Why is Homo in Homo sapiens capitalized?

This is the convention in the system of binomial nomenclature (two-part names) developed by Linnaeus in the 18th century. In the Linnaean system, the first Word of a binomial names the genus (a group of similar and closely related species) and is capitalized, and the second Word names the species within that genus. The genus name, but not the species name, can be abbreviated to its first letter. In some cases the binomial may be extended by the addition of a subspecies name.

In current anthropological thinking, the genus Homocontains modern humans (H. sapiens) as well as the extinct species H. erectus, H. habilis and H. eraster, and perhaps several others whose status is still in dispute. Neanderthal Man is variously considered a separate species, H. neanderthalensis, or a subspecies of H. sapiens. In the latter case, Neanderthals are known as H. sapiens neanderthalensis, and modern humans as H. sapiens sapiens.