For Rutgers University paleoanthropologist Robert Blumenschine and his international team, it was the fossil find of a lifetime.

On the fringe of Tanzania's Serengeti Plain in 1995, they uncovered a 1.8-million-year-old upper jaw from Homo habilis, one of the earliest humans.

Now their discovery is a bone of contention.

In a scholarly paper published today, Blumenschine's team is using its find to shake up the human family tree. It challenges whether a prior discovery, dubbed Homo rudolfensis, was a separate species after all.

Trivial stuff to many, maybe. But it's a big deal to anyone who has dodged lions and leopards under the sub-Saharan sun to sift through humanity's petrified past.

5 MILLION YEARS OF CONTROVERSY PAGE 22 "Paleoanthropologists will bicker what to name things till the cows come home," said Blumenschine.

The controversy centers on a 1972 find by Richard Leakey, son of famed paleoanthropologists Louis and Mary Leakey. He led a team in northern Kenya that uncovered a skull known in textbooks as ER 1470.

Russian anthropologist Valerii Alexeev thought the skull's larger size and longer face indicated a species distinct from Homo habilis. So he christened it Pithecanthropus rudolfensis, after a lake area where the skull was discovered. (Others substituted the more familiar genus Homo.)

But hold on. Blumenschine's team says its jawbone -- which includes teeth and some facial bone -- has proportions and features similar to the skull's.

So similar that Leakey's discovery should be lumped with the species Homo habilis.

"This should resign rudolfensis to the history books," Blumenschine told The Star-Ledger this week.

Carl Swisher, a Rutgers geologist who helped date the Tanzanian jawbone, is not as sure.

"If you talk to five different anthropologists, you will get five different views on that," Swisher said.

To further stir things up, Blumenschine colleagues Ron Clarke, a fossil surgeon from South Africa, and Charles Peters of the University of Georgia say other Tanzanian fossils described as Homo habilis really may be something else -- a smaller- brained species, as yet unnamed.

The process of naming species is called taxonomy, and it can produce these kinds of sparks among scientists.

"This problem is not unusual," writes South African researcher Phillip Tobias in a commentary that accompanies Blumenschine's paper in the journal Science.

"We are living at a time when hominin samples from just one or two sites are generating a flurry of new species and genera," asserted Tobias, who helped name Homo habilis in 1964. "Hominin" refers to humanlike primates.

Naming our ancestors is crucial to studying them, said Ian Tattersall, anthropology curator at the American Museum of Natural History. "You'll never understand the play if you don't know who the actors are," he said.

Tattersall welcomes Blumenschine's claims: "This will open the way for a lot of discussion. There's plenty of room for controversy."

There is room because precious little is known about Homo habilis. No complete skeletons have been found; most of what is known comes from a few dozen fragments.

Homo habilis was discovered in 1960 by a Leakey team, which unearthed a lower jaw on the eastern side of Tanzania's Olduvai Gorge, a fossil treasure trove in East Africa. Blumenschine's team made its 1995 find on the western side of this ancient lake.

Perhaps 5 feet tall, Homo habilis scavenged for food and probably sought refuge in trees, Blumenschine said.

The species' use of flaked- stone tools, for butchering fresh kills abandoned by predators, distinguished it from another cousin, Paranthropus (Australopithecus boisei).

Homo habilis also had a larger brain -- almost half the size of the modern organ, Blumenschine said.

His team uncovered stone tools and bones of gazelles, goats and extinct horses that, in some cases, showed signs of being sliced or hammered.

The jaw was discovered beneath a slope by Augustino Venance, a Tanzanian, and former Rutgers graduate student Amy Cushing.

Tagged OH65, the fossil awaits further study at the National Museums of Tanzania in Arusha.

Based on the fairly good condition of the jaw's 16 teeth, researchers believe the specimen came from an adult in the prime of life.

A "banding" condition of the enamel suggests growth disruptions, perhaps from disease or seasonal food shortages, said Blumenschine.

It took almost eight years to publish the discovery, partly because scientists wanted to be certain of the fossil's age. They used three methods.

Rutgers graduate student Lindsay McHenry applied geological "fingerprinting," comparing volcanic rock at the discovery site with grains already dated elsewhere at Olduvai Gorge.

Scientists with mass spectrometers estimated age based on known decay rates of radioactive isotopes, found in volcanic ash and glass at the site. The technique is accurate within 10,000 years, said Swisher.

Finally, Swisher used a magnetometer to determine the alignment of small magnetic grains within rocks at the excavation site.

Blumenschine, 48, grew up watching Marlin Perkins' "Wild Kingdom" and reading about the Leakeys. He has spent summers at Olduvai Gorge since 1989. His forte is prehistoric tools and their uses for preparing food.

Controversy aside, Blumenschine said he considers his work an antidote to modern strife.

"Here we have something from the very remote past, an ancestor of everybody on the planet," he said. "It helps us to better understand what humans are all about."