It is important to define what a species is, to understand what is implied by names like sapiens, neanderthalensis, heidelbergensis or erectus.
Usually, the definition used by anthropologists is that of the biological species, or a unit of animals that can interbreed. Since the inability to breed with related lineages, is unique to a lineage, they can define a species by the lack of evolutionary change, as a grade.
On the other hand, there are others define lineages by evolutionary uniqueness without considering shared features, like interfertility. They often also imply that a distinct lineage is also a biological species, but at least with the earliest members of the lineage, there was interbreeding. This definition gives more restrictive definitions than the other, which as an extreme, places Acheulians as part of the modern species.
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