Some medical scientists believe that the increase in metal ions in the human body is the cause of human aging. In 1978, several scientists performed an interesting experiment on an invertebrate rotifer. The healthy rotifer was divided into two groups, one Injecting a substance that can eliminate metal ions in the group, as a result, the rotifers injected with this substance have a much longer life span than the rotifers without injection. It can be seen that the increasing number of metal ions in the human body is the cause of human aging. From the above we know:. ['Many medical scientists are interested in a long life.', 'People should take as little metal as possible.', 'Metal ions are the cause of human aging.', 'Some scientists have studied the causes of human aging through experiments.']