The sagittal keel is a feature seen in many Homo erectus specimens. It refers to a ridge or bump that runs along the midline of the skull, from the forehead to the back of the head. The sagittal keel is formed by a thickening of the bone along this ridge. The sagittal keel serves as an attachment site for strong neck muscles. These muscles would have helped support and stabilize the head, especially during activities that required a lot of physical exertion, such as running or hunting. The presence of a sagittal keel suggests that Homo erectus had powerful neck muscles, indicating that they were likely adapted for an active lifestyle. This feature is distinct from the sagittal crest, which is found in some other hominin species like the robust australopithecines. While the sagittal crest also runs along the midline of the skull, it is a much larger and more pronounced ridge, indicating even stronger jaw muscles for chewing tough foods. In summary, the sagittal keel is a ridge along the midline of the skull seen in many Homo erectus specimens. It suggests the presence of strong neck muscles, indicating an active lifestyle.