The most primitive animals were (and are) capable of both sexual and asexual reproduction. Sponges, for example, can reproduce by fragmentation or by budding (both asexual), but they can also produce sperm and eggs.
Animals that act as both males and females are called hermaphrodites, and that is the condition of the first animals. Separate male and females in a species have evolved several times in different lineages and we see it still in the process of evolving in some species today!
For example, there are several species of sea slug where individuals may be hermaphroditic physically, but where a large number of individuals refuse to act as ';females';. That is, they will half-mate with other sea-slugs, only delivering sperm, refusing to accept sperm. See, being a male is easy. Producing sperm is biologically cheap, while producing eggs requires a lot of biochemical resources. By acting as males only, they cheat the system, reproducing multiple times, and never going through the expensive process of making eggs themselves.
In other species, we see the next step, the ';cheating'; males no longer bother with female equipment at all. So, you have a population of hermaphrodites and males. Once the obligate males become common enough, the hermaphrodites are wasting their time and energy making sperm and male equipment, so they become pure females as a cost saving feature.
And then you have separate sexes.
At least, that's how it seems to be working in several species of sea slug. Other systems are possible and we don't know about our own ancestors enough to know how they separated or exactly when. We do know that male frogs frequently have vestigial ovaries, however, and many fish actually change sex in the middle of their lives!
Life is full of wonderful possibilities.What were the first animal forms to evolve from asexual reproduction to true sexual reproduction.?
Not readily. Sadly, male and female parts tend to be soft parts (no jokes, please), and don't fossilize readily. The vast majority of fossils don't tell us whether the animal was male, female, or both.
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Sergeants Major in the US Army were the first animals to be able to do this.
It is the other way round. Sexual reproduction is the oldest form in animals. Asexual animals evolved from sexual animals.
Why did you assume asexual reproduction came before sexual? The reason I ask is due to the fact that Creationist insist that asexual reproduction came first. These folks obviously either don't know their biology are just plain ignore the biology of sex and reproduction.
It is fairly simple. Asexual reproduction is cloning. Sooner or later clone individuals will loose any and all abilities to compete and or survive in a changing ecosystem. Cloning is in direct opposition to the way Nature works. Nature is in chaos. Always changing. The physical factors effecting life on Earth are always changing. For the most part, very slowly. Asexual animals have a limited capability for mutation. If mutation is required for survival. Asexual animals will be out competed by sexual animals.
It is worth noting the rarity of true asexual reproduction in animals. Obviously that method of continuing the species is not very successful.
';Asexual animals arose from diversified sexually reproducing ancestors. Often they are hybrids between two different species, and may possess two complete sets of chromosomes.';
http://www.pbs.org/wgbh/evolution/librar鈥?/a>
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