Human evolution: when male and female fully developed into separate and distinct ?

[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.:uhh:
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.:uhh:
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

 

[pgardn]

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.

And females and males basically in early developmental stages are much the same which is of course explained by the above. If you "guys" really want to experience something interesting you should read about male reproductive parts and female reproductive parts and see how much they have in common even though the outcome in development from a purely physical view may make them seem drastically different.

 

[Jon Richfield]

... males didn't develop nipples that serve no function. Our nipples (biologically) serve the same function that they do for women.

Ahem... it isn't as simple as that. It is perfectly possible in principle for adaptation to produce either functioning, but non-functional (completely or nearly completely), or totally non-functioning organs in different castes or genders. Human nipples are a case in point. Some of us have been talking about human male lactation for example, but it is not always possible, even if the necessary hormones are supplied artificially. I don't have figures, but many or most males don't have the necessary physical ducts in the nipples by the time of puberty.
Also, I don't reckon that it counts as biological adaptation if it depends on "artificial" manipulation that does not occur "in a state of nature" as a result of adaptive selection. (I refuse to go into the question of where "natural" selection ends and "artificial" manipulation begins!)
You see, what matters is the inclusive burden of selection. As someone mentioned, the nipples are the product of an embryogenic process that begins similarly in both genders. To avoid that happening would require a huge selective burden, whereas a few abortive nipples are usually too trivial to reckon in. Notice that in some ways the adaptation in females is far more specialised than in males, because they must get it right. In us males, our non-functioning vestiges require no more adaptation than keeping out of the way. Now, our primitive nipple count is more like six than two, but it is unusual to see signs of more than two in women, where we breastless men commonly have supernumerary nipples, only the extra ones are so small that we usually don't notice them. I had been adult for years when I happened to notice that an old friend had six, of which the lower four were basically pigmented dots. I then inspected myself and found that I had two extra, about at the level of my lower ribs. I had always thought that they were freckles! Since ours don't generally develop further, it is not important to control their appearance, but for women it is important, so the controls are not often so lax. But not that those extra "nipples" really, really are not as a rule functional at all. They are essentially areolar freckles.

In short, I agree with Pgardn about the unity of male and female ontogeny.

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?

Certainly, more or less, but we are discussing way back. Tribes don't come into it. Lactation is part of a generic process of physiological secretion for feeding offspring, and in mammals at least, it started before our ancestors looked like tree shrews, let alone monkeys. We don't know just when the first true mammalian lactation began, but it might well have started with the development of hair and sweat glands, probably before the upper Jurassic if you ask me.
It was in many ways not a very striking development really. Some birds, some fish, and some insects, to grab a few examples just off the cuff, produce secretions for the feeding of their young. The said secretions are pretty well universally derived from other secretions that some ancestral baby began to lick up or swallow, or even bite off, along with the parental mucus when the parent fed it. To this day monotremes don't have proper nipples, just multiple little ducts opening into external hair tufts for the young to lick.
Fish such as tilapia and discus feed their fry on mucus, either inside the mouth, or a specially thick secretion around the lips and gill coverts. Pigeons' "milk" is from mucus glands in the throat. Get the picture? It is a coherent one, I think you will agree.

I'm just trying to understand what would be improbable about the way I'm envisioning it.

So? I, for one, am no physicist. If I'm not shy about asking for help with physics, or even arguing about it, I don't see why you should be shy about discussing biology. Your general line of thought was quite reasonable, and apart from a bit of cross-purposes, I don't think anyone thought anything else. Or begrudged you your thinking. It all is science anyway, no matter what Rutherford thought in his ignorance.

Go well, and enjoy.

Jon

 

[pgardn]

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

Which is why evolution on the whole can actually be very difficult to understand in its overwhelming richness. It requires a lot of background reading and critical thinking. One has to both understand the unity among all living things and throw in a dash of the complexity of the amazing array of diversity and how it might have come about from a historical biological perspective. I think the subject takes a long time to appreciate fully. A lot of reading...

I found that Steven J. Gould first, and then Ernst Mayr after, helped me quite a bit. A smidgen of Richard Dawkins for entertainment along the way. Lots of others but I found these fellows to be quite good with Mayr being very technical. The Growth of Biological Thought was very good but needs updating will all the "recent" advances in molecular biology.

 

[Jon Richfield]

Which is why evolution on the whole can actually be very difficult to understand in its overwhelming richness. It requires a lot of background reading and critical thinking. One has to both understand the unity among all living things and throw in a dash of the complexity of the amazing array of diversity and how it might have come about from a historical biological perspective. I think the subject takes a long time to appreciate fully. A lot of reading...

