One point we’ll return to repeatedly throughout the semester is that all
life forms on this planet are related to one another. It’s nAnd, therefore, everything that exists today on this planet represents
the descendents of that successful group of cells that existed a very
long time ago. Here we have all this family tree of the different
metazoan forms that have been created by the florid hand of
evolution. And we’re not going to study those phylogenies simply
because we want to understand principles that explain all of them. ot as if life
was invented multiple times on this planet and that there are multiple
independent inventions to the extent that life arose more than once on
this planet, and it may have. The other alternative or competing life
forms were soon wiped out by our ancestors, our single-cellular
ancestors 3 billion years ago.