I am studying the relationship between genetic diversity and epigenetic complexity and its role in common diseases and evolution. Biological phenotypes are determined by both genetics and epigenetics. Thus, the relationship between genetics and epigenetics is a fundamental problem but has yet to be studied. We proposed a novel hypothesis of genetic diversity and evolution, the Maximum Genetic Diversity (MGD) hypothesis. It is based on a self-evident axiom that we termed the First Axiom of Construction, stating that random variations in building blocks above atom level are inversely related to system complexity. The equivalent this fundamental axiom in the field of biology is what we call the First Axiom of Biology stating that genetic diversity is inversely related to epigenetic complexity. This axiom in turn led us to deduce the MGD hypothesis, a more complete account of hereditary changes or evolution. Genetic diversity of a species has an upper limit as set up by the epigenetic complexity levels, while the epigenetic complexity level also has an upper limit as set up by the level of genetic diversity. Each species has a specific maximum genetic diversity level. The MGD of simple organisms is greater than that of complex organisms. The MGD hypothesis is supported by numerous observations and has yet to meet a single contradiction. While genetic diversity in viruses and bacteria is adaptive, we will show that genetic diversity is mostly harmful or disease causing in humans. It is common sense that a machine may tolerate a certain amount of random errors in its buidling blocks but only to a limited extent. Thus, too much random errors/noises or genetic diversity in the DNA building blocks may interfere with the optimal function of a human being especially the brain. By studying fundamental problems of evolution and common diseases such as bipolar disorder, autism, schizophrenia, Parkinson’s, cancer, etc, our goal is to obtain more evidence to support the First Axiom of Biology and the MGD hypothesis and to gain fundamental insights into the past, present and future of life on Earth.