Final answer:
Paranthropus robustus existed until about 1 million years ago and had distinctive features such as a sagittal crest to support strong jaw muscles for chewing hard plant materials. This feature contrasts with gracile australopithecines, which lacked a sagittal crest.
Step-by-step explanation:
Paranthropus robustus, part of the robust australopithecines, was discovered by Robert Broom at Kromdraai in South Africa and is distinguished by its large features, including a sagittal crest and flared zygomatic arch. These robust australopithecines evolved to have stronger jaw muscles to chew tough plant materials, as evident by these physical characteristics and the wear patterns observed on their teeth through scanning electron microscopy (SEM). The robust australopithecines, including P. robustus, continued to exist until about 1 million years ago.
The sagittal crest seen in P. robustus and other robust australopithecines was a bony ridge on top of the skull that provided additional surface area for the attachment of large temporalis muscles, essential for their heavy-chewing diet. The presence of this crest is in contrast to gracile australopithecines, which lacked such a feature. The differences in the physical morphology of these two groups highlight the varied evolutionary adaptations among early hominid species.