When Did Male and Female Humans Fully Distinguish in Evolution?

[jackson6612]

I'm finding it hard to describe what I mean, but here it goes. According to the evolution theory monkeys, apes among others, and humans have a common ancestor - their hereditary line of ascent leads to a common forefather(s). As evolution predicts, let's say, male nipples is a proof that human species is a product of evolution. Where did that male and female beings got separated - became fully developed into separate and distinct entities? What I'm trying to say is: there would be times when, perhaps, male being also had nipples which somewhat looked more like female mammary glands which would simply mean male hadn't become a full male yet, on the other side, maybe female vestigial organ looked more of a male sex organ. Also, before they fully developed how did they reproduce? Were they able to reproduce before the sex organs fully developed? What about all those enzymes, chemicals - without the proper chemicals sexual reproduction is impossible, at least that's what I can possibly think?

It's possible I'm thinking along the wrong lines. I'm not a biology student neither am I very knowledgeable about the evolutionary theory. So, please, try to keep your reply as simple as possible so that I and many others like me can understand what you say and appreciate your effort to the fullest. Thank you.

 

[arildno]

Were they able to reproduce before the sex organs fully developed?

Definitely. Otherwise, we wouldn't be here. Thus, whatever sex organs they had were fully functional then, whatever degree of similarity they might have with our own.

As for when sexual distinction arose, I'd hazard for an extremely early age in mammalian history, long before there were any apes, let alone humans, around.

I'll leave to others more qualified than me to answer some of your other questions

 

[AlexB2010]

The sex differentiation of animals go far beyond you imagine on evolutionary line. Apes still have full sexual differentiation from their ancestral. Fishes already have full sexual separation.
If you study molecular evolution you will see that some proteins are very well the same in their folded forms from bacteria to humans.

 

[nismaratwork]

Probably the best place to learn about this is through the study of amphibians which can and do change their gender according to environment. It is not the beginning of the story, but it's a good place to start.

 

[Proton Soup]

aren't sea squirts the earliest chordates? it seems they were initially hermaphroditic.

http://marinebiology.suite101.com/article.cfm/sea_squirts_complex_sex_lives

http://sci-s03.bacs.uq.edu.au/biol/biol-news/december08/crean.pdf

 

[nismaratwork]

aren't sea squirts the earliest chordates? it seems they were initially hermaphroditic.

http://marinebiology.suite101.com/article.cfm/sea_squirts_complex_sex_lives

http://sci-s03.bacs.uq.edu.au/biol/biol-news/december08/crean.pdf

That sounds about right to me. Ugly little things aren't they? :p

 

[Proton Soup]

That sounds about right to me. Ugly little things aren't they? :p

i was always excited to find them on a piece of live rock in my old aquarium

 

[nismaratwork]

i was always excited to find them on a piece of live rock in my old aquarium

You had sea squirts in your aquarium? That's... actually pretty cool. Beats the hell out of brine shrimp in a bag!

 

[Proton Soup]

You had sea squirts in your aquarium? That's... actually pretty cool. Beats the hell out of brine shrimp in a bag!

i had those once, too. Sea Monkeys!

 

[jackson6612]

The sex differentiation of animals go far beyond you imagine on evolutionary line. Apes still have full sexual differentiation from their ancestral. Fishes already have full sexual separation.
If you study molecular evolution you will see that some proteins are very well the same in their folded forms from bacteria to humans.

I'm sorry for this off-topic question. I'm also an English learner, so it'd be nice if you could help with the language so that I can get the point.

I would have written: The sex differentiation of animals go far beyond you can imagine on the evolutionary line.Or, rather better: The sex differentiation of animals happened far long ago on the evolutionary line. Does my way match what you had in my mind?



Here, by apes , you mean monkeys, gorillas, etc? Yes, I have no doubt about it - there are a lot of male and female monkeys around! But I don't think this is what you exactly meant. You used two different words, 'sexual differentiation' and 'sexual separation', to refer to what appears the same feature to me. Did you do that on purpose, or, just for the sake of variety?

Thank you for all the help.

 

[russ_watters]

Not a biologist either, but I wonder if the OP's question isn't looking in the wrong direction: things like nipples and sex organs need not be completely separate sexual "parts". Thinking as someone who'se done a little computer programming, it strikes me that the genetic code could be simplified by making related functions or having certain parts of a program activated in different ways. Hence, you could differentiate male and female before nipples and breasts even evolve at all.

Just speculating though - I'm curious to know from a biologist how that actually works.

