Splitting & Lumping
It is hard to avoid the conclusion that under current taxonomic practice there is a distinct tendency to under-estimate the abundance of species in the primate, and notably the hominin fossil record. ~ American paleoanthropologist Ian Tattersall
Unless an animal can be observed, it is only a guess the degree to which anatomical variances define speciation. By their nature fossils forgo that inquiry; whence arose the constant controversy in paleoanthropology: how to split species versus lumping specimens together.
There were either multiple species of early human, such as Homo habilis, Homo ergaster and Homo rudolfensis, or one highly diverse species. ~ English paleoanthropologist Jay Stark
The frequent paucity of fossil specimens at any one site makes determining Homo speciation difficult. The geographic range of these hominins compounds the accounting for differences found. There is accumulating evidence of diversity in body sizes within early Homo populations, as is seen among modern humans. This augurs for fewer species (assuming you consider the sizable diversity of Homo sapiens to be a single species).