I found that Steven J. Gould first, and then Ernst Mayr after, helped me quite a bit. A smidgen of Richard Dawkins for entertainment along the way. Lots of others but I found these fellows to be quite good with Mayr being very technical. The Growth of Biological Thought was very good but needs updating will all the "recent" advances in molecular biology.

Quite right. I am fond of pointing out that there are certain subjects that are intellectual tarpits. They are treacherous because the basic concepts are simple, so people think that they understand the subject because they understand those ideas. But the way those concepts combine can be very complex indeed. Two examples are probability theory and evolution. Dead simple basic concepts, but heaven help anyone who gets too arrogant!
One mark of someone mature in such a field is that he is no longer embarrassed when he makes a mistake, but is very careful about committing to any hard and fast opinions; that kind of sucker bait is for the tyros.


Jon

 

[jackson6612]

Suppose a biological life started in a furnace. When a first living organism developed, it went on to change into more advanced forms of life and ultimately branching out into several different varieties of life - animals , birds, etc. Is that furnace still there? I assume that furnace had to be earth. I mean to say is there some kind of organism still on its way to become a human in future? Dinosaurs were also the result of evolution. If that furnace is still working, then there could be dinosaurs again in the future, perhaps. I hope you would be able to understand what I'm trying to say. Thanks for reading this.

 

[jackson6612]

I think that that furnace has ceased working because conditions on Earth have changed and are not favorable anymore for its working.

 

[AlexB2010]

I think that that furnace has ceased working because conditions on Earth have changed and are not favorable anymore for its working.

The "furnace" is still working, the selection environment changed.

Alex

 

[Pythagorean]

Ahem... it isn't as simple as that. It is perfectly possible in principle for adaptation to produce either functioning, but non-functional (completely or nearly completely), or totally non-functioning organs in different castes or genders. Human nipples are a case in point. Some of us have been talking about human male lactation for example, but it is not always possible, even if the necessary hormones are supplied artificially. I don't have figures, but many or most males don't have the necessary physical ducts in the nipples by the time of puberty.
Also, I don't reckon that it counts as biological adaptation if it depends on "artificial" manipulation that does not occur "in a state of nature" as a result of adaptive selection. (I refuse to go into the question of where "natural" selection ends and "artificial" manipulation begins!)
You see, what matters is the inclusive burden of selection. As someone mentioned, the nipples are the product of an embryogenic process that begins similarly in both genders. To avoid that happening would require a huge selective burden, whereas a few abortive nipples are usually too trivial to reckon in. Notice that in some ways the adaptation in females is far more specialised than in males, because they must get it right. In us males, our non-functioning vestiges require no more adaptation than keeping out of the way. Now, our primitive nipple count is more like six than two, but it is unusual to see signs of more than two in women, where we breastless men commonly have supernumerary nipples, only the extra ones are so small that we usually don't notice them. I had been adult for years when I happened to notice that an old friend had six, of which the lower four were basically pigmented dots. I then inspected myself and found that I had two extra, about at the level of my lower ribs. I had always thought that they were freckles! Since ours don't generally develop further, it is not important to control their appearance, but for women it is important, so the controls are not often so lax. But not that those extra "nipples" really, really are not as a rule functional at all. They are essentially areolar freckles.

In short, I agree with Pgardn about the unity of male and female ontogeny.


Certainly, more or less, but we are discussing way back. Tribes don't come into it. Lactation is part of a generic process of physiological secretion for feeding offspring, and in mammals at least, it started before our ancestors looked like tree shrews, let alone monkeys. We don't know just when the first true mammalian lactation began, but it might well have started with the development of hair and sweat glands, probably before the upper Jurassic if you ask me.
It was in many ways not a very striking development really. Some birds, some fish, and some insects, to grab a few examples just off the cuff, produce secretions for the feeding of their young. The said secretions are pretty well universally derived from other secretions that some ancestral baby began to lick up or swallow, or even bite off, along with the parental mucus when the parent fed it. To this day monotremes don't have proper nipples, just multiple little ducts opening into external hair tufts for the young to lick.
Fish such as tilapia and discus feed their fry on mucus, either inside the mouth, or a specially thick secretion around the lips and gill coverts. Pigeons' "milk" is from mucus glands in the throat. Get the picture? It is a coherent one, I think you will agree.



