This guide isn't supposed to be the tell-all story of the origins of life and evolution, but instead a gateway to the world of early Earth science and evolutionary biology. There is one theory that wasn't discussed: intelligent design. Intelligent design is the theory that life is too complex to have happened randomly, so it must have been created by some intelligent designer.
There is an ongoing battle between science and proponents of intelligent design over the origins of life. Science at its core is about taking observations, making hypotheses, and testing those hypotheses. Observations from those tests are then used to make new hypotheses, continuing the process. Observe, hypothesize, test, and repeat. Don't forget to rinse. All branches of science attempt to explain the natural world using the scientific method, including the study of evolution. However, the teaching of evolution continues to be a hot-button issue in some areas of the United States.
Many court cases have been fought over the right to teach intelligent design in schools, the most notable of which is Kitzmiller v. Dover Area School District. The case was brought up after a school board decision in Dover, Pennsylvania stating that intelligent design should be taught as an alternative to evolutionary theory, and that an intelligent design textbook be used as a reference for biology courses. The judge in the trial ruled that intelligent design was not science, and instead religion. Therefore, it could not be taught in the schools.
As discussed in this guide, there are many controversies in the evolutionary biology community: the neutral model of evolution compared to the selectionist model, the punctuated equilibrium model versus phyletic gradualism. Even Lamarckian evolution is making a comeback as evidence suggests that our experience affects the fitness of our offspring. Modifications of DNA that occur during our lifetime can be passed on to our offspring, and these modifications are a function of our behavior. Sounds Lamarckian.
Since there is some debate over the validity of Darwinian evolution, why is intelligent design not a valid scientific theory? Intelligent design isn't testable, and therefore doesn’t make use of the scientific method. The mantra of observe, hypothesize, and test isn't followed.