 

[russ_watters]

I'm sorry for this off-topic question. I'm also an English learner, so it'd be nice if you could help with the language so that I can get the point.

I would have written: The sex differentiation of animals go far beyond you can imagine on the evolutionary line.Or, rather better: The sex differentiation of animals happened far long ago on the evolutionary line. Does my way match what you had in my mind?

Yours is better.

 

[jackson6612]

Probably the best place to learn about this is through the study of amphibians which can and do change their gender according to environment. It is not the beginning of the story, but it's a good place to start.

Can you give me an example, please?

 

[AlexB2010]

I'm sorry for this off-topic question. I'm also an English learner, so it'd be nice if you could help with the language so that I can get the point.

I would have written: The sex differentiation of animals go far beyond you can imagine on the evolutionary line.Or, rather better: The sex differentiation of animals happened far long ago on the evolutionary line. Does my way match what you had in my mind?



Here, by apes , you mean monkeys, gorillas, etc? Yes, I have no doubt about it - there are a lot of male and female monkeys around! But I don't think this is what you exactly meant. You used two different words, 'sexual differentiation' and 'sexual separation', to refer to what appears the same feature to me. Did you do that on purpose, or, just for the sake of variety?

Thank you for all the help.

Unfortunately, I am Brazilian and English isn’t my native tongue, I am just struggling to be understood and trying to improve my English like you. Maybe a native speaker can correct me too.

 

[AlexB2010]

Not a biologist either, but I wonder if the OP's question isn't looking in the wrong direction: things like nipples and sex organs need not be completely separate sexual "parts". Thinking as someone who'se done a little computer programming, it strikes me that the genetic code could be simplified by making related functions or having certain parts of a program activated in different ways. Hence, you could differentiate male and female before nipples and breasts even evolve at all.

Just speculating though - I'm curious to know from a biologist how that actually works.

Sex has a stronger selection factor than just sexual mechanics. The generation of genetic diversity is the reason sex separated individuals are so successful. Sex mechanics, or how individuals exchange genetic material is a know cause of speciation.

 

[nismaratwork]

Can you give me an example, please?

Of course, I'm sorry I should have done that in my original post. Here is one very odd example, where the frog in question has some funky chromosomes.

http://www.genetics.org/cgi/content/full/164/2/613

Here is a less rigorous, but broader bit to read: http://www.thefreelibrary.com/Jumping+gender:+frogs+change+from+she+to+he.-a08784789

Jumping gender: Frogs change from she to he

Shrimp do it, orchids do it, even some tropical fish do it. Now biologists find that frogs do it, too--switch their sex, that is. A West German research team reports that females of two related frog species can become males without hormonal or surgical intervention. So complete is the transformation -- observed so far only in the laboratory -- that the newly male frogs breed successfully with members of their former sex.

Ulmar Grafe and Eduard Linsenmair detected the gender-bending while studying African reed frogs, Hyperolius viridiflavus ommatostictus, at the University of Wurzburg. The two were analyzing male life histories when a female began fighting with one of the males. "We were really excited, because that shouldn't happen -- females don't fight," says Grafe. In the days that followed, several females adopted the masculine mating stance, extending their forelegs forelegs

see forelimb.

--------------------------------------------------------------------------------

inherited thick forelegs
juvenile hyperostosis (inherited thick forelegs) of pigs. and emitting a low-pitched whistle.

During the next few months, seven adult females -- including six previously observed to lay eggs -- developed functioning testicular testicular /tes·tic·u·lar/ (tes-tik´u-lar) pertaining to a testis.
--------------------------------------------------------------------------------

tes·tic·u·lar
adj.
Of or relating to a testicle or testis.


--------------------------------------------------------------------------------

testicular

pertaining to the testis. nodulesNodules
A small mass of tissue in the form of a protuberance or a knot that is solid and can be detected by touch.

Mentioned in: Leprosy
... Click the link for more information. and aggressive behavior typical of male frogs, the researchers report in the current issue of COPEIA, released in January. Four of the seven "secondary males" copulated with females, fertilizing up to 70 percent of the eggs and generating normal offspring, the investigators say. Grafe and Linsenmair found that two females of a related species also changed sex in the laboratory terrariums.

and from the gov:

http://Newton.dep.anl.gov/askasci/bio99/bio99128.htm

Question: Why can frogs (some species) change sex?
Mike A Smola

Answer:
This is a complex subject. Several studies have exposed some of
the answers to this question, but a definitive answer is yet to be
made.

