Why would a single celled, asexual, organism, that was neat and tidy, and had everything it needed to survive and reproduce, all within a perfect enclosed system, suddenly decide it now needed something hugely vital from another of its kind (that didn't yet exist) and begin the transformation into a much more complex process of sexual interaction for the purpose of its reproduction and survival of its species, and for how many millions of years did it get this wrong and not have the ability to reproduce - stuck somewhere in between - and how could it have possibly survived during the 100's of thousands of limbo years as an organism that needed something from another organism to reproduce, while that other organism hadn't even begun to form yet?
The parameters demand that it would be one of many that were making this change over an extended period of time, but none of them would have survived that got it wrong, and the ones that came after could not have made further progress from the point of needing that something from another similar organism (which still didn't yet exist), and even if they did, the chances that another organism would be making a similar - but opposite - change at that same time and would get it right, and THEN they would just so happen to come together for a sexual exchange of vital materials - are a googol to one. The mathematical equivalent of ... absolutely impossible.
A lot of the theories that answer questions like these actually assume an inherent intelligence. But you can't borrow from Creationism to flesh out your evolutionary theoretical construction. Evolution is all about randomness. It's only Creationism that is allowed to have intelligence already inside the cells of organisms.
That's how a plant/tree can use very specific and finely tuned scents to call out to specific insects that feed on the insects that are attacking them in order to save themselves. How can a plant possibly know that it needs exactly that - while simultaneously having the very complex inherent intelligence to be capable of speaking the scent language of the specific insect that feeds on the insects that are attacking it? There is only ONE answer. And it is absolutely NOT random evolution. It is God and it is absolutely brilliant intelligent design.
Most Evolutionists don't even consider the impossibilities. They just take it on faith and embrace anything their heroes like Hawking, Dawkins, Einstein, etc. seem to embrace. Yes, faith. Science is a religion as well - don't forget.