backpack

It seems probable that humans have been carrying objects on their backs for almost as long as they have been walking on two legs. Our ancestors may have got the idea from watching animals carry their young or they may have discovered that much heavier objects can be borne on the back than carried in the arms. Carrying something on your back not only means that you can carry weight for longer periods, it also, crucially, means that your hands are free, to carry a spear, climb a tree or swipe your credit card. Of course, in order to free your hands, you need some form of backpack to do the holding for you.