How Did the First Sexually Reproducing Organisms Arrive?

This is from the top of my head, let me know if you want more specific information.First of all, bacteria participate in the swapping of DNA, so even the most simplest of life forms is capable of what we would call "reproduction". However, with bacteria this is usually limited to small bits of DNA instead of half the genome, as is seen in mammals for instance. Also, bacteria absorb extraneous DNA from their environment as well, and can incorporate this DNA into their chromosomes. This is called "horizontal gene transfer". The production of gametes (eggs and sperm) may have been a evolved pathway towards more specific DNA transfers, that is keeping DNA isolated to the same species instead of casting DNA "to the wind" so to speak.Secondly, we also see that many species are not strictly separated into male and female. Take earthworms for example, they produce both sperm and eggs, and fertilize each other during mating. Once the apparatus is there, it wouldn't be too difficult for the sexes to be separated permanently.So, with species alive today we see a gradation between simple DNA swapping---> gamete swapping ----> sexually separate species. There are probably many intermediate steps between these three, but the pathway seems to be well evidenced.

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Just a small point LM about earthworms. As I understand it gonochory (separate sexes) is the primitive condition in metazoans.

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Thanks for the correction.

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That is to say that hermaphroditism (both sequential and simultaneous) seems to be derived from the gonochoristic condition rather than the other way around. If you look at the developmental aspect of gonads in metazoans you see how hermaphroditism can independantly develop. For example, caridean shrimps have a structure called an ovitestis that is literally an ovary in the front and a teste posteriorly. In most shrimp one or the other portion doesn't develop at all. In those species which are sequential or simultaneous hermaphrodites hormones cause the ovarian portion to grow and the testicular portion to either remain functional or atrophy.

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So is sex controlled by the environment or by a genetic disposition (such as sex chromosomes in mammals)? Even in mammals, hormones control gonad development, however the timing of hormonal release is a mixture of genetic predispositions and mother hormonal feedback (IIRC). In shrimp, is it more of an environmental pressure or a genetic predisposition that decides gonadal development, be it sequential or simultaneous hermaphroditism?

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The most obvious and probably most important reason for sex is recombination. This is not saying that some unicellular eukaryote 3.7 BYA decided it needed to recombine. But the fact that some of them (unicellular eukaryotes) do shows there is a reason to comingle.

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Using antibiotic resistance as an example, bacterial recombinanation with exogenous DNA is an important mechanism. Plasmids carrying resistance genes can cross SPECIES barriers, even to the point of recombinantion between Gram positives and negatives (one of the major divisions in microbiological species). Some species have sexi pilli, which are transmembrane bridges that allow the transfer of small sections of DNA and plasmids. Even within the simplest organisms, horizontal gene transfer seems to be an important evolutionary and adaptational mechanism. Sexual reproduction in metazoans seems to be the next step towards taking advantage of recombination. That we see sexually dimorphic organisms (either simultaneous or sequential) is an important step that avoids the "chicken and egg" problem that separate sexes would pose as a first step in sexual evolution.