Identify the point where the common ancestor of the fly and spider would be located.

The common ancestor of the fly and spider would be located in the closest where B and C converge.

Explanation:

In phylogenetic trees, two species are more related if they have a recent common ancestor. On the other hand, two species will be less related if their common ancestor is not a recent one and is located far away in the tree.  

In the phylogenetic tree, the taxonomic groups of interest are located in the extremes of the lines called branches. In the present example, the fly and the spider are the species located in the extreme of the branches. The ramification point, which is also called "node" or divergence point, represents the location of the most recent common ancestor.

In the example, the point where the branches B and C get united, is the place where their common ancestor is located.      

The red point in the attached file "tree1" shows you where it is located the common ancestor of the two species.                      

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