You have asked a marvelous and wonderful question! Scientists called "Developmental Biologists" study this question. And guess, what, my area of research is exactly this question! In fact, the whole reason I became a scientist was because I was fascinated by the very question you have asked!

I am a professor at UCSB and I run a research lab in addition to teaching classes. My favorite class to teach is called Developmental Biology!

As you know, when a sperm fertilizes an egg, that starts the "development" of a new organism. Now, that newly fertilized egg, called a "ZYGOTE," has to divide to create lots of smaller cells. In some organisms, these new cells are "instructed" as to what they will become later in development; that is, what will be their "fate?" Brain? Heart? In other kinds of organisms, the instruction or "fate specification" does not occur until much later, once many thousand of cells have been formed from that one big zygote. Regardless of when this specification occurs, the instructions are the key thing. How does a cell "know" what to become and at what time and in what place in the developing embryo? That is another way to state your question. No one really knows the complete answer to this, but we do have ideas about it based on studying "model organisms" that are easy to see as they develop. These model organisms, like fruit flies, frogs, mice, nematode worms and sea urchins, also can be used in genetic and molecular studies to ask the question of "how."

Let's start with this: every cell in your body has the exact same DNA in its nucleus. That is your "genome" -- all of the genes are the same in each cell. Yet, each kind of cell is different. A brain cell is very different from a heart cell. Why? Well, as you probably know, a gene (DNA) is "expressed" in a cell in a form called mRNA. This mRNA is then "translated" to make a protein. It is the protein that does the "work" and actually functions in a cell. So, if a heart cell has a different function than a brain cell, what do you predict about the proteins in the two cells? Will they be the same or different? Try to answer the question before you read further-- I bet you can figure it out.

Right -- they will be different! So, the DNA (genome) is the same, but WHICH GENES ARE EXPRESSED AND MAKE PROTEIN IS DIFFERENT IN DIFFERENT KINDS OF CELLS. Scientists call this "differential gene expression" and understanding how it works and is regulated is a big area of research. Not only is it important to understand from the point of view of development (how an egg becomes a new organism) but also in terms of human disease -- in many diseases, like cancer, the pattern of gene expression is changed in an affected cell. Identifying the "molecular regulators" of gene expression is a very big goal for many scientists.

How a new organism develops specific cell types is very complex, because it happens in a very regulated way in space and time (in other words, it wouldn't be very good if the heart formed before the chest cavity was sculpted or if the heart formed in the wrong place). We have a long way to go before we really understand how this happens. If you can, visit the educational link from the home page of the Society for Developmental Biology

You can learn much more about this and you can also watch movies of eggs turning into new organisms. I encourage you to keep asking questions like this -- maybe some day you will be the one who figures it out!

I'm not a biologist but I suspect that no one has all the answers.Basically, what is known is that in the egg there is an embryo which is made up of what are known as stem cells. Stem cells are immature cells that have the ability to develop into many different kinds of cells like brain, bone, blood, or liver cells for example. The DNA inside the cells is a code that determines how the embryo will develop.

A lot of scientists are working on figuring out how stem cells are told what kind of cell to turn in to. The hope is that we could use stem cells to create a new liver or other organs for people who need them. Today, people sometimes get a stem cell transplant when they have a problem in their blood producing bone marrow such as leukemia, lymphoma, or certain types of anemia. Somehow, the new stem cells know to go to the right place and start producing blood cells.

This is a complicated question that people are still trying to figure out,but here's the general idea. We all started as a fertilized egg that divided to make more of itself. As you noted, that's not enough to make a person. The cells have to specialize and organize to make a baby (or a corn plant, or a salmon). Each cell has the full set of recipes for making the whole individual, the DNA. Some cells start using one part of the DNA,while other cells use another part. This means the cells specialize as muscle or liver cells for example. They also have to organize into eyes, muscles and liver.

Here's how the brain and eyes get started. We were all basically a flat disc of cells when we were about 2 weeks old. Picture a groove forming at the top of the disk. This could form by the cells in the groove dividing faster than the cells on either side of it. Now picture the sides of the groove gradually coming together to form a tube. This is called the neural tube and it will become the brain and spinal cord. This all happens in the first few weeks after fertilization. The eyes form from out-pockets of the brain.

In general, a lot of things form as tubes and fold up or get thicker. The main things that seem to shape us are differences in the actual cell types specialization) and differences in how fast cells divide, which causes shape changes. Cell death is important too. We started off with paddle-shaped hands and feet, but some cells were programed to die, leaving us with separated fingers and toes. Some cells have to move around to find their targets. A nerve cell may have to grow from the spinal cord to a finger muscle.

The fields of "embryology" and "developmental biology" study questions like these. You might want to explore a career in one of these fields.