Everything in our bodies is made of cells - bones, muscles, brain, blood. How do cells know how to build a body? Granted, each cell has a blueprint of the whole body in its nuleus, the genome. Does that help the cell know where it is in the grand scheme of things and how it should behave (divide?) for the end result to look like a human body?
If someone told ten million people to form the word "Help!" that's 100 miles wide and each person got the exact blueprint of what the end result should look like, I don't think the aliens would see a convincing message in a few months or maybe even years.
If those people are to finish their task in any forseeable future, they need some kind of a feedback loop. For instance, they could build an app that showed everyone's position on the map, so they could say "Hey, that blob looks like they're trying to be the dot in the exclamation mark. I need to move 400 meters to the south-west if I want to join them."
Do cells have similar feedback loops? Or am I too naive to think that individual cells have any say in how our bodies are built? If it's not the cells, then what is it that controls the growth of our bodies making sure we don't grow a second head and everything works in perfect harmony? Obviously, this doesn't work 100% of the time - that's when we get cancer or Siamese twins. But it works most of the time.
How does the body do it? What and where is the conductor of this billion-cell orchestra?
I am not a biologist, so please be condescending. After all, the question What controls the size of breasts? got a decent answer. I don't see why mine shouldn't.