So incest.
Well... I recently read an article about Mycetozoa. They're slimy organisms that fall between plants and animals. And they haven't changed much since the first ones appeared, actually they're much alike the stuff that's thought to be the very first life form.
So when the very first slimy cell was born by a lightning hitting a puddle (Darwin) or near a hot submarine spring or a comet hitting the Earth, that cell didn't travel around to find a mating partner. Likely it duplicated itself by splitting which is the simplest form of reproduction. And they split again, and again, and again until the place was full after which some of them found other ways and places to live through molecular evolution. I'm not saying that all life has begun from one single surviving cell but as there's evidence of heat being involved at an early stage of the development process it's safe to assume that the earliest life forms have appeared in a certain surrounding. Also as about 8% of our DNA is of viral origin and that a bunch of bacteria is essential for our well-being it's most likely that already the earliest living cells affected each other in one way or another, improving their ability to survive even outside their cradle.
Thus, when going back some 4 billion years we all are kin, offspring of the first single cell organisms that split.
