How come all life on earth came from one single-celled organism, but it didn't die out from a genetic similarity?

I am a biology major in undergrad, and I have come to understand that organisms such as Cheetahs are endangered because of a genetic bottleneck event, which has caused their DNA to become overly similar to one another. This leads to a lack of genetic diversity and can cause inbreeding, which is bad for several reasons, like passing on potentially negative traits and limiting genetic diversity in future generations. I've heard that if there were only 2 humans left on Earth, that amount of inbreeding would likely cause us to die out as a species. If this is the case (which I'm not sure about), how could all life have come from a single organism? Wouldn't there be too much genetic similarity to get out of that inbreeding cycle? If there were no other organisms at that time, then how would this be circumvented?