This is a two part question.

Is it true that all humans are conceived as female, and it is the Y chromosome that makes them male?

If this is true, is it possible that mankind in the early stages of development were all originally female and the males were a random mutation as is seen in an area which is populated by an all-female group of amphibians?

answer: sort of yes, and no. I don't think "conceived" is the right term, because at conception the fetus is either male or female, depending on its sex chromosomes. However it's true that in early embryological development, they all begin with female characteristics, and those with the Y chromosome then develop male ones. Female, you could say, is the "default" condition. (Or, if you want to get into a war, you could say maleness is a step beyond femaleness). As to the speculation, sexual differentiation and sexual reproduction developed a gazillion years before "mankind" so that the idea that there were a bunch of female-only hominids running around doesn't make sense: once you have a line of organisms that use sexual reproduction (which is just about everything beyond one-celled animals) then you are stuck with needing male and female to produce offspring, else the line dies out.

On an evolutionary time scale, a-sexual and hermaphroditic creatures would have been the norm until the sexes seperated. This would have happened before there were even vertabrates.

One of the persistent misconceptions about gender differentiation in fetuses is that the male is in some way a form of modified female. That this notion is false has been known for quite some time, and the truth of the matter, a part of basic medical knowledge. The following site shows how the two sexes develop, and how the male is as individually developed as is the female.


http://www.people.virginia.edu/~rjh9u/sexdev.html

All-female groups of amphibians reproduce by a kind of cloning. The female's eggs develop into individuals without the contribution of the male genetic material, so are identical to the parent.

It's not a favorable reproductive method, because there is little room for the development of great variety in the members of the group, which prepares the way for adaptation and evolution. Colonies of clones are easily eradicated by the intervention of, say, a new bacteria into the habitat.

That is why parthenogenesis is not more widely spread.

My post was indeed less than correct: it would have been better (I was being overly simplistic) to say that in the absense of the Y chromosome a fetus develops into female, and with the presense of it, it becomes male. In the earliest development sexual differentiation is not visible; they look the same, male and female. Which way it goes depends on the presense or absense of the Y. Which is what I meant when I said that it's not true that all fetuses are female at conception. This may or may not get me off the hook

Sid, rather than defining femaleness as the 'absense' of anything, would it not be as true to say that XX is a female at conception and XY is a male?

Also, isn't YY possible? How does this come about?

Females have the XX chromosome pair. Males have the XY chromosome pair. When a child is being conceived, eggs only have an X to offer because females have no Y chromosome. Sperm can either have an X or a Y to fertilize the egg with, depending upon the cell. So, the child ends up being XY (male) or XX (female), depending upon which sperm fertilizes the egg. There is no YY because the egg doesn't have a Y to combine with the sperm's Y.

This is why it's ironic that a long time ago, if a man's wife didn't give birth to any sons, it was her fault, she was considered a bad wife. In fact, it was a result of the sperm that reached the egg that had an X chromosome.

I believe there's an XYY occurance. YY would die before birth (spontaneous abortion).

Re: the good old days. In those days, many people died before they passed their reproductive prime. There must have been cases of women who were barren, or bore only girls, who became widows, remarried, and later bore boys. You'd think this would throw some doubt on the old beliefs, wouldn't you?