In this task, we ask you to elaborate the sentence without changing its general meaning. You can do so by explaining further the input sentence, using more precise wording, adding qualifiers and auxiliary information etc.

Example input: The Inheritance Cycle is a series of fantasy books written by Christopher Paolini.
Example output: The Inheritance Cycle is a tetralogy of young adult high fantasy novels written by American author Christopher Paolini.
Example explanation: The output sentence elaborates on the input sentence without changing its general meaning e.g. "tetralogy of young adult high fantasy novels".
Q: In humans , the pigment of the iris varies from light brown to black .
A:
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 .