Final answer:
The earliest species of robust australopithecine is Paranthropus aethiopicus, which dates back to about 2.5 million years ago and exhibits features of both robust and gracile australopithecines.
Step-by-step explanation:
The earliest species of robust australopithecine is Paranthropus aethiopicus. This species has been dated to about 2.5 million years ago (MYA) and is known from the discovery of the "black skull" in Ethiopia. Paranthropus aethiopicus is characterized as having features of both robust and gracile australopithecines. A distinguishing feature of robust australopithecines like Paranthropus aethiopicus is the large grinding teeth and the adapted morphology for a diet of hard nuts and seeds, as opposed to the more varied or even partially carnivorous diet of gracile species.