It all boils down to the level of which genes are activated.
Studies have shown that the sex chromosomes are not necessarily the
determining factor. The traditional display of a female is when
an individual has two X chromosomes present. When one of these
genes is a Y, it is a male. This tends to be the case for most
organisms that sport individuals that are different sexes.
However, the Y chromosomes has been almost always a male determining
chromosome, but with molecular biology and genetic engineering
techniques, a male can be made from a XX combination and females
have developed from XY combinations. These are extremely rare, but
they have given insight to the fact that there are other factors
beside X and Y chromosomes that determine sex, probably a gene
found in both chromosomes.

As far as frogs are concerned (and other organisms that display this
Phenomenon), apparently there are chemical triggers that respond to the
number of members in a population that will activate the gene(s)
that will allow for the disintegration of one set of sex organs and
the development of the other. This is an advantage to a species
whereby they have evolved the ability to assure their reproductive
success.
Steven D Sample

Some fish do this to an astonishing degree, where there are all females, save for one male. If the male dies, another female undergoes major morphological changes and rapidly grows, and becomes male!

 

[pgardn]

It is also possible for different sexes to arise independently in different types of organisms. There are clearly male and female plants in some species. There are lizards that are essentially all female... So having differentiation of sexes can be thought of just like any other trait that might help, or sometimes not, in the continuing evolutionary success of a certain population of organisms. There are some protist species with about 13 different sexes I believe.

 

[nismaratwork]

It is also possible for different sexes to arise independently in different types of organisms. There are clearly male and female plants in some species. There are lizards that are essentially all female... So having differentiation of sexes can be thought of just like any other trait that might help, or sometimes not, in the continuing evolutionary success of a certain population of organisms. There are some protist species with about 13 different sexes I believe.

13 sexes, that must be one fun orgy, but the cleanup! Yuck. :)

 

[pgardn]

13 sexes, that must be one fun orgy, but the cleanup! Yuck. :)

Biology is a very messy science. Thats why most people who like a tidy world choose Physics:)

 

[cesiumfrog]

There are some protist species with about 13 different sexes I believe.

You mean, several (discrete) types of individual, which can pair (binary coupling) and exchange genetic material readily (or at least far more readily than with individuals not classed as types of this species)? Not that several individuals, one from every type, all combine (each contributing less than half their individual genome) in the process of creating a complete offspring?

 

[pgardn]

You mean, several (discrete) types of individual, which can pair (binary coupling) and exchange genetic material readily (or at least far more readily than with individuals not classed as types of this species)? Not that several individuals, one from every type, all combine (each contributing less than half their individual genome) in the process of creating a complete offspring?

I have not been at this for a while. But the rules for exchanging genetic material was quite complex and not at all worked out. At the time I looked at it they had assigned the protists mating types. The rules of which type exchanges with which type and how exchange was taking place was still being explored. They may have figured out more about this. Molecular genetics and such is absolutley exploding. Its mind boggling compared to what I learned. I am sure its somewhere out there in all its glory now explained in more detail. I just don't want to make my head hurt trying to understand it. So if you want to explain further about mating types v. true sexes (however that is defined now) be my guest.

The major point I was trying to bring is that sexual reproduction has advantages and disadvantages from an evolutionary point of view. So sexual reproduction, just like any other mechanism that is under selective pressure, has arisen independently more than once. And in some species sexual reproduction has "dissappeared". We tend to look at the world from our point of view, boy meets girl, and its just so much richer than this. I remember reading a Steven J. Gould article about the life of the male in a species of deep sea Angler Fish and feeling completely humbled, being a male myself. My wife has continued the humbling process...

 

[nismaratwork]

Biology is a very messy science. Thats why most people who like a tidy world choose Physics:)

That is a very keen observation, and one I am not prepared to refute. I myself, enjoy a wide variety of worlds to play in.

 

[SHAMSAEL]

I'm sorry for this off-topic question. I'm also an English learner, so it'd be nice if you could help with the language so that I can get the point.

I would have written: The sex differentiation of animals go far beyond you can imagine on the evolutionary line.Or, rather better: The sex differentiation of animals happened far long ago on the evolutionary line. Does my way match what you had in my mind?.

The sex differentiation of animals goes farther down the evolutionary line than you might have imagined.

On a related note, there was an article here on physorg a few months back about a breed of flower in north america, i think, that is in the process of evolving into a two-sex reproductive pattern, that is the mutation occurred at some point in the past but across the entire population, many of these flowers are still asexual. It was, supposedly, proof that not all animals inheretid sex from the same ancestor but that it could've arisen at different times among different species.

