Brown adipose tissue is present in small mammals (e.g., mice) and in newborn humans. Most of it disappears in adult humans. The cells in both types of fat are called adipocytes although they differ in origin, structure, and function in the two types of tissue.
On the left hand, index finger. It's not the index finger you wear it on. Left hand, ring finger. The finger that you would wear a wedding ring on.
In Western countries yes it means the person is single. It could be a parent or relative or the person bought the ring themselves and it's for show so it goes on the middle finger or sometimes people wear 'pinkie rings' on their little finger. However, some cultures such as European cultures can wear a wedding ring (generally on the right hand) on any finger.