Interchangeable!

 Supreme nature has created various forms of creatures (micro to macro), rivers, oceans, deserts, mountains, glaciers, moons, planets, stars, and other heavenly bodies. These all are interchangeable. 

Since the big bang, natural evolutionary and catastrophic changes have been taking place in various heavenly bodies in the universe. We are living on our planet Earth so we will focus on the natural laws and processes that have been governing the planet Earth internally and externally for millions and billions of years. Since the creation of the Earth, all things (living and non-living things) are moving in their own circles. To understand this process, you need to think over the water cycle, oxygen cycle, the cycles of entire elements, and finally life cycle. In the natural processes, you cannot disturb the cycle. How much total water has been formed on the planet Earth you cannot increase or decrease the ratio of the water, you cannot increase the ratio of oxygen or any other element's ratio. You can change the forms of these elements into various forms but the natural ratio you cannot change.

Also, you can observe the landscape of your area, it changes because of weathering processes, erosion, seismic activities, and many other causes but these processes cannot finish the nature of the material. We have observed and learned that everything is interchangeable. 

Now, it depends on the natural processes and the luck of living or non-living bodies that affects them in their life cycle. Suppose you have been born in the USA, this is just because of natural processes and your static luck. Having born, your education system, your way of living, your deeds, etc are coming in your dynamic luck. Therefore, you need to understand these three things, 1. Natural processes, 2. Static luck, 3. Dynamic luck

You have been bestowed some characteristics, parents, birthplace, height, color, etc on the basis of your past deeds, from here onward now it's up to you that in what direction are you moving? For that, you have to thank supreme nature for everything and then you need to utilize your dynamic luck to change yourself, and what form of creature you need?  If you need a knowledgeable, pious, and honest human form, then you need to adopt and developed such habits. If you need the form of an animal such as a rat, then you need to adapt and develop such habits that are corruption, stealing, dishonest, etc, then you can get the form of a rat. If you need the form of a snake then you need to adapt and develop anger, enmity, etc then you can get it easily.