I would still agree, though, that our line acquired sex long before we crawled out of the sea.

 

[nismaratwork]

The sex differentiation of animals goes farther down the evolutionary line than you might have imagined.

On a related note, there was an article here on physorg a few months back about a breed of flower in north america, i think, that is in the process of evolving into a two-sex reproductive pattern, that is the mutation occurred at some point in the past but across the entire population, many of these flowers are still asexual. It was, supposedly, proof that not all animals inheretid sex from the same ancestor but that it could've arisen at different times among different species.

I would still agree, though, that our line acquired sex long before we crawled out of the sea.

Given how useful sexual reproduction seems to be, I find the possibility of parallel evolution of that trait believable. Not proven, but not absurd either. Asexual reproduction may well leave a species open to extinction, but a little mutation that nudges the species from asexual to some kind genetic swap-meet could save it. It makes sense that if this is the case, we find ourselves with the survivors, and asexual organisms will have to evolve or die as changes occur in their environment, and now at a high rate with the aid of humans.

 

[jackson6612]

Thank you very much, everybody. I will need some time to assimilate the information, and make some sense out of it because I'm not a biology student - just a stupid lazy person! Shamsael, thanks for better version of the sentence.

 

[Mkorr]

This is probably a horribly unfair accusation, but the whole issues sounds like the young Earth creationists Ray Comfort and Kirk Cameron's canard that the first human male did not have a female to mate with, so therefore, evolution is false. This absurdity can be read on his "raycomfortfood" blogspot blog in the articles "the atheist and first dog" and "Evolution's explanation for male and female". I won't link to them, because the blog posts are, in my opinion, horribly cranky.

 

[nismaratwork]

This is probably a horribly unfair accusation, but the whole issues sounds like the young Earth creationists Ray Comfort and Kirk Cameron's canard that the first human male did not have a female to mate with, so therefore, evolution is false. This absurdity can be read on his "raycomfortfood" blogspot blog in the articles "the atheist and first dog" and "Evolution's explanation for male and female". I won't link to them, because the blog posts are, in my opinion, horribly cranky.

Huh? Who are you accusing, and where do you get the notion that any of this is anything but support for evolutionary theory? I'm completely baffled here.

 

[jackson6612]

Is Mkorr accusing me of being a young creationist?

 

[mheslep]

aren't sea squirts the earliest chordates? it seems they were initially hermaphroditic.

http://marinebiology.suite101.com/article.cfm/sea_squirts_complex_sex_lives

http://sci-s03.bacs.uq.edu.au/biol/biol-news/december08/crean.pdf

I would have said plants invented preproduction through sexual separation? What ever the original source, I believe the evolutionary motivation is that spreading genes around between organisms of the same species provides an evolutionary advantage over asexual reproduction.

 

[pgardn]

I would have said plants invented preproduction through sexual separation? What ever the original source, I believe the evolutionary motivation is that spreading genes around between organisms of the same species provides an evolutionary advantage over asexual reproduction.

Assuming that sexual reproduction produces more genetic variation when necessary (whatever conditions might warrant the need). But asexual reproduction could also claim to be much a much more efficient way of reproducing. There appear to be advantages and disadvantages for both. Both mechanisms are still around. So it is interesting to study populations of organisms that go back and forth between both I should think.

 

[jackson6612]

Assuming that sexual reproduction produces more genetic variation when necessary (whatever conditions might warrant the need). But asexual reproduction could also claim to be much a much more efficient way of reproducing. There appear to be advantages and disadvantages for both. Both mechanisms are still around. So it is interesting to study populations of organisms that go back and forth between both I should think.

Hi PG

Would you please tell me which animals/organisms reproduce asexually? I have always thought that animals reproduce sexually - copulation. Plants reproduce asexually because they have no other way. Pardon me for my ignorance, if there is something utterly wrong with what I have said. Please keep your reply simple, I'm not a biology or science student.

Thanks
Jack

 

[Pythagorean]

I'm glad someone mentioned plants. Perhaps sexual differentation was something coded by a common ancestor between us and plants?

(not a biologist)

 

[arildno]

I have to reiterate that I personally think that sex differentiation (leading to our own) must be somewhere in the early history of mammals, rather than even further back (say, to plants).

Why?

Answer:
Nipples.

That already sex differentiated males should develop nipples that serve no function, is meaningless.

