How does evolution lead to both the diversity and the unity of life?

Simple. All of the millions of different life forms (the diversity of life) have come about by natural selection, whereby random mutations in the genes have either succeeded or failed. But all life can be traced back to a common single-celled origin. Thus we have the unity of life where we (possibly) share more than 50 % of our DNA with a crocodile. eg:analogous organs, homologous organs.