Task: Text Simplification
Input (English): In humans , the pigmentation of the iris varies from light brown to black , depending on the concentration of melanin in the iris pigment epithelium ( located on the back of the iris ) , the melanin content within the iris stroma ( located at the front of the iris ) , and the cellular density of the stroma .
Output (English): In humans , the pigment of the iris varies from light brown to black .