The heart of this family tree of life is
interesting. There we find the
Last Common Ancestor.
http://www.ucmp.berkeley.edu/education/events/eukevol1.html
The process of trying to figure out the
characteristics of the Last Common Ancestor
is called rooting the tree.
Why do we care about the Last Common Ancestor?
We believe that properties common to all life
on Earth may devolve from that organism.