For almost all of us, the heart beats on the left, but there are exceptions. One person in ten thousand is born with his or her organs reversed, as if through a mirror. This condition, which usually causes no trouble, is called situs inversus. Still, the patient should wear an alert bracelet, so that an emergency medical technician will listen for the heartbeat on the right, and look for the appendix on the left, etc.
Things do get complicated however, if the patient ever needs an organ transplant. It's hard enough finding a compatible donor, and when you do, he will almost certainly be oriented normally, left to right. The heart and liver have asymmetric shapes, and do not easily fit into a cavity that is the mirror image. It's a bit like putting your right hand into a left glove. Furthermore, these organs attach via veins and arteries at particular locations, and the surgeon must improvise when the point of attachment is on the other side, like stretching a cord all the way across the room to plug a toaster into the wall.
The mystery isn't that situs inversus exists, the mystery is why it doesn't happen more often. How does the developing embryo determine left and right with such fidelity?
The order of orientation is front back, up down, left right. The point of attachment between the embryo and the uterus becomes the front of the baby. When the embryo separates from the uterin wall, the umbilical cord retains the connection, still at the front of the fetus. Thus the front back axis is established.
While the embryo is attached to the uterus, gravity determines head and tail. The mother spends most of her day in one orientation relative to the earth. chemical markers separate slowly within the embryo, directed by gravity, whereupon the top becomes the head and the bottom becomes the tail. That establishes the up down axis.
Determining left from right, an event known as symmetry breaking, is still a matter of research. According to studies in mice, cilia play an important role. They twirl clockwise, and that by itself doesn't favor left or right, but they also tilt down towards the tail. Somewhat unexpectedly, this produces a net flow of fluid to the left, which in turn establishes left from right. The heart develops on the left and the liver grows on the right, and it works almost every time. See the references below.
Since gravity plays a key role, can an embryo develop in space? Could a woman get pregnant on the Space Station or on the way to Mars? Early experiments suggest the answer is no. Mouse embryos were sent up on the Space Shuttle, STS-80, and none of them developed properly, relative to a control group here on earth. In fact they all degenerated. Another experiment on Cosmos 1129 involved mature rats who were free to mate, but produced no pups. Post flight examination found that two of the females had achieved pregnancy, but their embryos appear to have been resorbed. Based on these preliminary results, it looks like the first extraterrestrial pregnancy will take place on Mars, assuming one third G is good enough. Not in my lifetime though. Even if NASA sends humans to Mars by 2040, nobody is willing to risk a tubal pregnancy so far from home.
A chicken egg is another story altogether. There is no point of attachment to determine front from back, and the mother hen turns the eggs on a regular basis. I have no idea how the embryo develops here on earth, or how it would develop in zero gravity.
Returning to our original theme, assume Jane has situs inversus. Her organs are reflected, but her molecules are not. Most of life's molecules, sugars, proteins, enzymes, vitamins, etc, come in left and right handed versions. Almost all biology on earth uses left handed proteins and right handed sugars. If you eat left handed sugar, it will pass right through you undigested. The chemicals that built Jane as an embryo are the same as those used to build any other embryo; she simply put her heart on the other side. Thus you can take Jane out to a nice restaurant, and she can eat all the same food, and digest it, just like you and me. Things are quite different if she was truly reflected, by a transporter malfunction or a trip around the universe.
Take a long strip of paper and put the two ends together to make a loop. Now twist one of the two ends 180 degrees and then staple the ends together to make a mobius strip. Draw a fish on the strip, with his head and tail oriented along the strip as though he was swimming forward like a car on the road. At the start, his dorsal fin points to the left edge of the strip. If he swims all the way around and back to start, his dorsal fin now points right. He has been reflected by traveling all the way around his universe. Assuming our universe is finite, (which it might not be), it could have a similar twist. Ride your space ship for 20 or 30 billion light years, and come back to earth, and you might be reflected, in your organs, in your molecules, and in your atoms. Of course the sun would be a frozen ball of carbon by that time, so this isn't a feasible experiment. However, there may be microscopic dimensions, curled up so tightly that we can't see them at all. Suppose Bob found a way to whip around the fifth dimension and return to start. From our point of view he would wink out of existence for just a femtosecond and then return. The trip would be too fast to measure. If the fifth dimension has a twist, and Bob returns reflected, he's in trouble. He can breathe oxygen and drink water, because those are symmetric molecules, but as soon as he tries to eat he will derive no nutrition from his food, and he might, in certain situations, become sick. A lab could construct reverse sugars and amino acids for him, but it's not clear if Bob could be kept alive long term. He needs to jump around the loop again to return to the proper, biological orientation. Then he can write a paper, because he has personally demonstrated the existence of extra microscopic dimensions, first suggested by Kaluza and Klein in 1926.
If the fifth dimension curls up and twists, then our universe has a midline. This goes against the steps taken by Einstein, demonstrating over and over again that there is no center, no special place in the universe. With this in mind, I'd bet my bottom dollar that there is no twist in any dimension. However, if there was, Bob would certainly not lie on the midline when he ran his circle of infinitesimal length. He would return to a position that is his original location reflected through the midline. This is probably a billion light years away, between galaxies, in the vacuum of space. His reflected biology is the least of his worries, as he gasps for air that isn't there, and dies in 28 seconds, the first 15 of which are agony. And the rest of us - we're not off the hook either. Bob leaves a vacuum behind, where he once stood. Air slams together to fill the void, creating a sonic boom just two meters in front of us. Windows shatter for several blocks in all directions, and our hearing is damaged for life. Damnit Bob!
Well no matter, there probably is no twist, hence Bob returns to his original location, oriented as he was, and has no idea he even made the trip. Maybe we all make this trip from time to time, shorter than a proton, faster than a fleck of light, and always stopping back here in this plane of existence for some quantum mechanical reason. We would of course be none the wiser.
I've gone way off topic, from situs inversus to quantum loops, but that's the fun of these articles.