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Why is KNM-ER 1805 difficult to classify as a hominin species?

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An illustration of Homo habilis, an early human ancestor, shown walking upright with a hairy body and ape-like features. A scale beside the figure indicates height in inches and centimeters. — Homo habilis Artist's rendering of *Homo habilis*, which lived from 2 to 1.5 million years ago.

The specimen KNM-ER 1805 is difficult to classify because it exhibits a combination of conflicting anatomical features that blur the lines between different hominin groups.1

While the braincase of ER 1805 is expanded to approximately 600 cc—a size moderately larger than what is expected in Australopithecus—it also possesses a sagittal crest along the top of the skull and a prominent crest across the rear. These ridges indicate powerfully developed chewing and neck muscles, a pattern of cresting typically seen in robust australopiths rather than in the genus Homo.1

Because the skull displays these "robust" features alongside other characteristics that are considered Homo-like, anatomists have disagreed on its proper taxonomic assignment. Despite these anomalies, the specimen is frequently grouped with Homo habilis.1