How did the first sexual animal come to exist? [closed]

Question

I am more or less familiar with the evolution theory based on mutations.

Now, starting with a nonsexual being, how did the first organism that reproduces sexually come to exist.

Answer 1

Big topic yes, but lets hit some high points.

First, sexual reproduction is a no brainer in evolution. In its basis, sex only requires a segment of the chromosome to be heterogamous - that it will not be recombined. In male humans this is both X and Y chromosomes, in other animals it is the female which has heterogamy. Its not the entire chromosome either - the pseudo-autosomal regions of X and Y in humans do recombine.

Sexual reproduction allows some individuals to combine their genomes by donating some but not all chromosomes to offspring. The heterogamous regions are always donated by one parent and the same sex is not allowed to combine with others.

Sexual reproduction is found in plants, single celled eukaryotes as well as animals. The forces that drive the emergence of sex show up frequently enough and are persistent enough for sexual chromosomes to have evolved hundreds of times in plants.

Even in animals, where asexual reproduction is very rare there are multiple configurations of sex determining chromosomes.

Sex goes back to a very ancient trait in eukaryotes related to the haploid / diploid life cycle. In fungi, who are related to the original metazoans, sex shows up when its needed. There the combination of haploid phases may be limited by more than one combination of specific elements. Fungi can have many genders (mating types) - each of may only allow mating to specific types. Others like Saccharomyces allow recombination between any two different mating types.

The earliest single celled eukaryotes related to animals show mating types. The various sexual characteristics related to sexual reproduction, such as placentas, eggs, sperm form and function are all embellishments from sexual selection which vary tremendously from one form of animal to another. The chromosomal functions which became sexual traits are ancient and predate metazoans. They are not necessarily conserved though - they are simple enough to reproduce that they change their characteristics often in evolutionary history.

I should say that all this doesn't apply to bacteria and eubacteria. Why? Because bacteria are much older and have their own systems of gene transfer and recombination with their own rules. They usually have a single copy of their genome (I'd like to hear about exceptions - I think I know of maybe one) which makes quite gender different. The do have something called 'sex' where a pili gene can be found on a plasmid which allows the bacteria to transfer DNA directly, which is like a sex determining locus. But a bacteria may catch gender like a cold - it can not have it, it can get it, it can lose it.

Bacteria are also capable of transferring a single gene or a segment of DNA by various means, including pili, plasmids, phage, and simply taking in the naked DNA that they find laying around.

Answer 2

This is a very broad field, with many complementing, overlapping, and contrasting theories. I suggest that you read this part of the Wikipedia article on this topic:

http://en.wikipedia.org/wiki/Evolution_of_sexual_reproduction#Origin_of_sexual_reproduction

Here is what I got from the Wikipedia article (this is just one hypothesis, read the article for all the others):

There are many advantages to sexual reproduction, two main ones are greater genetic variation and recombinational DNA repair (occurs during meiosis).

There are certain species that can reproduce both sexually and asexually. They choose to reproduce sexually during times when the environment is uncertain (changing). This provides evidence that greater variation is one reason for sexual reproduction.

Asexual organisms only have one copy of each gene, therefore if this copy is mutated in a bad way, they are out of luck. Organisms that sexually reproduce have two copies of each gene, when one goes bad, they can use the other copy to repair the original copy, during meiosis.

As you may know, stress has many harmful effects, one being that it leads to DNA damage. There are certain species of bacteria who, in response to stress, uptake/integrate nearby bacteria cell's DNA into their own. They can then use this new copy to repair their own copy in case they experience DNA damage.

This "trait" of up taking/integrating other's DNA may have been evolutionary selected for (because these organisms repaired their DNA, they survived and passed on their genes), therefore eventually leading to organisms that sexually reproduce.