The first reasonably complete skull of the earliest recognized human ancestors after the split-off from the great apes has been found near the bank of a dry riverbed in Ethiopia's arid badlands.

The skull, with its apelike heavy brow, jutting jaw and small brain case, is apparently that of a large male who lived three million years ago.

The remarkable find, which fills a serious gap in understanding early human evolution, gives a face to the species first identified and made famous by the discovery in 1974 of the headless "Lucy" skeleton. Without a skull, scientists had not been sure what these creatures looked like or exactly what Lucy's position was in the human lineage.

The discovery could thus settle some of the hotly debated issues over whether the varied fossils from this time, between 3.9 million and 3 million years ago, actually belonged to a single species, known as Australopithecus afarensis and considered the common root of the human family tree, or represented two or more species of different sizes and behavior.

In a report to be published today in the journal Nature, the discoverers said the skull confirmed the "taxonomic unity of A. afarensis," that is, their original hypothesis that these creatures belonged to one species and not two, as other paleontologists had contended.

The discoverers described the skull as not only the youngest and largest but also the only relatively intact one of the afarensis species, which lived for almost one million years in the region from Ethiopia in the north to Tanzania in the south. The famous fossil footprints at Laetoli, in Tanzania, were presumably made by an afarensis adult and child out for a walk 3.5 million years ago. This is the earliest direct evidence for upright walking by human ancestors.

The longevity of the afarensis species was remarkable in itself, the discovery team said, noting how few detectable evolutionary changes seemed to occur between the first known afarensis specimens from 3.9 million years ago and the skull and other recently discovered fossils that are 3 million years old.

The team was headed by Dr. William Kimbel, director of paleoanthropology at the Institute of Human Origins in Berkeley, Calif.; Dr. Donald C. Johanson, president of the institute, and Dr. Yoel Rak, a paleontologist at Tel Aviv University in Israel. Dr. Johanson was one of the discoverers of the Lucy fossils at a site about one mile away from where the skull was found, near the Awash River, and finally to discover a skull was a personal triumph for him.

Although their report emphasized the skull's scientific implications, Dr. Johanson said in an interview that its emotional effect could not be discounted.

"Until you have a skull, until you can look into one of those big orbits or hold a cranium in the cup of your hand," he said, "some people are not really satisfied that you have a new species."

Commenting on the skull's importance in an accompanying journal article, Dr. Leslie C. Aiello, a paleontologist at University College, London, said the skull and other recent findings "provide persuasive support for the idea that A. afarensis is a single, highly dimorphic species," that is, one with a large difference between men and women. He said this should "go a long way to settle some of the most heated controversies surrounding the earliest species in the human lineage." Hypothesis Challenged

The single-species hypothesis had been challenged by scientists who studied the striking variations in the size of afarensis fossils and decided that they were too pronounced to be included in one species. In the alternative view, larger-boned individuals represented a separate "robust" species, now extinct, which lived at the same time as the smaller species, represented by Lucy, which evolve into the Homo lineage, leading eventually to modern humans, Homo sapiens.

In this view, the two distinct lines -- one leading to humans and the other to later australopithecines, a branch that became extinct one million years ago -- had already diverged by three million years ago. Like many paleontologists, Dr. Kimbel's group thinks that primitive upright-walking hominids -- humans and their extinct ancestors and relatives -- did not diversify into discrete lineages until some time in the half a million years after the three-million-year-old skull.

After an analysis of recent fossil discoveries, Dr. Kimbel's group attributed the size differences to sexual dimorphism. The males in the fossil record were considerably larger and heavier than the females like Lucy. The females tended to be no more than three and a half to four feet tall and weigh about 75 pounds. The males seemed to be at least five feet tall and weigh at least 110 pounds.

As Dr. Aiello said, "It would seem that all australopithecines were highly dimorphic and that the range of size dimorphism in A. afarensis is not particularly unique." The characteristic is even more pronounced among the great apes and continues to a lesser extent among modern humans. Caution Is Advised