This is an extremely important question for understanding life . The idea of descent with modification explains many things in biology, like why whales have hips, why bacteria are becoming resistant to antibiotics, why house cats look a lot like tigers, or why vertebrate embryos all look really similar in the earliest stages.

It looks like you have some misunderstandings about evolution. This is very common. First, individuals don’t evolve. Populations do. When gene frequencies change over time, that’s evolution. Gene frequency is how common certain varieties of genes (alleles) are.

There’s a lot of variation between individuals in any population. This is due to random mutation (errors when DNA copies itself). We notice variation in humans, but it’s true for salmon and pine trees and bread yeast too. Even when single cells divide, their DNA can undergo mutation. Any population not made of clones has a lot of variation. Some of those differences in genes don’t affect an individual’s chances of reproducing. Some improve its chances. Some decrease its chances.

Here’s another important fact, evolution does not happen because something decides to evolve. We humans are pretty darn smart, but we can’t change ourselves into other species. Mutations do not happen because of trying, or needing them, or wanting them. Mutations are totally random. Natural selection, the process in which individuals with genetic advantages are more likely to pass on their genes, is NOT random.

So when the first cells were dividing, their DNA copied itself. Mutations would have occurred. As the cells spread and multiplied, some of those cells would have had advantages for particular conditions and places. Imagine that some cells ended up in colder water. Cells with genes that help them survive in colder water would have had an advantage there. Every now and again, some random mutation may have happened that made them even better at living in cold water, and worse at living in warmer water. Eventually, the cold and warm water populations would become very different. They didn’t decide to do anything. Random mutation created variation and cells with helpful mutations were more likely to keep dividing, leaving more cells with the variations that are helpful.

Single-cell organisms can exchange DNA, but let’s fast forward to multicellular species. As populations get isolated from each other due to geographic, climate, geological changes, and major mutations, they become so different that they can’t interbreed anymore and become different species. We can trace which species are more or less closely related by counting the number of differences in their DNA . By knowing the average number of mutations per a period of time, we can estimate how long ago two lines started changing in different directions. Evidence from the fossil record helps validate our estimates. For example, tigers and house cats are the descendants of an extinct species of cat that lived about 11 million years ago. House cats and cheetahs are both descended from an extinct cat species that lived about 6 million years ago. By the way, we did not descend from chimps or any other living species, we just share common ancestors.

Can you think of examples of species that are in the process of becoming so different that they don’t usually reproduce, but they still can under certain circumstances? Try looking at hybrid species.

I believe you are asking about how multicellularity evolved which is a very interesting field of research!

Our bodies are made up of cells that have specific jobs. Your heart cells have to rhythmically beat, cells in your intestines have to absorb nutrients, and brain cells have to secrete signals and communicate with each other…the list goes on and on. How did all of this cellular diversity arise?, and furthermore how do these cells come together and form an entire organism like a monkey or a dinosaur!?

Single celled organisms are still around and thriving today. Some people argue that they can make “decisions”; a simple example is a motile cell that will move toward a source of nutrients or away from heat. Single-celled organisms can be quite complex; look up Tetrahymena. It breeds with other Tetrahymena which leads to the exchange of genetic information. Even bacteria can have sex so this, as you allude to, provides an increased opportunity for evolutionary selection to take place. So, what selected for single cells to stay together? Nowadays there is still evidence of this taking place: microbial mats (you can look that up too). Some single celled organisms will come together and form complex layers (“mats”). Depending on where the microbe is situated in this configuration, it can have a different job, like making and secreting different molecules to communicate with other bacteria, or absorbing and processing things from the environment. This division of labor among the same species of bacteria is similar to how all our different cell types (which all contain exactly the same genetic information) make up a functioning body.

This is a link to a really cool video narrated by Sir David Atenborough who explains the history of life: history of life

You're confusing growth with evolution.

Every cell in your body has the exact same genetic makeup, except for your sperm (if you are male) or eggs (if you are female), which have half the DNA of any of your other cells. Your cells respond to certain stimuli, such as temperature, light, or chemicals, and proteins inside of the cell then tell the cell to do things, which can include sending other chemical signals to other cells. This is how your cells "know" to divide or not to divide, or what to turn into, be it muscle, nerves, etc. Cells do have Golgi aparati, which act as a sort of nerve center within a cell, but they still need information from the outside in order to specialize to build your multicellular body, and that information comes from either environmental effects or from chemical signals from other cells.

