This is a topic that will bring about a fair bit of arguments and disagreement, however the consensus (More among anthropologists than historians) is that pre-class, hunter-gatherer societies were rather egalitarian in terms of gender. Although there were definitely gender roles in many of these hunter-gatherer societies the role of women was not viewed as inferior, and in some instances were actually valued more than men. If you want a quick and easy overview there was a recent article in the Guardian that summarizes some recent findings by anthropologists.

Of course hunter-gatherer societies were very varied and diverse, but there are general principles that you can apply to the majority. Although women were mainly performing that gathering and men doing the hunting, there is very little evidence to suggest that these societies held hunting above gathering. In fact, which role was valued more depended heavily on seasonal change; while there might be a large amount of animals to hunt for one season, they might become far more scarce as the seasons change and thus the main source of food is based upon the gathering. Thus, interdependence between the hunting and gathering was the norm which shows that women's work wasn't given a "secondary" status to that of men. Additionally child-raising wasn't a task that was burdened onto the mother, this role was generally a community role that the entire tribe would engage in.

With the agricultural revolution some 9,000 or so years ago these nomadic hunter-gatherers became settled communities, and here we have a definitive shift from relatively egalitarian tribes to a class society where you can produce a surplus of goods (unlike hunter-gatherers) and actually own property (farms and livestock). Men, who traditionally performed the hunting role, took over the ownership of the livestock and thus the land associated with that form of labour. As these agricultural communities grew, so did competition with other communities and thus there came a need to protect your livestock and other goods. Here we have the beginning of the subjugation of women who, as child bearers, were taken into the home for the main purpose of raising (male) children to be the successor and next owner of their fathers land. This trend continues on and develops as class societies develop.

Do I think the question of women's oppression comes down to physical differences between men and women? No, I don't think so. That theory doesn't explain why men and women lived essentially as equals for the vast majority of human existence. I also think that the physical abilities difference is quite exaggerated and generalized , since to reinforce an idea that women have inherently modern feminine values and cannot fight or defend themselves properly (you can look at the numerous powerful women warriors throughout recorded history for an example). I think a far more plausible explanation is that women's oppression arose from a change in hunter-gather societies to class based agricultural communities.