Thus, we are left with two alternatives:

A) Sex differentiation in mammals correlated with a reduction of capacity for lactation in the emergent males, and, probably, an enhanced capacity for lactation in the emergent females.

B) Nipples arose from an entirely different reason than lactation, and sex differentiation had already taken place. The already differentiated sexes then developed their nipples into different functions, crossing the line from non-mammalians to mammalians in the process.


As for now, I haven't heard any good argument for advocating B)-type histories.



It follows from my tentative adherence to A)-stories that I think sex differentiation is a fairly easy trait to evolve, and that has done so a number of times independently.
Perhaps I an totally wrong on this.

 

[pgardn]

Hi PG

Would you please tell me which animals/organisms reproduce asexually? I have always thought that animals reproduce sexually - copulation. Plants reproduce asexually because they have no other way. Pardon me for my ignorance, if there is something utterly wrong with what I have said. Please keep your reply simple, I'm not a biology or science student.

Thanks
Jack

Animals cover a wide range of organisms that you might not consider animals. For example there are numerous sponges, jellyfish, worms that can reproduce asexually. Copulation actually means male "parts" enter female "parts". There are many animals in which this does not happen. To copulate you have to "find" a partner. Another more random way would be just laying eggs and having males "spray" sperm rather randomly. Many fish do this... as there is a medium to help cell meet cell (fertilization) and that medium is water. Land animals are more likely to copulate as the medium for transport of sex cells might just be air (I think you can see problems with male animals just spraying sperm randomly into the air; the Catholic Church does anyway...). Plants of course have no problem with air as a transport medium. But to make it more efficient one group of plants, the flowering plants, often enlist the help of insects. So we have the birds (copulate) and the bees statement. Bees really are the transport medium for many plants to have sexual reproduction.
Hope this kinda makes sense.

 

[pgardn]

I'm glad someone mentioned plants. Perhaps sexual differentation was something coded by a common ancestor between us and plants?

(not a biologist)

That is possible but based on the wide variety of mechanisms of both sexual and asexual reproduction, it has been greatly modified over time, or arisen independently which basically says your statement above is unlikely. Last I read, the above is probably not correct, the processess have arisen independently. But your assertion is a valid point and thought.

Oops... arildno's post above pretty much covers my blanket statement through a specific example.

 

[Pythagorean]

I have to reiterate that I personally think that sex differentiation (leading to our own) must be somewhere in the early history of mammals, rather than even further back (say, to plants).

Why?

Answer:
Nipples.

That already sex differentiated males should develop nipples that serve no function, is meaningless.

Thus, we are left with two alternatives:

A) Sex differentiation in mammals correlated with a reduction of capacity for lactation in the emergent males, and, probably, an enhanced capacity for lactation in the emergent females.

B) Nipples arose from an entirely different reason than lactation, and sex differentiation had already taken place. The already differentiated sexes then developed their nipples into different functions, crossing the line from non-mammalians to mammalians in the process.


As for now, I haven't heard any good argument for advocating B)-type histories.



It follows from my tentative adherence to A)-stories that I think sex differentiation is a fairly easy trait to evolve, and that has done so a number of times independently.
Perhaps I an totally wrong on this.

That sounds reasonable. I wonder then, if it's an element of genetic code that we share with plants (or a property of genetic code in general) that allows this kind of adaptation.

Have there ever been three-sex species? What's the advantage of two?

 

[arildno]

That sounds reasonable. I wonder then, if it's an element of genetic code that we share with plants (or a property of genetic code in general) that allows this kind of adaptation.

I don't know. I think that this requires a knowledge of how sex-differentiation is coupled to chromosomal structures in plants, and if some similarities exist there with our X&Y way of distinguishing between the sexes.

Have there ever been three-sex species?

I don't know, but vaguely recall to have read somewhere that multiple sexes do, indeed, occur.

What's the advantage of two?

Perhaps the more correct question would be:
"Why more than two sexes?"

Already, a two-sex population greatly enhance its capacity to compat parasites/viruses by swift DNA-recombinations (one of the advantages with a sexed population)

Developing more than two might:
1. Increase whatever costs there are with sexed underpopulations (perhaps instability of genomes, heightened vulnerability to mutations? Here, I have no idea at all!)

2. Only providing diminishing returns in terms of multiple-sex advantages.
Somewhere along the line, developing new sexes just isn't worth it anymore..

