Final answer:
Biological anthropologists explain diversity in human physical forms by studying biological variation across time and space. They observe that racial categories are scientifically unsound and that human traits vary on a spectrum.
Step-by-step explanation:
Biological anthropologists explain the difference in the diversity of human physical forms by studying human biological variation across time and space. They analyze differences in morphology, such as height, jawline, eye sockets, ear and nose shape and size, as well as biochemical differences like variations in skin pigmentation, sense of smell, and genetic mutations. They observe that there is more variation within racial categories than between them, and conclude that human populations cannot be scientifically divided into distinct races. Instead, human traits vary along a spectrum and are mixed and matched among individuals.