Which species of the genus Homo was the first to be discovered outside Africa?

The species of the genus Homo that was first discovered outside Africa is Homo erectus. This species is significant in the study of human evolution due to its wide geographic distribution and its role in understanding human migration patterns. Fossils of Homo erectus have been found in various locations, including parts of Asia and Europe, which supports the idea that this species was among the first to leave the African continent.

Homo erectus is often recognized for its advancements in tool use and adaptation to different environments, which likely contributed to its ability to populate areas outside Africa. In contrast, other species mentioned either have more limited geographic distributions or are not noted for early migrations outside Africa. For example, Homo ergaster is considered to be primarily an African species that predates Homo erectus. Homo rudolfensis is mainly known from East Africa, and Homo floresiensis, while remarkable for its unique characteristics, was discovered on the island of Flores, Indonesia, much later in comparison to the initial migrations of Homo erectus. Thus, the importance of Homo erectus in the context of human evolution and migration makes it the correct answer.