 

[jackson6612]

Animals cover a wide range of organisms that you might not consider animals. For example there are numerous sponges, jellyfish, worms that can reproduce asexually. Copulation actually means male "parts" enter female "parts". There are many animals in which this does not happen. To copulate you have to "find" a partner. Another more random way would be just laying eggs and having males "spray" sperm rather randomly. Many fish do this... as there is a medium to help cell meet cell (fertilization) and that medium is water. Land animals are more likely to copulate as the medium for transport of sex cells might just be air (I think you can see problems with male animals just spraying sperm randomly into the air; the Catholic Church does anyway...). Plants of course have no problem with air as a transport medium. But to make it more efficient one group of plants, the flowering plants, often enlist the help of insects. So we have the birds (copulate) and the bees statement. Bees really are the transport medium for many plants to have sexual reproduction.
Hope this kinda makes sense.

PG, thanks a lot for all the explanation.Please keep in mind I'm not a biology or science student.

Of course, I will never consider them animals, that's against my common sense!

When a male animal copulate with a female, there is reason for this. He is under the pressure of hormones. The release of sperms is, say, a by product of an attempt to overcome that pressure. Spraying eggs with sperms doesn't make sense. Even if he is trying to release pressure, he can do it anywhere else not just on eggs. I hope you get the point.

Best wishes
Jack

 

[pgardn]

PG, thanks a lot for all the explanation.Please keep in mind I'm not a biology or science student.

Of course, I will never consider them animals, that's against my common sense!

When a male animal copulate with a female, there is reason for this. He is under the pressure of hormones. The release of sperms is, say, a by product of an attempt to overcome that pressure. Spraying eggs with sperms doesn't make sense. Even if he is trying to release pressure, he can do it anywhere else not just on eggs. I hope you get the point.

Best wishes
Jack

So there is probably a "pressure" to release milt (sperm) when eggs are about. You see that it is important not to just spray milt anywhere in a random manner. There is a chemical in the water that tells some male fish to release milt and that chemical is released by females when laying eggs.

 

[nismaratwork]

So there is probably a "pressure" to release milt (sperm) when eggs are about. You see that it is important not to just spray milt anywhere in a random manner. There is a chemical in the water that tells some male fish to release milt and that chemical is released by females when laying eggs.

...And as chemical signaling predates sexes... it would make sense that they could work together. Chemical signals that "we are part of the same group" or "holy **** a predator, swim!" could mutate.

 

[pgardn]

...And as chemical signaling predates sexes... it would make sense that they could work together. Chemical signals that "we are part of the same group" or "holy **** a predator, swim!" could mutate.

This is exactly why molecular evolution is so cool. And why evolution on the whole is so interesting. The surprises that pop up are amazing. You look at a particular feature of an organism that is visibly impressive and surmise it must be for so and so and then you do some background and find out its nothing like you assumed.

The old Steven J. Gould articles in Natural History were great for illustrating so many interesting features of evolution.

 

[nismaratwork]

This is exactly why molecular evolution is so cool. And why evolution on the whole is so interesting. The surprises that pop up are amazing. You look at a particular feature of an organism that is visibly impressive and surmise it must be for so and so and then you do some background and find out its nothing like you assumed.

The old Steven J. Gould articles in Natural History were great for illustrating so many interesting features of evolution.

You'll hear no arguments from me, I find it fascinating and very compelling.

 

[Jon Richfield]

For me, having come into this discussion very late indeed, it is frustrating and confusing to skim over all the points that have been raised.
I accept that "sex", in particular in the context of "gender", referring largely to maleness, femaleness and the like, is such a pervasive concept in recent times (say, the last 600 million years or so) that it is easy to get it out of perspective.
To me, not counting my personal life as a mammal (of masculine gender and leanings) the biological concept of sex amounts to an elaboration on the basic concept of genetic interchange and recombination.
Now that concept, not that the fossil record offers as much detail, mostly some admittedly very impressive work on stromatolites in (by terrestrial standards) really old rocks (some 3 billion years plus) goes back, not merely to before the advent of mammals or for that matter fish, or even chordates, but perhaps even before the existence of well-organised cells as we know them today.
It seems likely that the first development along these lines was the ability to reproduce in some manner resembling what we now call mitosis. In other words the genetic material would have been duplicated, after which the cell would split. Primitive or defective early mechanisms could permit a partial reversal of the procedure, in which cells adhering to each other could unite and exchange genetic material.
Now, the only fundamental aspect of sex remaining to develop after that point, is an asymmetry between the two participants in the process. Everything else is icing on the cake. Sometimes very pleasurable icing of course, and often downright baroque, but fundamentally superficial all the same. To see how superficial, simply consult textbooks on elementary biology, particularly microbiology, for examples of say, the reproduction of fungi, algae and similar organisms in which some classes of gametes accumulate material supplies such as food stores, and either stay home with Mama, or drift off comparatively passively whereas others specialise in transport and take only the bare minimum of supplies, plus propulsive mechanisms such as cilia.
Careful consideration of these aspects can be very useful in developing insights into the principles.
As organisms became more complex, particularly in their bodily organisation into metazoa or metaphyta, the dizzying radiation into thousands and thousands of reproductive strategies occurred and re-occurred time and time again down the ages. They included such things as gender specialisation in many forms. For example, would you believe, all the girls I know have bigger nipples and breasts than almost all the boys I know.:!) :!)
I do not disparage the study of the function and evolution of gender and sex in macro organisms, but to do so other than in the context of the early history of sexual reproduction simply invites the kind of confusion implicit in many of the exchanges in this thread so far.
Cheers,
Jon

