A recent study analyzing the evolution of bees has supported, to some degree, existing theories about their evolution. For instance, when provided with a pollen-generating food source, bees will develop the capacity to feed on pollen, known as pollinivory; this finding does help to explain the explosion in bee species some 120 million years ago. The study also validates the theory that bees evolved from a species of carnivorous wasps. While pollinivory was confirmed as an important step in the process of bee speciation, the study overturned the previous hypothesis that this factor by itself was the cause of the diversification of bee species. Instead, the research indicated that this increase was more accurately explained by including the complementary process which saw bees moving from being specialized in only a few host-plant species to becoming generalists able to feed on a wide number of hosts. This generalization enabled bees to move into new ecological regions and rapidly expand speciation. This finding helps explain why pollen-eating wasps, who specialize in only a few plant hosts, failed to demonstrate the same rapid diversification as bees. The Masarinae family—the subfamily of wasps that feeds exclusively on pollen—contains around 350 known species; by contrast, there are almost 20,000 known species of pollen-eating bees in the world, divided across only nine families.