Scientists announced yesterday the discovery of skull and leg bones of an upright ape-man who lived in East Africa 4 million years ago, the oldest evidence ever found of what some anthropologists believe was the forerunner of the family of man.

The fossil bones predate by nearly a half-million years the previous oldest evidence of a creature that walked upright like a man but had a brain smaller than a chimpanzee's.

The discovery also amasses new and significant evidence on one side of a vigorous and occasionally bitter argument that has raged for several years among anthropologists, according to Tim D. White, one of those who made the announcement. The argument revolves around how many types of ape-man existed millions of years ago, and which of those led to man.

The discovery was made in November and December last year during an expedition to north-central Ethiopia. The scientists delayed announcing the find until the age of the bones could be confirmed by experiments.

Bones from the skull of one creature and the top of a thigh bone from another were found in the Middle Awash River Valley in Ethiopia, at a site that was being mapped and prepared for several years of searches.

The bones indicate that the creature stood about 4 1/2 feet high and walked as upright as modern man.

But in other ways he remained like an ape, with a brain smaller than a chimpanzee's, and no evidence of tool use was found in association with the bones.

According to the leaders of the expedition, White and J. Desmond Clark, both of the University of California at Berkeley, the creature belonged to the species called Australopithecus afarensis.

They said that this line of ape-man leads directly into the line of development that runs from Homo habilis to Homo erectus and finally to Homo sapiens.

It is the first substantial fossil find of this sort since Donald C. Johanson of the Cleveland Museum of Natural History found in 1974 the extraordinarily complete and well-preserved skeleton of a young ape-woman he called Lucy.

Johanson created the new species-name "afarensis" to apply to her and creatures of her type, which he said stand as direct ancestors to man.

This view suggested that afarensis was the sole type of ape-man from which man descended, and challenged the wider view that two or more species of ape-man existed about 3 to 4 million years ago, and that one of these others might be man's ancestor.

Among Johanson's opponents, those most visible are the Leakey family: anthropologist Mary, the widow of pioneer anthropologist Louis B. Leakey, and their son, Richard, of the National Museum of Kenya, also an eminent researcher on the history of man.

The debate peaked on a CBS television show last September, when Johanson and Richard Leakey were asked to draw their versions of the human descendancy.

Johanson drew his radical version, then Leakey crossed it out. Asked what he would replace it with, he replied:

"A question mark."

The discoverers of the new fossils announced yesterday said the bones supported the existence of the separate species afarensis, which they said had become an upright walker long before evidence of a large brain or the use of tools appeared in the human family tree.

White said:

"What the debate ends up with now is, on the one hand you have Johanson with a hypothesis and a number of fossils to support it.

"And on the other hand you have the Leakeys with a hypothesis and a family tradition, and they are still out there looking for the evidence.

"Is it possible the Leakeys will find it? Yes."

Harvard anthropologist David Pilbeam said that the fossil evidence can be interpreted in more than one way, and that while the find announced yesterday may be very important it does not necessarily support White's and Johanson's view exclusively.

White said that one scenario among many possible ones to explain the emergence of an upright ape-man is that afarensis, in competing with monkeys and other animals for food, began to learn an early form of provisioning and storing, behavior which made him walk upright while carrying foods.

The Ethiopian expedition reported yesterday was the beginning of a several-year project planned for a 20- by 50-square-mile area in the Awash River Valley.

The expedition has been sponsored by the National Science Foundation and the L.S.B. Leakey Foundation, and has included among its more than 20 members scientists from Ethiopia, Japan, Canada, England, Australia and New Zealand.

The team brought back not only the two premier fossils but new evidence of human tool use as long ago as 1.5 million years, and a thorough fossil record of a large variety of animals from 6 million to less than 1 million years ago.