 

[Pythagorean]

Ok, wait a minute, I've been thinking about this while I was away from technology:

That already sex differentiated males should develop nipples that serve no function, is meaningless.

This is actually somewhat incorrect. Men can lactate and feed children, they just haven't for a long time (and possibly may have never, but it could easily have been a socially influenced change. Perhaps lactation has nothing to do with sex, biologically).

Anyway, if you really wanted to lactate to feed a child, you could through regular nipple stimulation.

But if we went ahead with your stipulation, there's also a C)
Development in the womb is somehow more efficient if there's no discrimination about nipple placement.

or D)

it's an artifact of development, tied to the fact that we were all females in the womb at one time (even though our gender has already been determined).

 

[arildno]

Men can lactate and feed children, they just haven't for a long time (and possibly may have never, but it could easily have been a socially influenced change. Perhaps lactation has nothing to do with sex, biologically).

Not quite sure if I get your point:

That lactating capacity could precede sex differentiation is basically what I said.
One might then think of a long time in which there was adaptive pressure to differentiate into sexes, for example one remaining at a sheltered place with the young, the other developing skills for long range hunting, losing gradually the capacity to co-feed the child through lactation.

 

[pgardn]

For me, having come into this discussion very late indeed, it is frustrating and confusing to skim over all the points that have been raised.
I accept that "sex", in particular in the context of "gender", referring largely to maleness, femaleness and the like, is such a pervasive concept in recent times (say, the last 600 million years or so) that it is easy to get it out of perspective.
To me, not counting my personal life as a mammal (of masculine gender and leanings) the biological concept of sex amounts to an elaboration on the basic concept of genetic interchange and recombination.
Now that concept, not that the fossil record offers as much detail, mostly some admittedly very impressive work on stromatolites in (by terrestrial standards) really old rocks (some 3 billion years plus) goes back, not merely to before the advent of mammals or for that matter fish, or even chordates, but perhaps even before the existence of well-organised cells as we know them today.
It seems likely that the first development along these lines was the ability to reproduce in some manner resembling what we now call mitosis. In other words the genetic material would have been duplicated, after which the cell would split. Primitive or defective early mechanisms could permit a partial reversal of the procedure, in which cells adhering to each other could unite and exchange genetic material.
Now, the only fundamental aspect of sex remaining to develop after that point, is an asymmetry between the two participants in the process. Everything else is icing on the cake. Sometimes very pleasurable icing of course, and often downright baroque, but fundamentally superficial all the same. To see how superficial, simply consult textbooks on elementary biology, particularly microbiology, for examples of say, the reproduction of fungi, algae and similar organisms in which some classes of gametes accumulate material supplies such as food stores, and either stay home with Mama, or drift off comparatively passively whereas others specialise in transport and take only the bare minimum of supplies, plus propulsive mechanisms such as cilia.
Careful consideration of these aspects can be very useful in developing insights into the principles.
As organisms became more complex, particularly in their bodily organisation into metazoa or metaphyta, the dizzying radiation into thousands and thousands of reproductive strategies occurred and re-occurred time and time again down the ages. They included such things as gender specialisation in many forms. For example, would you believe, all the girls I know have bigger nipples and breasts than almost all the boys I know.:!) :!)
I do not disparage the study of the function and evolution of gender and sex in macro organisms, but to do so other than in the context of the early history of sexual reproduction simply invites the kind of confusion implicit in many of the exchanges in this thread so far.
Cheers,
Jon

We should expect that those who have considered sex and sexes to start their questioning with humans I suppose. You are correct of course in your major theme, but getting this across to others that don't start with a wide ranging understanding of genetic diversity and evolution sometimes requires we start from the wrong position in order to have some context. It may not be the way to explain this subject, but sometimes its better than completely leaving them down the road. Its not confusing to me to find that people are confused when we are such an egocentric species that happens to wonder.

