Branch Lengths Expanded
Lets use our time calibrated primate phylogeny to gain a better understanding of branch lengths. Remember, in time calibrated phylogenies the branch lengths represent time. For example, the branch leading to the Gorillas tip has a branch length of 8, meaning there are 8 million years of evolutionary history contained in that branch.
But on a time calibrated tree, we can actually add branch lengths. Lets start at the MRCA for Gorillas and Humans,, indicated by the red circle, and follow the flow of information to humans tip. This is illustrated by the blue arrows. If we add the branch lengths starting from the MRCA and leading to the Human tip, we can determine the amount of time humans have been evolving since our common ancestor with Gorillas.
We first add 2 million years for the branch leading from our starting MRCA (red circle) to the MRCAof humans, bonobos and chimps. We then add 6 million years for the branch length leading from that MRCAto humans. When we do this we see that we also have 8 million years of Evolutionary history!
This reveals a critical nuance. Any tip which connects to our MRCA also contains 8 million years of evolutionary history. If we follow the principle of “Information flows from the root to the tip”, we find the sum of the branch lengths from that MRCA to any of the Gorillas, Humans, Chimps, or Bonobos are all 8 million years!
Given a time calibrated phylogeny, we know all the tips from a given common ancestor will result in the same summed branch length. This tells us that all life on this planet has been evolving for the same amount of time because we all share a common ancestor. As you continue your studies this fact will help you make proper interpretations of the trees provided to you.