What we do know is that man's sperm has evolved to go war with other competing sperm.
For ages, when scientists looked at male sperm under a microscope they thought we had 3 types of sperm. Regular proper functioning sperm that raced to the egg to fertilize it and two types of malfunctioning sperm. 1 that was labeled 'lazy sperm' as it hardly moved at all. And the other 'crazy sperm' as it basically ran around in circles but never went to the egg.
And it is often when a man has too many of the second and third type that he struggles to get a woman pregnant.
it was only when they introduced a second man's sperm ( i always wonder if that was an 'Oops' accident) that they saw that the lazy and crazy sperm both had jobs. The lazy sperm was left in the canal as blocking guards to hold and shut down the line. They would attach to incoming sperm and slow them down. The crazy sperm would literally kill any foreign sperm they came in contact with. Just the same way sperm fertilizes an egg by driving its head into the egg they would ram their head into the competing sperm, pin them and kill them.
That is some pretty crazy shit that they say 100% of males who have functioning sperm have this type of sperm. It means it was needed everywhere and regardless of circumstances (good times, bad, feast or famine or other) that if men did not have sperm that could compete they got pushed out of the evolutionary pool. No exceptions.