The common ancestor.
A phylogenetic tree shows how a species
originates (by evolution) other species. It’s very similar a genealogic tree. If
you want your family tree, you should write down your name and the names of all
your brothers and sisters and then draw a line from all those names to the
names of your parents, and then draw a line from the name of your father to the
names of your gramma and grampa, and then do the same with the names of your aunts
and uncles. If you repeat the process with many, many generations, you will
have the common ancestor to all of those relatives. You can do the same with
species: for example, humans are supposed to have a common ancestor with
monkeys, so their lines will end together in the same point in the graph. Then
you have to draw a line from that point to another point that will be shared
with other mammalians, then Mammalians share a point with other vertebrates,
and so on… If you keep on going, you will get to LUCA someday.
