When did early man first realise the link between sex and birth?
Early man probably knew immediately being that the child would have the traits of both parents such as brown hair. But if she had sex with another man who had red hair and a red haired baby was born, it wouldnt take too much rocket science to draw the conclusion. Also consider the link could be established by the fact that only women having sex are getting pregnant. So if an early woman decided that she didnt want to have sex but she wanted a baby she would eventually figure out what other women were doing.
