All species on Earth, both living and extinct, are related. We know this because of a biological tool called the tree of life. This "tree" takes the form of a diagram that maps the relationships between plants, animals and other organisms. Its "trunk" successively forks into millions of ever-more intricate "branches," each of which represents a new strand of life.
The origins of this biological metaphor are difficult to trace, because scientists throughout history have tried to visually depict the relationships among organisms in tree-style diagrams. But most scientists credit the modern inception of the tree to Charles Darwin.
