The skull of the hominid Homo floresiensis appears above that of a modern human. (Peter Brown/University of New England)

The small skull of a diminutive hominid fossil found on a remote island in Indonesia seven years ago is consistent with primate evolution and not likely the result of disease, researchers say.

Scientists at Cambridge and Durham universities in Britain say the tiny brain cases of Homo floresiensis, a small hominid species first found in 2003 are part of the normal evolution of the species. The species was dubbed "the hobbit" after the diminutive creatures in J.R.R. Tolkien's Lord of the Rings series of novels.

The hobbit's tiny brain has sparked a debate among paleontologists, with some saying the size could be the result of a hormone deficiency or other disease.

The researchers used body and brain size data from 60 living and extinct primate species, including humans, and mapped the evolution of these traits over time.

They found that while both absolute brain size and brain size relative to body size generally increase over time, in some branches of the evolutionary tree, brain size did not decrease.

"We conclude that, while [natural] selection has acted to enlarge primate brains, in some lineages this trend has been reversed," the researchers wrote in their study, published this week in BMC Biology.

"Under reasonable assumptions, the reduction in brain size during the evolution of Homo floresiensis is not unusual in comparison to these other primates," said Nick Mundy of Cambridge's department of zoology, in a statement.

Since its discovery on the Indonesian island of Flores in 2003, the fossil, known as LB1, has become the subject of intense scrutiny and debate in the scientific community.

LB1 consists of the remains of an adult female less than one metre tall with a brain the size of a grapefruit.

The remains are estimated to date back some 15,000 to 18,000 years.

Critics have suggested the fossil's skull was too small to have accommodate a brain with the tool-making intelligence attributed to the humanoid after stone-flaking technology was found at the same site.

An article in Science in 2006 suggested the skull of LB1 was likely to be found in someone with a genetic illness that causes dwarfism.