Unskilled, perhaps, but very versatile

Before squirrels arrived, the trees were jumped on by Plesiadapis, a primatomorph – that is, a close relative not only of primates like monkeys and lemurs, but also of colugos, small mammals vaguely resembling flying squirrels that live in the forests of Southeast Asia. Vegetarian or omnivore, they moved on the trees of four or two legs. Unlike large-brained primates, their gray matter was not much, according to recent studies.