of re-programming cells is that they can be induced to 'forget' how old they are. Aging will become a thing of the past if you can afford enough new pieces. We will then discover the extent to which our minds arise from perceptions of our bodies and the passage of time. If you give an old person the body of a teenager do they start to behave and think like one? Who knows, but it will be game-changing to find out.