significant cultural change, which in turn may have accelerated human evolution. Agriculture brought about an abundance of cereals, enabling women to wean their babies earlier and have more children over shorter periods of time. Despite the vulnerability of densely populated communities to diseases, this led to a population explosion and thus more genetic variation, the raw material on which natural selection acts. Diets in early agricultural communities were deficient in many nutrients, including vitamin D. This could be one reason why natural selection has favored light skin among Europeans, as it increases UV absorption and synthesis of vitamin D. The light skin pigmentation characteristic of modern Europeans is estimated to have spread across Europe in a "selective sweep" during the Mesolithic (5,000 years ago). Signals for selection in favor of light skin among Europeans was one of the most pronounced, comparable to those for resistance to malaria or lactose tolerance. Eumelanin, which is responsible for pigmentation in human skin, protects against ultraviolet radiation while also limiting vitamin D synthesis. Variations in skin color, due to the levels of melanin, are caused by at least 25 different genes, and variations evolved independently of each other to meet different environmental needs. Over the millennia, human skin colors have evolved to be well-suited to their local environments. Having too much melanin can lead to vitamin D deficiency and bone deformities while having too little makes the person more vulnerable to skin cancer. Blue eyes are an adaptation for living in regions where the amounts of light are limited because they allow more light to come in than brown eyes. They also seem to have undergone both sexual and frequency-dependent selection. A research program by geneticist Hans Eiberg and his team at the University of Copenhagen from the 1990s to 2000s investigating the origins of blue eyes revealed that a mutation in the gene OCA2 is responsible for this trait. According to them, all humans initially had brown eyes and the OCA2 mutation took place between 6,000 and 10,000 years ago. It dilutes the production of melanin, responsible for the pigmentation of human hair, eye, and skin color. The mutation does not completely switch off melanin production, however, as that would leave the individual with a condition known as albinism. Variations in eye color from brown to green can be explained via the variation in the amounts of melanin produced in the iris. While