Almost all forms of complex life have both male and female.?

Great question and the right place for it.

I don't know exactly what the consensus of scientists is on this particular matter, but it's not difficult to come up with a plausible situation.

I know that the science agrees that all life started out as single-celled organisms, which reproduced asexually.

I'm guessing that the single-celled organisms began by swapping strands of dna through their membranes, thus providing a sexual reproduction, but without genders.

From there, it would be possible for some single-celled organisms to expel their dna, even without a single-celled organism being specifically next to another cell, which would allow the kind of fertilisation that fishes engage in. Specifically, the female lays the eggs, and the male fertilises them afterwards. This would be similar, except instead of eggs, there would be partially (or perhaps even completely) fertile cells, and instead of sperm, there would be simple dna strands, maybe wrapped in some membrane.

From there the genders could form, as some cells naturally become efficient at expelling DNA, while others become more efficient at receiving DNA and using it to produce offspring. This would have a natural advantage over asexual reproduction, because there would be a greater variety in DNA, and the cell population would be more resilient to environmental changes.

The reproductive procedure is first and foremost in natural selection, so mutations which make the process more efficient will be quickly selected for. So, as creatures become more complex, certain clusters of cells would naturally become more and more dedicated to producing reproductive material.

It's not too hard to see that it's at least plausible. All you have to do is think about it a little while.

I am aware of animals that reproduce asexually, and I am also aware of animals that can change genders within their life cycle. I'm thinking that the change was gradual.

Evolution is not about things coming into being spontaneously, so your question about "male and female each spontaneously came into being" has nothing to do with evolution. "If each sex was able to reproduce without the other." Animals that reproduce with only one gender are called asexual which means the absence of a male and female sex. Again, you are asking a bad question.

It seems that you aren't interested in evolution so much as you are trying to make a point. Why don't you prove creation first, then you can start working on disproving evolution? Even if you disprove evolution, which it doesn't look like you are close to doing, you've got a long way to go before you can prove creation. Be positive, not negative.

Initially forms of life that reproduced without any sort of sexual aspect being involved.

Later it seems likely that each organism would have had BOTH male and female parts (with separate sexes coming later - but being kept in life forms that evolved after the separation).

Sexual reproduction allows for a much higher chance of variation, which means that species with sexual reproduction are likely to be better able to compete in changeable environments. Separation of the sexes allows each sex to develop differently which can have many advantages - including reducing competition between the sexes, and allowing specialisations which make caring for offspring easier.

Interestingly, there are some known examples of organisms which, although having a clearly defined sex, can 'switch' and become the other sex under certain circumstances.

Sexual reproduction has been around for a long time. The first differentiation between male and female was probably quite small and the differences became more pronounced over time.

It's utterly ridiculous to think that male and female would have had to evolve separately. That's one of the most bizarre lies creationists have come up with, but apparently their scientifically illiterate audience thinks it's a great argument.

All you're doing by asking this question is showing how little you know. Only those who know nothing about how biology works could possibly think that was a good argument.

That's not so if by complex life you mean multicellular. Most flowering plants are hermaphrodite. Many animal phyla are primarily hermaphrodite. Separate sexes are probably a form of specialisation - one sex spreads the genes for the species, maximising diversity, and the other specialises in guaranteeing circumstances which maximise the chances of the offspring surviving to reproduce.

neither...

2 facts that would change your conception of this world

most life on earth is microscopic

most life on earth doesn't Reproduce by sexual Reproduction

most animals do reproduce by regular sexual Reproduction (male and female) but not all (snails for example are Hermaphrodite, both male and female)

it's basic biology

if you don't know the answer

go back to school...

http://en.wikipedia.org/wiki/Asexual_reproduction

Sexual selection did not emerge in it's present rigid form as seen in us mammals. We're the end product of 2+ billion years of refinement of the system.

Sex likely originated as a modified form of bacterial conjugation - a practice bacterial species continue to carry out to this day. It's not sex, at least as we would understand it as it does not necessarily precede procreation and is not required for fertility, but it allows bacteria to connect with one another to swap genetic material for the purpose of passing on beneficial genetic information.

Sexual reproduction has a number of forms and is hardly a rigid male-female system. Plants tend to be both. Amphibians retain the ability to swap sexuality as required by the pressures of habitat. A number of reptiles opt for parthenogenesis when sexual reproduction is difficult. Imagine if that were us - being able to get ourselves pregnant (male or female) if we have trouble getting a partner! It would be a strange world.

Even in mammals and in humans especially, sexuality is not as deep as was once believed. All that stands between a foetus becoming female or male is a differing level of hormones fed to them in the womb. Change the hormone doses, you change the sex. Female is the default setting which is why men come with useless nipples, and also why men suffering from hormonal imbalances tend to have undersized genitals and breast development.

Sexual selection is not the only method of reproduction but evolutionarily it has been highly successful. The struggle involved in finding a suitable mate has likely seen a great increase in the fitness of offspring in most species. The major drawbacks with non-sexual selection is lack of genetic diversity and species stagnation which slows down the species' ability to respond to drastic changes in their habitats. With sexual selection, changes in habitat can be responded to rapidly (or what passes for rapid in evolutionary terms - say 25,000 years or so) with most carried genes being from the creatures which adapted to the changes and survived long enough to have offspring. In a single sex system everyone has offspring regardless of fitness to a changed environment, so survival rates in these unoptimised offspring will be low. There's a far higher degree of trial and error single sex systems which means a lot of wasted energy and potential over the species as a whole. Evolution loves efficiency, and sexual selection is efficient in this regard, if not perfect. (we only need to look at the vast list of genetic disorders to realise how many imperfections are inherent to sexual selection).

You ask what came first, female or males. As should be clear from what I've written above, the true answer is neither. Species first began to display some aspects of male and female and would take millions of years to develop into the bipolar system now observed. However given that female is the default "setting" to which life is created, then technically I suppose you could say that the female was first. Remember though that evolution works on populations, not on individuals. The line of what counts as female and what counts as male can be even today a tricky question. If I ask you, what sex was the male born toad that created it's offspring by swapping it's own gender - what can we say? Male, female, both, neither?

Hackett out.

Science doesn't have every possible answer, but it does not give up looking for them. You seem like you're happy with a "goddidit" answer. Many of us are not happy with that answer because it gives us no useful information.

This question will be more properly addressed in the BIOLOGY section, it has nothing to do with religion and spirituality......

"evolution is all lies" because if it's true, my sweet dear bible is all wrong, and if the bible is wrong I CAN'T GO ON LIVING because life is meaningless unless I'm judged by a supernatural overlord after I die.

... or is it...?