How Did the First Sexually Reproducing Organisms Arrive?

is to remember that there are not all that many key Evolutionary steps needed.There are the steps involving asexual ----> sexual critters. The sexual critters seem then to break down into two categories, those that are male or female, and a very large body that is hermaphoditic. IIRC, there are over 100 species of hermaphrodite fish as one example.The descendants of those early critters, both plant and animal, are what we see today.One interesting thing for me is that in the sexual community, there seems to be only two breeding methods that are common to all of the organisims, both plant and animal. There are those that breed as individuals, that mate as individuals, and there are the broadcasters, those that simply release sperm and eggs (or polen) and let them mix.Since these two methods seem to be common to both plant (although the mating plants do use an intermediary) and animal, they are likely traits that predate either.

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Aslan is not a Tame Lion

You gave me a well thought out answer. I'm just throwing out questions to see where it goes. I have no other thoughts on the subject for now. I usually get my thoughts from reading something. Not very original thinker.rickrose

First, just for fun, here is an article I wrote for Of Sea & Shore magazine on mollusk reproduction that details major aspects of sexual biology and then talks about specific mollusk reproductive strategies (entitled "Behind the green operculum"). Be forewarned, it is not from a peer-reviewed journal and was written to amuse and inform. also, it is by far the worst written piece I have ever submitted (I had less than two weeks two write it from when it was suggested). Check out: http://www.ofseaandshore.com/greenoperculum.htm.I think Extremophile's comments are most relevant to this topic. A large number of protists have what are called isogametes. To reiterate, these are gametes that are merely packets of DNA contained in a membrane with varying amounts of supportive cellular contents. In those species that have pure isogamy there is no such thing as male or female. Within the phyla of protists we see all stages of the evolution of sexual reproduction. The most primitive is hologamy. This is where two single celled organisms combine, exchange DNA, then split and then asexually produce clones. This is probably the first eukaryotic "sex". The flaw with it is that genetically both parents are dead. The resultant beings are a mix.The next step is to meet a conspecific, clone yourself with a minimal amount of cytoplasm and a full complement of DNA and let that 'minimee' comingle with your partner's progeny and resort into two individuals, leaving the parents intact. At this step we have true isogamy. Once we have isogamy then selective factors as mentioned by Extremophile take over and lead to separate sexes as we see them today.I think it is important to remember that our definition of male and female in a biological sense is only related to gamete size. Mammal sexes are defined by XX and XY chromosomes, but this is not the case in most organisms. Males are those individuals who produce motile gametes that tend to have reduced cytoplasm and associated organelles. Females have large gametes with supportive organelles and a large amount of cytoplasm.Because of the great diversity of reproductive patterns we see in protists, even within different phyla, I think it is likely that sexual reproduction evolved separately, possibly several times within the existing multicellular kingdoms or their immediate protist ancestors.

What about things like the slipper limpet (Crepidula fornicata) a protandrous hermaphrodite which starts off as male but which become female as they near the bottom of a mound of limpets. Also many reef fish which change sex during their life.I suspect that most of these are also later adaptations of the basic sexual strategy and not neccessarily informative in terms of what might be the prototypic sexual organism.Maybe I missed it but I was surprised no one has described the red-queen hypothesis yet given how close to it Lithoid mans early hypothesising was.TTFN,WK