never a woman on her period is not fertile when she's on it. or if you ment as in age wise i have no idea huh...

No period at all. Female fertility was a recognised fact thousands of years before Man learned to write, thus before "history" began. But man's contribution to pregnancy was not discovered until recently, about 1,800 BC. Men thought that women conceived just because they could do so. They thought that it was MAGIC.

Long before there was history. Assuming that consciousness evolved with the species, I'm sure by the time they were fully conscious, women always just knew. They might not have shared that little fact with men for thousands of years, but given the known olfactory capabilities of primitive man, I'd be pretty sure men had figured it out on their own too by the time they'd achieved consciousness as well. It's also been shown that a man's sexual interest in a woman and arousal naturally increases through her cycle, peaking at ovulation, and then diminishes reaching its nadir during menstruation. (It's also been shown that if she's on the Pill, his interest remains at the level it is naturally when she's menstruating.) But this little "discovery" of modern researchers was perfectly well known to pre-Columbian Indians, Australian Aboriginees, and more than a few tribes in New Guinea, not to mention all of East Asia. Contra Sandman, that 1800 BC date was invented by feminist historians/anthropologists and it's been shown to be bogus. (A good indicator of that is the presence of adultery laws and their categorization as a kind of estate fraud in the Ancient Near East.) However, we have discovered a handful of primitive isolated tribal societies where men were evidently ignorant of their part in making babies, but all other living "stone age" cultures we've discovered to date, have understood the facts of life as well as we do... even if it is also seen as "magic" (and most modern folks see it as magical/miraculous too).