Evolution is the change in the DNA of a lineage. Because all of your cells have the same DNA, your cells are not evolving, and neither is your body. You cannot evolve. The human species can evolve, but only because your children do not have the same DNA as you do (in particular, they will have half of their DNA from you, and half from your husband or wife).

The ancestor of all animals did have only a single cell. By evolving, that species of protozoan began to be able to send chemical signals to other members of its species, and in time they began to organize themselves into larger and larger groups. Eventually, the signals they were sending became complex enough and the cells' responses became complex enough that some cells behaved in a different way and turned into different types of cells from the others in their population. This is how multicellularity evolved. All of the cells in a multicellular animal are still descended from a single cell, however, and that cell is the product of a sperm meeting an egg from a previous generation. Your father, for example, is a multicellular animal, and your mother is also a multicellular animal, but your father's sperm and your mother's egg combined to create a single cell: you. You then divided into multiple cells. That was not evolution, but growth. The only evolution that happened was when the half of your father's DNA combined with the half of your mother's DNA combined to become your DNA, which makes you different from your parents. However, the process by which single-celled protozoans evolved into multicellular animals was not simply growth, and involved many generations, and many combinations of sperm with egg.

As for what a zygote (a cell created by the union of sperm and egg) turns into, it depends on the cell's DNA. Your DNA told the zygote that was you to become a human, because you have human DNA. A monkey zygote has monkey DNA that tells it to grow into a monkey, and a dinosaur zygote has dinosaur DNA that tells it to grow into a dinosaur. The exact way in which this happens varies from animal to animal (and from plant to plant, since they use an analogous system), and many of the details of how it happens remain unknown. What we do know is that there are cascades of regulatory genes, where gene A activates genes B, C, and D, while gene D then activates E, F, and G, and so on.

At the most fundamental level, evolution is always acting on genes - whether or not you're looking at monkeys, humans, avocados, or bacteria, the traits that make an organism more or less adapted to their surroundings are dictated by the DNA.

Single-celled organisms such as bacteria don't live in isolation. They can compete with each other (some inject their neighbors with toxins, for example) or cooperate with each other. Over millions of years, colonies of single-celled (genetically identical) organisms eventually became tissues that could out-compete their non-cooperative neighbors for resources. Over time, these tissues became organs that were interdependent on one another for specific functions. Ultimately, the function of every single one of our complex organs is to get our genes into the next generation; our cells have simply evolved a much more complicated way of getting there.

It is important to remember that evolution occurs on geological timescales that are much longer than humans can observe. Here is an example: I am sitting in Santa Barbara, California right now, and I don’t feel that I am moving at all. But if I could somehow fast-forward time I would see that I am actually sitting on a continental plate that is mobile and drifting across a sea of mantle. Here is a picture of just the last 250 million years
continental drift

If I could go back several hundred million years, the land where I am sitting now would actually be in Antarctica. The reason I don’t see the continents moving is because they move too slow compared to human perception.

Evolution occurs on the same geological timescales. A single cell did not become a monkey or a human, it lived and divided, passing on its genetic material. From its perspective nothing changed. The first single-celled organisms appeared 3.5 billion years ago, but the first multicellular organisms didn’t appear until 600 million years ago. That is an incredibly long time even by evolutionary standards. For almost 3 billion years there were only single-celled organisms. Evolution proceeds by natural selection, meaning that the cells which had a better chance of survival were more likely to pass on their genetic information. No one knows exactly what triggered the arrival of multicellular organisms. One can speculate that eventually, different cells began specializing and cooperating in a symbiotic relationship, which was mutually beneficial to both cells. These cooperating cells were more likely to survive and pass on their genes, and eventually, over millions of years these cooperating cells became the earliest multicellular organisms. Again, it is important to realize that this occurs on geological timescales.

Scientists use the fossil record which provides snapshots which are like still frames in this fast-forward movie of time. No single organism decides to become a monkey or a dinosaur; evolution occurs through natural selection which only becomes noticeable after many generations.