 

[Pythagorean]

Not quite sure if I get your point:

That lactating capacity could precede sex differentiation is basically what I said.
One might then think of a long time in which there was adaptive pressure to differentiate into sexes, for example one remaining at a sheltered place with the young, the other developing skills for long range hunting, losing gradually the capacity to co-feed the child through lactation.

Well, I guess I thought your argument was based on:

That already sex differentiated males should develop nipples that serve no function, is meaningless.

And my response is that males didn't develop nipples that serve no function. Our nipples (biologically) serve the same function that they do for women. If lactation is independent of sexuality (biologically anyway, obviously not socially) than what bearing would that have on sexual differentiation?

In other words, isn't it completely possible that sexual differentiation happened first (say in reptiles or therapsids) and nipples came later (with mammals) and was applied indiscriminately to both sexes; and that eventually, through socially driven epigenetics, male and female nipples took on different shapes? Or maybe even because the tribes who's males were out hunting instead of lactating survived, for instance?

I'll reiterate that I'm not a biologist. I'm just trying to understand what would be improbable about the way I'm envisioning it.

 

[Proton Soup]

Well, I guess I thought your argument was based on:



And my response is that males didn't develop nipples that serve no function. Our nipples (biologically) serve the same function that they do for women. If lactation is independent of sexuality (biologically anyway, obviously not socially) than what bearing would that have on sexual differentiation?

In other words, isn't it completely possible that sexual differentiation happened first (say in reptiles or therapsids) and nipples came later (with mammals) and was applied indiscriminately to both sexes; and that eventually, through socially driven epigenetics, male and female nipples took on different shapes? Or maybe even because the tribes who's males were out hunting instead of lactating survived, for instance?

I'll reiterate that I'm not a biologist. I'm just trying to understand what would be improbable about the way I'm envisioning it.

yes, nipples are peculiar to mammals, AFAIK. also, i think it's important to remember that we males also have both X and Y copies of genetic code. we are essentially both male and female. and our maleness is simply an expression of that Y-ness (perhaps some suppression of X-ness?). indeed, males can, and do lactate, given enough of the proper hormones like prolactin. and with diseases like androgen insensitivity syndrome, we males can look pretty indistinguishable from females.

 

[nismaratwork]

yes, nipples are peculiar to mammals, AFAIK. also, i think it's important to remember that we males also have both X and Y copies of genetic code. we are essentially both male and female. and our maleness is simply an expression of that Y-ness (perhaps some suppression of X-ness?). indeed, males can, and do lactate, given enough of the proper hormones like prolactin. and with diseases like androgen insensitivity syndrome, we males can look pretty indistinguishable from females.

...especially if the pool is really really cold.

I also believe that nipples (used to feed young) are unique to mammals. There are organs which perform a similar task in other animals, but not nipples and mammary tissue.

 

[Jon Richfield]

We should expect that those who have considered sex and sexes to start their questioning with humans I suppose. You are correct of course in your major theme, but getting this across to others that don't start with a wide ranging understanding of genetic diversity and evolution sometimes requires we start from the wrong position in order to have some context. It may not be the way to explain this subject, but sometimes its better than completely leaving them down the road. Its not confusing to me to find that people are confused when we are such an egocentric species that happens to wonder.

You are right of course, that we have a perennial dilemma in conveying technical subjects (not necessarily biological). Ideally we should begin with the fundamental concepts and build from there, bottom up. In practice, that often does not work when dealing with people who are not going to deal with the subject at some depth. Then top-down is commonly the best, in fact the only approach. Actually, even for specialist students who are going to take the subject to advanced levels, one usually needs a mix of bottom-up and top down. To do it one way only takes, not just a brilliant teacher, but an inspired one with brilliant and motivated students!
Sorry, slight digression that was! What I had been aiming for was to convey a basic concept in the light of which the original question would appear in a slightly simpler perspective. Unfortunately, my experience is that if you simplify a matter in a way that your audience does not understand, they will think that you had complicated it...
Well, I feel another wave of simplification coming over me; wish me luck!

Jon