So? I, for one, am no physicist. If I'm not shy about asking for help with physics, or even arguing about it, I don't see why you should be shy about discussing biology. Your general line of thought was quite reasonable, and apart from a bit of cross-purposes, I don't think anyone thought anything else. Or begrudged you your thinking. It all is science anyway, no matter what Rutherford thought in his ignorance.

Go well, and enjoy.

Jon

Well, thank you for a thorough reply!

I guess I had been under the impression that there was no hormone therapy necessary, and that if we played with our nipples consistently enough, we would begin to develop mammary glands.

Well, bummer, I guess I don't have an excuse anymore...

 

[nismaratwork]

Well, thank you for a thorough reply!

I guess I had been under the impression that there was no hormone therapy necessary, and that if we played with our nipples consistently enough, we would begin to develop mammary glands.

Well, bummer, I guess I don't have an excuse anymore...

On the bright side, think of how you'll avoid the need to wear little band-aids over them to avoid chafing!

 

[Jon Richfield]

Suppose a biological life started in a furnace. When a first living organism developed, it went on to change into more advanced forms of life and ultimately branching out into several different varieties of life - animals , birds, etc. Is that furnace still there? I assume that furnace had to be earth. I mean to say is there some kind of organism still on its way to become a human in future? Dinosaurs were also the result of evolution. If that furnace is still working, then there could be dinosaurs again in the future, perhaps. I hope you would be able to understand what I'm trying to say. Thanks for reading this.

Jackson,

as I read your question, it is not clear to me whether you are referring to the nature of the environment as the "furnace" that supplies the power and material for the development of life, or whether you are referring to the mechanism and process of biological evolution in a more abstract line of thought.

Either would be reasonable in context.

So, firstly the nature of the environment and the populations in which adaptation occurred (and occurs) has changed repeatedly in the last few billion years on this planet. Some quite subtle changes have led to huge consequences in the history of our biology. My own bet is that we neither know of, nor understand one in 10 of the major changes of this type. And I say that in full appreciation of some very ingenious work and remarkable revelations.

Personally my attitude towards palaeontology is one of fascinated grief. It is a stressful subject to me. I just cannot stand the idea of all those marvellous creatures that I shall never see alive, nor understand the biology of. And yet there are excellent (if speculative) arguments for why some very impressive organisms died out. Sometimes it was because of changing conditions. For example some of the giant arthropods of the Carboniferous period might not have been viable in the lower oxygen levels that have reigned since then. Maybe we have no giant dragonflies or millipedes, simply because they could not breathe our air. Maybe seed plants took over simply because they did not need liquid water for their pollination.

Some people have made a great fuss about the explosion of new phyla in the Cambrian period, but actually it might be a better way of looking at it, to see the period as a loss of phyla. Late Precambrian life, such as the organisms that left the fossils of the Burgess shale and the Ediacaran fossils seem to have been wildly varied. By the end of the Cambrian we were left with a few dozen phyla.

These remarks are not so much a simple as simplistic, misleadingly so. All the way from the first development of cells, to actual modern times, we have developed into narrower and narrower basic plans of construction, what the comparative morphologist would call a bauplan. Every now and then a new bauplan, such as the arthropods all molluscs would take over and eliminate whole classes of rivals. Sometimes the bauplan that took over would be extremely unobvious, for instance the early chordates were not at all very promising. However once bony fish became dominant in the ocean, many other lines simply vanished. With each such an advance there would be dramatic radiation into wide varieties of lines of specialisation, but the number of bauplanne tended to reduce. For example, varied though they be, our modern birds are a rather thin sample of the fellow dinosaurs that they survived.

Such tendencies are not universal of course, we still have microbes apparently nearly identical to the dominant forms of two or 3 billion years ago, and worms very similar to those of five or 6 hundred million years ago and so on.

It is a large subject.

The second possibility was that the question you had in mind concerned the process of evolution as such. If that is what you meant, yes, it is still going strong and in principle it is hard to see how it could stop as long as anything like a living ecology based on reproduction with variation existed. It is a principle only arguably less fundamental than say, thermodynamics. In fact the two are very closely related, because like thermodynamics, Darwinistic evolution can be seen as a consequence of information theory.

Does that help?

Jon

 

[jackson6612]

Does that help?

Jon, it would be a lie to say I completely understood it. But I genuinely admire and respect people like you who so much sincerely strive to help others without any monetary incentive. I offer you and others my thanks and best wishes.

 

[Jon Richfield]

Jon, it would be a lie to say I completely understood it. But I genuinely admire and respect people like you who so much sincerely strive to help others without any monetary incentive. I offer you and others my thanks and best wishes.

