Most of the cells in your body have a nucleus and within that they possess all of the DNA necessary to serve every function of every cell. Individual, specialized cells don’t use most of the DNA because they only serve a few specific functions. When cells reproduce, they must double everything first and then divide to maintain the same number of chromosomes. However, if they divide a second time, they will only have half the DNA from their parent cells. Cells with all their DNA are called diploid and cells with half are called haploid.
After the first eukaryotic cells formed, a random mutation caused cells to divide an extra time to produce four haploid cells instead of two diploid ones. Usually, a mutation like this would be devastating for the organism and it would quickly die.
Symbiosis saves the day! The haploid cells did not die since they were supported by the rest of the functioning diploid cells of the organism. The trait to create some haploid cells continued because they were not weeded out by natural selection.
Nature doesn’t waste. Two organisms with haploid cells, descended from a common ancestor, could combine them via sexual reproduction. We are not sure the exact mechanism of the first sexual reproduction but it is likely that haploid cells were expelled by the organisms and they were able to recombine in order to reach their diploid amount of DNA.
With the recombination, these new cells had a much different genetic code than their parents. This makes sense because there is no direct copying and any differences between the two organisms could account for differences in the newly formed offspring.
The new cells are able to divide and, with all the necessary DNA, reform all the specialized cells in order to become a functional organism. The offspring were similar to both parents, yet different.
Recombination allowed for greater genetic diversity than any random mutation and greater diversity opens up new niches to explore. The adaptation of sex cells was a likely cause of the Cambrian Explosion, a time that saw the rise of more new species than ever before on Earth. There is no direct link between the adaptation of sexual reproduction and this explosion of diversity, though they did occur around the same time period so we have to make an assumption.
Sex not only allowed for much more diverse organisms, but also helped to remove disadvantageous traits from the gene pool. If an organism had a harmful mutation on one of its chromosomes, it could sometimes continue to function if the chromosome pair had a more dominant allele.
This means that the disadvantage of recessive traits could remain hidden within an organism and still be passed down to the next generation. With the separation of DNA in the second cell division, chromosomes with disadvantageous traits can no longer hide. This meant the negative trait had a greater effect on the organism and it would very likely die. Death isn’t always a bad thing since it prevents the negative trait from passing down to the next generation.
Plants and fungi split from the our animal line fairly early, though they still adapted sexual reproduction. These organisms differ because each could produce multiple types of haploid cells that could recombine with themselves. Because these organisms were unable to move around to find mates, they produced huge numbers of each type of haploid cells. The individuals producing huge amounts of recombining cells had a greater chance that more of their offspring would survive. We still see that plants have two types of sex cells: many trees produce both pollen and seeds in the form of fruit or cones.
Animals are more mobile and can choose their mate. This is not to say that early animals stayed up late into the night thinking about their soul mate. Rather, they would pass up mating with individuals who seemed unfit, instead choosing to mate with individuals with favorable traits. Some of the first animal sex was nothing more than one animal shooting haploid cells through the water and into the mating animal or on top of the mate’s already expelled haploid cells (think about fish eggs). The individuals who recombined haploid cells within their bodies gave a greater chance of survival to their offspring.
One mutation that turned out to be very advantageous came from an error in copying one chromosome fully and a shriveled up version of it was all that remained within a few of the haploid cells. Now, the offspring were able to exist as two different forms, each with specific instructions to form one type of haploid cell. This was the birth of the sexes. In mammals, the female is the name we gave to the individuals with two identical copies of that chromosome. The male is the one with the mutant chromosome.
This is the same with some insects. However, many male insects have only one sex chromosome while the females have two. Males are not always the ones with different chromosomes: some birds, fish and insects have different chromosomes in the female. Other organisms, like bees and ants, don’t recombine at all. Females of these species have only diploid cells and male workers have only haploid cells. You’ve probably heard of these animals having a single female queen.
With two different sexes, structures could form to ensure effective delivery of sex cells between organisms. Many males developed outward-growing structures while females co-evolved an inward cavity to accept the male structure. Pleasure releasing neurons were also connected to these sex structures to encourage the organisms to reproduce, helping the species to survive.
I think the most interesting part is that males are just a mutation from the original female. That means mutants have assumed dominance over half of the species. Male or female, we are all human. It is time for us to realize that as a species.