Cumulative culture can refer to cumulative cultural evolution as well as to the products of this evolution. Cumulative culture simultaneously increases the efficiency of the trait as well as preserving changes to the form of the trait itself. It is often intergenerational and can lead to traits that are no longer inventable by a naive individual. The exact necessary and sufficient mechanisms of cumulative culture are still debated. While modern humans clearly exhibit cumulative culture, it is unclear both when this started in the human lineage and whether any nonhuman animals exhibit cumulative culture.