We all have have cells in our body, that is what our body is made of. Our body is made from two cells and those two cells break into a million cells and those cells turn into layers of tissue that turn into organs to make our body work. The organs turn into the organ system and you need every single system in your body to make your body run.(you don't need one system but you have it)
Humans Adapting to Earth
Humans had to adapt to this planet a lot. The first thing that I could think of would be sleeping because the first people on Earth didn't know to go to sleep when the sky turned dark and the second adaption is that they didn't know what poison was and so a lot of people might have died because of it. it would be very hard to live on earth if you were clueless about it. There was a LOT of adaptations for the first humans!!!!!