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How do scientists diagram evolutionary relationships?

Scientists represent evolutionary relationships with tree diagrams (also called cladograms) because new life forms appear the way new branches appear on a growing tree. So, you can read a tree diagram like a picture story of evolutionary history.

The trunk of the tree is the ancestral line, made up of generation after generation of the same type of organisms. As long as the same type of organisms appear in each generation, the trunk remains unbranched. If the Earth's evolutionary history had been this simple, the tree of life would just be a straight line consisting of countless generations of the same kind of bacteria-like organisms that were Earth's first life forms.

But this is not the case. After 3.8 billion years of evolution, the tree of life has branched again and again, with some branches dying out and others with living descendants out at the branch tips. As scientists piece together the finer details of the branching patterns (that is, evolutionary relationships), they are able to construct more and more accurate tree diagrams. But how do they figure out relationships between living things?

 
  -> How do scientists figure out relationships between living things?
  Tree of Life | One Common Ancestor | Diagramming Trees | Comparing Traits | Why It Matters
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