Almost 3.5 billion years ago, before humans, trees, and even dinosaurs, the only living things on Earth were single cells. As the years passed by, these cells reproduced, divided over and over again and filled the planet with lots of single cells. They lived on land and in the sea, but they stayed really small for millions and millions of years.
But then all of a sudden, creatures made up of many cells started to appear. Today we see these "multicellular" species all around us. Just look around you—there are cats and dogs, horses, lizards, mushrooms, and trees. You can even see them when you look at your reflection … the world is full of them.
But wait, in the beginning of time there were simply single-celled creatures and then, suddenly, they became multicellular creatures? What? What happened? What makes single cells form a multicellular creature?

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2019.00020