In 2008, scientists discovered that humans with blue eyes have one single, common ancestor. They believe a genetic mutation took place about 6,000 to 10,000 years ago and caused the blue-eyed people we know today. Originally, all humans had brown eyes, but a genetic mutation of what is called the OCA2 gene in our chromosomes that “turned off” the ability to create brown eyes—not turning it off entirely, but essentially diluting brown eyes to blue. The variation in color is actually due to the amount of melanin (the pigment that gives color to our hair, eyes, and skin) in the eye’s iris, with blue-eyed people having a small degree of variation in the amount of melanin in their eyes.

The frequency of a sound is the pitch. Frequency is expressed in Hertz (Hz). Sounds are classified as infrasounds (below the range of human hearing), sonic range (within the range of human hearing), and ultrasound (above the range of human hearing).