Skull proportions cannot reliably identify the race of a skull. Kenniwick man was originally identified as a skull belonging to a Caucasian, but DNA evidence shows that it is most closely related to Native Americans from Asia instead. Organisms adapt to the local environment. Therefore a wide ranging species can show different adaptations to different local environments. In some cases, these adaptations may lead to the evolution of new species. In many cases the regional differences may only allow scientists to classify them as different subspecies.
Humans originally evolved in Africa about 150,000-200,000 years ago. About 60,000 to 70,000 years ago, a small group of Africans migrated out of Africa, and they became the ancestors of all non-Africans. Some of these descendants found themselves in an environment that is very similar to those found in Africa, so they did not need to change to adapt to their environment. They as a result still have dark skin, a rather flat and wide nose, full lips and curly hair. The Negritoes, Australian aborigines and the Melanesians are the descendants of these early migrants from Africa that still look like Africans, and yet they are more closely related to other non-Africans.
However, some of the early migrants ended up in cold climates. There they evolved cold climate adaptations such as a tall and narrow nose bridge, more facial and body hair, thin lips, shorter arms and legs, more fat under the skin, a more rounded torso, and light skin tones. The northern Chinese, the Mongolians, and the Europeans appeared to have independently evolved these adaptations. If humans stopped migrating from that point onward, then we can probably classify humans into 3 or even 4 races or subspecies. Unfortunately for scientists, people keep on migrating throughout history and prehistory. In fact, since the invention of agriculture, the migration has increased because of population expansion.
When people migrate, they interbreed with the locals, at least in the beginning, before they start being more picky later on, and their descendants show a mix of cold and warm adapted features. For example, the northern Chinese migrated south to southern China and SE Asia, and there they mixed with the darker skinned locals to produce a variety of different people with mixed race characteristics. Europeans also migrated to West Asia, and there they also interbred with dark skinned locals. The Chinese also admixed with the Melanesians and the results are the Polynesians. As a result of such extensive admixing, it is simply no longer possible to classify the people of one region as one race or another race. Many southern Chinese and SE Asians have a mix of racial features so that they can neither be classified as African nor Mongoloid, for example. The same is true of most people in India.
As a result scientists threw up their hands and say, we cannot find any trait that can reliably identify people as being one race or another. In fact, since the invention of ships that can travel around the globe, the admixing has increased. Many Europeans have spread their genes to the Americas and Asia and even Africa. Therefore there is really little hope that we can classify people into different races from this point forward.