With brothers you become friends. Some you hang out with more than others. You talk to one about the other a little more. You get mad at them. Then, you love them. Then, you apologize. You have to apologize whether you want to or not. You have to. That's your brother.
Darwin theorized that mankind (both male and female) evolved alongside each other over millions of years, both reproducing after their own kind before the ability to physically have sex evolved. They did this through asexuality (without sexual desire or activity or lacking any apparent sex or sex organs). Each of them split in half: Asexual organisms reproduce by fission (splitting in half).