So far, more than 20 hominid skull fossils have been found at sites in Java. The latest, dubbed Sm 4 (see image), was recovered from the bed of the Solo River in central Java and is one of the largest yet discovered on the island. Hisao Baba of the University of Tokyo and his colleagues analyzed the skull, comparing it to previously discovered specimens. They found that Sm 4 shares certain characteristics, such as a flat top, with skulls dating to more than a million years ago. But Sm 4 also exhibits similarities to much younger fossils from Eastern Java--the shape of a nerve opening near the temple, for example. The researchers thus conclude that Sm 4 is an intermediary between earlier and later Javanese H. erectus. This indicates that the species lived on the island continuously for more than a million years, contrary to the hypothesis that distinct, consecutive migrations to the area occurred.
Sm 4 also presents the best look yet at a particular feature of H. erectus's head known as the cranial base, a bony shelf behind the eyes that helps to support the brain. Using computer imaging to probe the skull's interior, the team found that the cranial base is surprisingly modern in being sharply angled, or flexed. This, the researchers note, suggests that the larger brains of modern humans evolved independent of changes to the support on which they rest.