| Let's imagine a few eukaryotic cells in a primitive pond. Floating freely, taking food with the apparent sole purpose of dividing and propagating their genetic information generation after generation. Now, let's suppose a period of food scarcity comes. The cells best adapted to starvation will be those that survive and can hold out until periods of abundance return. It's logical to think that these repeated situations over time will lead to the development of sophisticated mechanisms for adapting to starvation. Well, one of these mechanisms is autophagy, a process millions of years old that we only began to understand about 50 years ago.
Autophagy taught us to recycle and cope with hunger
Since we were little, we were taught the rule that plastic went in the yellow bin, cardboard in the blue bin, and glass in the green bin. This simple division makes recycling materials much easier so we can reuse them, right? Well, it turns out our cells already knew this, because they've been recycling since we were born.
The cells in our body (and any eukaryotic cell in general, like the yeast that makes bread) are capable of wrapping and trapping organelles and proteins they no longer need within membranes and taking them to the organelle that is responsible for degrading and recycling (autophagy): the lysosome.
In this way, they are able to reuse "garbage" components in the most convenient way for them at any given time. When cells fast, they recycle many more molecules. Why? Because they need to adapt to the situation and survive, so they use everything they can reuse.
A tool for transformation
We all come from the zygote, which is the cell resulting from the union of sperm and egg. It's impressive to think how, from that single cell, an organism as complex as the human being can be generated. It's therefore inevitable that during development there are transformations of some cells into others, which is known as cellular differentiation | 