Only a pleasure, but I wish I could put it more clearly and usefully to the enquirer. If you can at any time identify paragraphs and themes that gave you problems, please feel welcome to ask again accordingly,

Cheers,

Jon

 

[Ken Natton]

My problem is that I come to this thread very late in the conversation, and my contribution may not seem terribly relevant to the active discussion. But I have a perspective to offer on the theme of the thread, one that offers some possible indicators rather than absolute answers to some of the questions jackson6612 asked right at the head of this thread. And one that enables me to offer an informed, but nonetheless partly speculative answer to a question that has recurred at different points throughout the thread – why do men have nipples?

Within the last day, I have posted a reply on another similar thread about my understanding of just how early in the evolutionary chain sexual reproduction developed. Along with that understanding, I have cited the text from which I took that understanding, so people are free to challenge my interpretation of the text or to challenge the text itself. The understanding that I am about to offer here is also not just something I have thought up myself. It comes from something I read, written by someone who seemed to know what he was talking about. It will be much more difficult, but not necessarily impossible for me to recover the precise reference. It was actually a medical textbook that I took this understanding from. I am not, and have never been a medical student. I did not read the whole book, just, in fact this one passage, and I read it in an idle moment. But I found it fascinating and it did seem to make a great deal of sense in explaining much about what is actually the difference between male and female, to which I would offer this taster – a great deal less than you might think.

In any case, enough of the justifications, here’s the point. Though it is clear that the ‘y’ chromosome is present in male embryos and absent in female embryos right from conception, it is not the case that male embryos develop as males and female embryos develop as females right from the start. Neither is it the case that the initial stages of embryonic development are essentially androgynous and the separation into female or male occurs later in the development. The key is that all embryos begin to develop as females, and would develop all the way to maturity as females but for the intervention, where present, of the ‘y’ chromosome. Human embryonic development takes about eight weeks. I cannot honestly remember the exact point in the course of that development that this text identified as the point where the ‘y’ chromosome begins to exert its influence, but I do remember that it is astonishingly late.

So jackson6612’s speculation ‘which would mean male hadn’t become full male yet’ was prescient. He was just looking in the wrong direction. Don’t look for the answer back down the evolutionary development chain. Look for it back down the embryonic developmental sequence. Although, drifting away from the point a little, there are cases where our modern embryonic development does replay little bits of our evolutionary history.

In any case, my guess at the answer to the question, why do men have nipples? Because they develop before the ‘y’ chromosome does its thing. Mammary glands are, after all, the very thing that define us as mammals.

 

[Jon Richfield]

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

Since you ask, no they are not. No more so than say, humans, hummingbirds, orchids, octopus, redwoods, spiders, or medfly.

And incidentally, no, sex did not start with the chordate descendants of sea squirts. It is far older than that. The discussion in this thread is very frustrating because it begins with misconceptions several steps removed from the fundamentals of sex and its many implications in evolution. I reckon that some of the most confused participants need to go back to basics and do a LOT of reading and thinking.

In case anyone reads this and thinks that I am being either patronising or derogatory, forget it. The plain fact of the matter is that it is about as easy to make sense of such matters without developing a perspective of the basics, as to understand quantum theory without mastering bra-ket maths.

Sorry, naught for your comfort. Have as much fun as you like, but, until you have done your homework, don't delude anyone, including yourself, that you have any coherent question, or any coherent understanding of what it would take to recognise the incoherence.

 

[Jon Richfield]

In any case, my guess at the answer to the question, why do men have nipples? Because they develop before the ‘y’ chromosome does its thing. Mammary glands are, after all, the very thing that define us as mammals.

Sorry Ken, I don't buy this. Most of what you wrote had merit, but no, except in a trivial, largely etymological, sense, no, they are not, not any more than say our nitrogenous excretion biochemistry. The fact that it is adequate to argue that if one finds a mammary gland one knows that a mammal was involved somewhere, does not imply that it is definitive. The same could be said of hair (distinct from say chetae or filoplumes, right?) or sweat glands, or enucleated erythrocytes, no?

Cheers,

Jon

 

[nismaratwork]

Since you ask, no they are not. No more so than say, humans, hummingbirds, orchids, octopus, redwoods, spiders, or medfly.

And incidentally, no, sex did not start with the chordate descendants of sea squirts. It is far older than that. The discussion in this thread is very frustrating because it begins with misconceptions several steps removed from the fundamentals of sex and its many implications in evolution. I reckon that some of the most confused participants need to go back to basics and do a LOT of reading and thinking.

