When Charles Darwin first sketched how species evolved by natural selection, he drew what looked like a tree. The diagram started at a central point with a common ancestor, then the lines spread apart as organisms evolved and separated into distinct species.
In the 175 years since, scientists have come to agree that Darwin’s original drawing is a bit simplistic, given that multiple species mix and interbreed in ways he didn’t consider possible (though you can’t fault the guy for not getting the most important scientific theory of all time exactly right the first time). Using a tree-like structure is a great way to show the history of the evolution of a species, or its phylogeny. But it’s not so great for showing the population history of groups within a single species, such as humans, who can move around and interbreed with each other.