Final answer:
Diseases brought by Europeans caused the highest number of deaths among American Indians during the early years of European colonization.
Step-by-step explanation:
The highest number of deaths among American Indians during the early years of European colonization was caused by diseases brought by Europeans. The Native Americans had no immunity to diseases such as smallpox, diphtheria, and measles, which were introduced by the Europeans. These diseases decimated the Native American tribes, as they had no exposure to the diseases and no ability to fight them.