In case anyone reads this and thinks that I am being either patronising or derogatory, forget it. The plain fact of the matter is that it is about as easy to make sense of such matters without developing a perspective of the basics, as to understand quantum theory without mastering bra-ket maths.

Sorry, naught for your comfort. Have as much fun as you like, but, until you have done your homework, don't delude anyone, including yourself, that you have any coherent question, or any coherent understanding of what it would take to recognise the incoherence.

Beyond your diatribe, would you care to offer an actual contribution as to when this divergence developed?

 

[Jon Richfield]

Beyond your diatribe, would you care to offer an actual contribution as to when this divergence developed?

Way back. Very possibly as a side effect of the nature of the first mitotic divisions that cells systematically achieved in the first few hundred million years of life on the planet. And as for gender, probably well within two billion years of that time. We can distinguish gender nowadays even in certain fungi and algae. Various classes of gender in fact; it is a very general concept and seems to have been developed in many ways independently in various biological kingdoms, including several times independently within single kingdoms.

Does that give you any clues? Pretty elementary biology, I should have thought. If you would like anything more specific, please suggest some of the points to cover; I do not have the time at the moment for a detailed work on the subject (or range of subjects actually.)

And incidentally, what is this ugliness thing of yours? Or shouldn't I ask?

 

[Jon Richfield]

In any case, enough of the justifications, here’s the point. Though it is clear that the ‘y’ chromosome is present in male embryos and absent in female embryos right from conception, it is not the case that male embryos develop as males and female embryos develop as females right from the start. Neither is it the case that the initial stages of embryonic development are essentially androgynous and the separation into female or male occurs later in the development. The key is that all embryos begin to develop as females, and would develop all the way to maturity as females but for the intervention, where present, of the ‘y’ chromosome. Human embryonic development takes about eight weeks. I cannot honestly remember the exact point in the course of that development that this text identified as the point where the ‘y’ chromosome begins to exert its influence, but I do remember that it is astonishingly late.

Ken, apart from some remarks I have made in other postings, some in answer to you, I would like you to consider the fact that gender determination is not a function of "Y-chromosomes" specifically. Nor are all Y-chromosomes homologous in this sense. We certainly have had well-defined gender, not only in chordates, from well before there were anything like modern Y-chromosomes. And even in modern chordates (Craniates in fact) Y-chromosomes are not either necessary at all, nor do they have the same effects. In birds, insofar as they are homologous, which is not so far (they are not in all way truly analogous even!) the Y-chromosome determines the female gender! And in many reptiles, not necessarily closely related, such as chelonians and crocodilians, gender is temperature-determined during incubation.

So certainly much of what you say is true about the way in which mammalian development of (especially male) gender occurs, and how it affects the interesting, but to my mind rather minor, issue of male nipples, but at a slightly deeper and correspondingly more general and more interesting (to my taste) level, if one is looking for more generally meaningful, insight-giving views of gender, one really does have to go far, far back into our past. The male nipple is pretty trivial in such a perspective.

Go well,

Jon

 

[nismaratwork]

Way back. Very possibly as a side effect of the nature of the first mitotic divisions that cells systematically achieved in the first few hundred million years of life on the planet. And as for gender, probably well within two billion years of that time. We can distinguish gender nowadays even in certain fungi and algae. Various classes of gender in fact; it is a very general concept and seems to have been developed in many ways independently in various biological kingdoms, including several times independently within single kingdoms.

Does that give you any clues? Pretty elementary biology, I should have thought. If you would like anything more specific, please suggest some of the points to cover; I do not have the time at the moment for a detailed work on the subject (or range of subjects actually.)

And incidentally, what is this ugliness thing of yours? Or shouldn't I ask?

If it's so elementary, I'm sure you'll have no problem providing sources which confirm your statements; I look forward to seeing them. As for the ugly comment, it was a joke. You need to relax in the worst way. You've provided your view of when sexual division began, now you get to back it up with something solid, and if not, go home.

 

[Ken Natton]

Hi Jon,

For certain, I agree with you that the male nipple thing is trivial, I only raised it because it has been a subject of discussion on this thread, and it does seem to be one that a lot of people latch on to as a mystery of evolution.

I’m surprised that you reject the idea of mammary glands as a defining mammalian feature. Apart from the obvious etymological connection, is not suckling the young the key defining feature of a mammal?

And, connected with another one of my very favourite stories from the annals of human evolution, my understanding was that sweat glands, or at least a profusion of sweat glands, was very much a specifically human feature. The story, as I heard it, is closely connected with the reason why we became the naked ape. It’s all about our big, energy hungry, heat producing brains.