system
Given the following post, respond as if you're commenting on Reddit:
human
Why does our hair continue to grow?

I was told our hair is what's left from fur after evolution. But why does our hair continue to grow whereas animal fur stays roughly the same length (and regrows to the same length after cutting it). Also: Are there any animals that loose their fur with old age?
gpt
Hair length is regulated by controlling the timing of hair growth (anagen), cessation (catagen) and rest (telogen). 

During anagen, the hair root dives into your scalp, giving your hair something to hold onto as it grows. 

During catagen, that hair follicle (which produces the hair) becomes attached to the hair and forms a "club" at the end that prevents more growth.

Telogen is the last stage, when growth has completely stopped and the hair just sits there, waiting to be forced out by another hair growing from below, or pulled out by the elements.

In reality, scalp and facial hair don't grow endlessly; for most people, hair can only grow for 2-7 years; the exact length of time is genetically determined in large part. The reason our hair seems to never stop growing is because we are always cutting it before it can reach that maximum length.

Hairs on the rest of your body have much shorter cycles. Fur is similar; it grows for a much shorter period of time, so their hair length is defined by that length of time.

Hair loss/alopecia does occur in other mammals, often having to do with illness. But even men with severe male-pattern baldness maintain their body hair, and often their facial hair. In that sense, we maintain most of our "fur" but lose only a small portion atop our heads.