By Will Dunham

WASHINGTON (Reuters) - Fossils unearthed on the Indonesian island of Flores may resolve one of the most intriguing mysteries in anthropology: the ancestry of the extraordinary diminutive human species dubbed the "Hobbit."Scientists on Wednesday described bone fragments and teeth about 700,000 years old retrieved from an ancient river bed that appear to belong to the extinct Hobbit species, previously known only from fossils and stone tools from a Flores cave ranging from 190,000 to 50,000 years old.

The species, called Homo floresiensis, stood about 3-1/2 feet tall (106 cm), possessing a small, chimpanzee-sized brain.

The new fossils "strongly suggest" the Hobbit evolved from large-bodied, large-brained members of the extinct human species Homo erectus living in Asia, said palaeoanthropologist Yousuke Kaifu of the National Museum of Nature and Science in Tokyo.

Homo erectus, which first appeared in Africa roughly 1.9 million years ago, is known from numerous fossils 1.5 million to 150,000 years old from Java, an Indonesian island west of Flores, and the new Flores fossils bear similarities to those, said paleontologist Gerrit van den Bergh of Australia's University of Wollongong.

The fossils included four adult and two baby teeth, a piece of jawbone and a cranial fragment from two children and either one or two adults who may have died in a volcanic eruption. They were dug up during excavations in grasslands nearly 45 miles (70 km) east of the cave where the first Hobbit bones were discovered in 2003.

The jawbone's size suggested the individual was even a bit smaller than the later cave remains.

Previously discovered stone tools suggest the Hobbit's big-bodied ancestors reached Flores a million years ago, indicating the species shrank during 300,000 years of evolution.

"It now appears that the Flores 'Hobbit' is indeed a dwarfed Homo erectus," said archaeologist Adam Brumm of Australia's Griffith University.

The research was published in the journal Nature.

Size reduction that occurs over many generations of larger mammal species, such as elephants, that somehow reach a new island habitat is called the "island rule," driven by limited food resources on islands.

Brumm said the 700,000-year-old fossils rule out claims by some scientists that the Hobbit was a member of our species with a medical condition causing small size. Homo sapiens first appeared in Africa about 200,000 years ago.

Characteristics of the fossils also do not support the idea the Hobbit evolved from even more ancient members of the human family tree like Homo habilis or Australopithecus, the researchers said.

(Reporting by Will Dunham; Editing by Paul Simao)