At one point in time, after the emergence of the genus Homo two millions years ago, Homo Heidelbergensis or other close candidate became obligate foragers and scavengers under ecological pressures (desertification) driving to resources scarcity. Individuals able to avoid free-riders and to divide the spoils with collaborative partners would have gain an adaptive advantage over non cooperators. The heightened dependence on joint effort to gain food and the social selection of partners are taken to account for an evolution toward better skills at coordinating individual's roles and perspectives under a common attentional frame (that of the hunt or scavenging) and under a common goal, giving rise to joint, interpersonal intention. Later, around 200 000 years ago, new ecological pressures presumably posed by competition within groups put those comprising "loose pool" of collaborators at a disadvantage against groups of coherently collaborative individuals working for a common territorial defense. "Individuals ... began to understand themselves as members of particular social group with a particular identity".