Taxonomy shows us the vast diversity that exists among species through its study and description of all organisms, extant (currently existing) and extinct. Scientists have catalogued almost 2 million extant species and they think that there are at least twice as many waiting to be discovered and described. When scientists then do their best to estimate how many species may have come and gone out of existence over 600 million years, they come up with something around ten billion. The huge array of adaptations that each species evolved for thriving within its unique environment is really mind-blowing.
It is easy to compare a bacterium and a human and see that we are very different. A bacterium is always a single cell; a human is only a single cell on his/her first day of existence. Bacteria are loners, surviving on their own; humans are very social and dependent on others during their entire lifespan. Bacteria reproduce more or less by splitting in half and they become full size in a matter of minutes; humans reproduce sexually and have a 9-month period of development before being exposed to the outside world, which is then followed by years of continued development before reaching full maturity. Bacteria can only react to their environments; humans can shape their environments. And the list goes on.
Taxonomy shows us the breadth of biodiversity on Earth. But it also helps us to visualize the unity that exists among all living things by categorizing organisms according to their similarities. Phylogenetic trees give us a graphical representation of how all living organisms are related to each other.
Molecular systematics is a particularly strong tool for showing the unity of life. We mentioned earlier that there are certain fundamental processes that all cells must go through. Instead of different mechanisms being created for each species for the same processes, the mechanisms have been largely conserved over time. We see this especially when we analyze the DNA sequences of genes involved in these processes. What then becomes clear are our similarities at these molecular levels. That an organism like a bacterium can have any genetic similarities to humans is really quite remarkable. And it's true.