Answer
Monkeys apes and human share common ancestors. Indeed, we share distant ancestors with all vertebrates-- including mammals, birds, amphibians, reptiles and fishes. We study their anatomy, and physiology, because knowledge of the evolutionary relationships of their features give knowledge of the structure of our own bodies .
We can do experimental explorations of the "lower" species that we would not be permitted to do on other humans,and species closely related to us.-- the Nazi human experiments and the Tuskegee experiments were ethical aberrations.
Some experimentation is still being done on monkeys. However, this is increasingly unpopular, and permission to conduct such studies is increasingly difficult to obtain.
Work Step by Step
Although we are related to all animals, evolutionion is not linear, and relationships between species are complex. We need to pick the right animal model for studying the anatomical and/or physiological issue of interest. For though all vertebrates are related, we can be led astray without a proper understanding of convergent and divergent evolution. In order to acquire this it is necessary to understand both convergent and divergent evolution. Bats and birds have wings, and they use them for the same purpose, but these structures have different evolutionary origins; also, not all primates are susceptible to the same diseases, nor are all mammals.
The animal model chosen to study a problem must be suitable from an evolutionary point of view.
If the animals selected for studying a human anatomical, physiological or behavioral problem are not related to humans evolutionarily ( with regard to the feature being studied) then the study would not yield useful information about our own biology. That is the value of understanding evolution by natural selection.
It enables us to choose the right experimental animals for studying biological and biomedical issues.
