Imagine being there when the first dog evolved. Let's say it's the African hunting dog (Lycaon pictus), the wild canid of Africa. There was a big bang, and millions of years later an animal with a tail and four legs, a liver, heart, kidneys, lungs, blood, ears and eyes evolved into the first dog. Fortunately for him, his eyes had evolved to maturity after millions of years of blindness, so that he could see the first female dog that had evolved standing by him. It was actually very fortunate, because if the female dog hadn't evolved also and been at the right place at the right time, with the right parts and the willingness to mate, he would have been a dead dog. He needed a female to keep the species alive.
When some thinking person on an atheist blog said, "It is my understanding that the evolution of sex remains something of an unexplained mystery," there was a quick reply: "Nope. For example, the selective advantage has recently become clear: it serves to keep transposable elements from completely taking over the genome."
Let me paraphrase what our "I haven't a clue, so I will blind him with science" atheist just said: "The evolution of sex is not an unexplained mystery at all. For example the natural selecting choice for the best that evolution makes has recently become clear. It serves to keep the movable material from completely taking over the genome (a genome is a full set of chromosomes; all the inheritable traits of an organism). If you are not sure what chromosomes are, they are a threadlike linear strand of DNA and associated proteins in the nucleus of eukaryotic cells that carries the genes and functions in the transmission of hereditary information. If you are not sure what eukaryotic cells are, they comprise all of the life kingdoms except monera. Eukaryotic cells also contain many internal membrane-bound structures called organelles. These organelles such as the mitochondrion or chloroplast serve to perform metabolic functions and energy conversion.
"So that's how you can know for sure that the female dog evolved at just the right time, at the right location, at just the right age, with the exact reproducing female parts to mate and keep the species alive. Keep repeating to yourself that evolution makes sense, and that it's extremely scientific. I know that it is intellectually beyond you, but we have all the answers. Trust me."
