luvluv

Can anyone link me to any information about how scientists believe sexual reproduction came about? Is there a first sexual being that can be pointed to? Do we know which animal that was? How, if mutations involve individuals, could a "mutant male" be certain to have a "mutant female" around at the right time? Did this first happen on the cellular level or the multicellualr level?

I'm sure I'm sounding real dumb, but you get the point. Teach me.

Dr.GH

Acually, there are lots of living examples of hermaphroditic animals; molusks, and annelids for example, that probably pre-stage animal hetrosexual organisms.

The Lone Ranger

Matt Ridley's The Red Queen: Sex and the Evolution of Human Nature contains a good overview of current theories regarding sexual reproduction -- how it may have evolved in the first place, and why it tends to persist.

Cheers,

Michael

Doubting Didymus

The first sexual organism would ceratinly have been unicellular, so the problem of mutant 'males' finding mutant females is not a problem. My guess is that the sexual process started with a mutation that caused a single celled lifeform to undergo meiosis instead of normal mitosis (i.e. dividing into four instead of two). The four cells that would have been produced would not have been able to reproduce themselves, but two of them could certainly have had the capacity to fuse together into one single cell, which could then do it again.

Probably, the cells that were produced by this early form os sexual recombination would not neccesarily always divide into four, but each would carry the genes that caused the mutation in the first place, so as the strain became more common, you would eventually have a population where individuals occasionally divise into four instead of two, and then each of the four combine with one of the others, producing two cells. If enough members of the population do this at the same time, then instead of fusing with one of its 'brothers' to create a diploid cell, they might fuse with the haploid cell from a different parent.

This life cycle, where normal cells produce clones of themselves most of the time, but occasionally divide into four little cells that go off and fuse with others, is actually a common life cycle found in many organisms, from fungi to algae to certain unicellular organisms. It is not too difficult to imagine meiosis (again, that's where a cell divides into four 'half' copies of itself) being a mutant mistake that turned out useful for mixing the gene pool.

lpetrich

I agree that diploid->haploid is straightforward: instead of normal

duplication / division

this happens:

duplication / division / division

-- the division control slips up a bit.

But the next question is how the resulting haploid cells form diploid ones. But there is an intermediate there also. Conjugation.

Some protists, such as paramecia, meet and exchange genetic material. It's not too difficult to imagine this becoming a cell-fusion phase.

And bacteria have a simpler version of conjugation; one bacterium inserts some genetic material into another bacterium.

So one can reconstruct the sequence:

Simple bacteria

Bacteria with pore structures for more efficient transport of materials across the cell membrane/wall

Pore structures get used for spreading genes

Eukaryotic cells develop larger-scale gene exchange: conjugation

Conjugation gets turned into cell fusion

Alternation between haploid and diploid phases emerges, with cycle

diploid - meiosis - haploid - fusion - diploid

Protists and fungi sometimes have more than one "mating type", essentially outwardly-identical sexes. And sometimes several mating types -- several sexes! Their only difference is most likely certain membrane proteins; a cell will refuse to conjugate with or fuse with a cell that has a membrane protein that is too much alike. This feature helps prevent inbreeding; sexually-reproducing organisms often have anti-inbreeding adaptations.

The next question is what would produce outwardly-different sexes.

These happen only in multicellular organisms, AFAIK. They produce gametes that meet each other and fuse, starting off new organisms. The earliest ones may have had lookalike gametes with more than one mating type, however here is a way non-lookalike gametes can emerge:

To give a multicelled organism a good start, a gamete ought to be bulky, with lots of food for its descendant cells.

But being bulky means being slow, and if one mating type chooses to scrimp on food, its gametes can swim fast. This swimming means that the food-filled gametes can pack more food -- and get away with being essentially stationary.

Thus, one type of gamete becomes big, food-filled, and stationary: eggs, and one type become small, foodless, and fast: sperm.

Nic Tamzek

I wrote some reasonably intelligible comments (I thought) on this back in February on an II thread here involving DNAunion, the origin of eukaryotes, and, I think, meiosis/sex.

Except I can't find the URL...anyone know the thread I'm talking about?

Gluttons for detail should type "Cavalier-Smith" into the Pubmed search engine, he's written several long articles on just this topic over the past year.

nic

PS: Another possibility is that the "original" "eukaryotes" were haploid, and diploidy was simply reproduction that was halted e.g. under starvation conditions. From there it's not too far for haploid cells to merge under nutrient-starved conditions (as happens today in many unicellular creatures), and in addition to surviving starvation longer the cells might gain an adaptation through recombination.

theyeti

<a href="http://iidb.org/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=58&t=000459&p=" target="_blank">Here</a>.

Nu

Do non-humans find sex to be an insanely pleasurable orgasmic experience?

theyeti

I do!

tronvillain

I would definitely leave the bacteria out of Ipetrich's sequence - the mechanisms do not appear to be related.

Nu:
Well, primates certainly appear to, and I suspect that it at least extends to all other mammals. Why do you ask?