Evolution, because it can prove how organisms evolved from past lives.
Testosterone is a steroid. Its physiological functions are:
Determines the the gender of a developing embryo
Development of reproductive organs and the prostrate gland in males
Responsible for secondary sexual characteristics in males such as deeper pitch, increased muscle bulk, hair on the upper lip
Regulates normal sperm development
Another is cholesterol. Physiological functions are:
Helps maintain the structure of cells and vessels improving overall health and function in the body.
Percursor to important sex hormones such as testosterone and estrogen.
Used as an insulator around nerves and is absolutely essential for brain function.
Precursor to Vitamin D, which supports a healthy immune and nervous system
I would think because there are animals that live in the freshwater and also the animals that use the freshwater to travel. Also the pollution from people littering and all that kind of stuff.
They are all reproductive cells