Whereas historiography of the debates on “early man in America” isolates Florentino Ameghino's ideas on human evolution from his paleontological and geological work, this paper presents Ameghino's ideas on human ancestors in regard to the controversies over the origin and dispersion of mammals. Therefore, this paper analyzes the constitution of paleontology in Argentina at the end of nineteenth century by describing, firstly, the Ameghino brothers' organization of research. By tackling this aspect I want also to discuss the place of science in late nineteenth-century Argentina. Secondly, I will sketch “Ameghino's ideas” about Patagonia as a center of distribution of mammals, the age of Patagonian strata, and the South American origin of humankind. The Ameghino brothers' logistics of fieldwork created not only the means for finding a remarkable fossil fauna but also a trap that undermined their scientific credibility. Therefore, I will focus on the problem of fieldwork in “distant” places and of scientific wandering in Patagonia. In the polemics presented here, language, transportation systems, visual representations, and technical devices were crucial elements for the creation of paleontological objects.