Short answers, pulled from the story.
The first evidence of distinct male and female reproductive types dates back 1.2 billion years to the fossilized remains of Bangiomorpha pubescens, a green alga that proves sexes evolved early in eukaryotic history.
Humans and most mammals follow the XY system, where the presence of a Y chromosome triggers male development, making the male gamete the determinant of the offspring's sex.
Crocodiles and most turtles lack sex chromosomes entirely and rely on temperature-dependent sex determination to decide whether an embryo will become male or female.
Female southern black widow spiders are typically twice as long as their male counterparts, a size difference linked to the high nutritional cost of producing eggs.
Clownfish live in groups where the largest individual becomes female, and if she dies, the dominant male changes sex to take her place.
The primary function of meiosis appears to be the conservation of genomic integrity, allowing for the repair of DNA damage and the masking of deleterious recessive mutations through genetic complementation.