The tree is often used as an analogy for ancestry, as in a family tree. Think of the trunk as yourself: the ground level; and the roots as your ancestors: basement level one, two, etc. The branches are your descendants, your children and grandchildren; second floor, third floor, etc.
The problem with the analogy is that the roots under the trunk (you) grow unseen, ever spreading, ever larger. There are your parents, your grandparents, your great grandparents, and so on. But it is impossible for the tree to represent humankind because the system can not increase forever. At some point it must begin to shrink down to just two ancestors: Adam and Eve. Even the theory of evolution must start decreasing down to the original “evolved” humans or set of “evolved” humans.
I wonder what the mathematics behind all that process is? Some factors would be: average reproductive age, average life span, development of medical science, development of farming technology.
Another interesting view using a tree trunk to represent yourself is: where I stand (ground level) I see many other trees. They each have roots and branches. At my children's’ level (first floor) they are trunks with roots (my wife and I) and branches (my grandchildren). They can see many other trunks, each of those with roots and branches.
When we see other trunks like ourselves this is called a “generation”.
