Humankind gradually evolved from early members of the genus "Homo"—such as "Homo habilis", who used simple stone tools—into anatomically modern humans as well as behaviourally modern humans by the Upper Paleolithic. During the end of the Paleolithic Age, specifically the Middle or Upper Paleolithic Age, humans began to produce the earliest works of art and to engage in religious or spiritual behavior such as burial and ritual. Conditions during the Paleolithic Age went through a set of glacial and interglacial periods in which the climate periodically fluctuated between warm and cool temperatures. Archaeological and genetic data suggest that the source populations of Paleolithic humans survived in sparsely-wooded areas and dispersed through areas of high primary productivity while avoiding dense forest-cover.