The field was most level when our weapons were hands, feet, and teeth. It has become steadily more uneven since then: the guys with more money generally have the best defenses and deadliest weapons, and technological advances have only amplified the difference.
No. It might have been nearly level between young, fit men but age a bit, get sick or be a child or female and you start getting power imbalance.
This species of primate used to live in families and small groups where old and children were protected; fights were limited by instinct, etc. There were few things to fight about, and no band would try to "own" everything or enslave other groups, because it simply did not make physical sense. So, by and large, no group was much better off than any other.
Weapons made the playing field much more unequal. Those who had swords, guns, bombers etc. could "own" whole countries or continents, taking the best of the resources and forcing millions to work for their benefit.
Now, throw a few inexpensive firearms in there and suddenly even the weak and unskilled can put down several oppressors without letting their physical advantage get anywhere close. It's a game-changer (physical advantage still plays into things but its importance is greatly diminished).
I should know better than getting into firearm discussion, but...
Firearms give advantage to those who use them first; they make attackers more powerful, but are very poor for defense. Those who spend all their time, money, and energy to acquire bigger guns will always prevail over those who try to live a honest life of productive work, even if they have guns. A machine gun trumps a revolver, a bazooka trumps a machine gun, a missile trumps a bazooka, and so on.
Several of my relatives and acquanitances have been victims of armed robberies in their homes. (A student frat next door to my house was robbed a couple of months ago, just after the students moved in. Frats are popular targets, since the robbers can count on collecting 6-12 laptopts and that many smartphones, at the very least.) Some of the victims had guns. One was a gun collector. But not one was able to use a gun for defense; instead, the robbers took their guns too.
In the typical armed robbery, one household member is made hostage when he/she enters or leaves the home, and is forced to open the doors for three or four bandits, guns in hand. Usually handguns, but sometimes heavy machine guns. Perhaps one or two family members could use the house gun that is stored somewhere; but only a totally demented person would try to, in those circumstances. In all those cases, fortunately, the robbers left without harming anyone.
It would make no difference if the victims had machine guns, bazookas, or portable tactical nuclear devices in their homes. The robbers will always be one step ahead, and improvements in weapons technology and availability will only make the gap bigger.
Even in the Wild West, the (relatively) best way to keep robbers at bay was to have the State nearby and on your side, with the biggest guns available. That solution has its risks, of course, but by and large ordinary people with governments with guns have always fared better than outlaws with guns.