This is a perennial illustration of mine, the theory of evolution shown in a silhouette of a man’s head, It’s in a head because I want to emphasize that this is a theory, a mental construct.
As the story goes, 4.5 billion years ago the earth is formed. 3.5 billion years ago there was organic soup. Life springs forth, first as plants about 2 billion years ago. Then at 600 million years ago there was a massive “explosion” of life – the Cambrian explosion. Sponges, worms, jellyfish, trilobites come on the scene, and through a process of mutation and Natural Selection (i.e. the Synthetic Theory of Evolution) life evolves into higher level animals. Finally, about 4 million years ago, man comes into being.