How old is man?
Anthropologist Phillip Tobias, who has spent a career studying fossils in Africa looking for an answer, no longer tackles the question head-on. Wary and sensitive to the debate on creationism vs. evolution, he smiles slyly and asks, ''What is your definition of man?''
The definition of man that concerns Professor Tobias, dean of medicine at the University of the Witwatersrand, is confined to physical structure and its evolution. Professor Tobias regards Africa as the most likely source of the physical origins of homo sapiens. He figures man, broadly defined, and ape ''parted ways'' sometime between 4 and 8 million years ago.
That homo sapiens' origins might lie in Africa came as a jolt to the world in 1924. In that year a small skull found at Taung, South Africa, suggested to evolutionists a creature between ape and homo sapien.
It took another find in 1936 at the Sterkfontein caves to convince a skeptical scientific world that the ''Taung Baby'' was not a fluke and that upright ancestors of homo sapiens may well have taken their first steps in Africa. Conventional wisdom had placed these ancestors in Asia.
Today, some 46 years later, Professor Tobias is plunging deeper in time in Sterkfontein for new clues to early man. Field work here resumed in 1966 and has gone on ever since, making Sterkfontein the oldest continuous excavation site in the world, according to Mr. Tobias.
The area has been the richest source of Australopithecus (southern ape) Africanus (Africa) -- the species of early homo sapien that some anthropologists believe was the main branch of evolution leading to man as he appears today. It is also a popular tourist attraction, drawing 40,000 visitors a year.
However, there has been a strong challenge to the positioning of A. Africanus as the main ancestral line of modern man.
Donald Johanson's 1974 discovery of ''Lucy'' in Ethiopia led him to conclude she represented a new species dating back 3 or 4 million years. Mr. Johanson claims Australopithecus Afarensis -- named for its discovery site in Afar, Ethiopia -- is the ''true'' ancestral line to homo sapiens.
One thing Professor Tobias hopes to finddeeper in Sterkfontein is a version of A. Africanus, comparable in age to Lucy. Such a discovery might help support his belief that Lucy is not a new species but an early version of the species at Sterkfontein.
Sterkfontein is the summit of a low hill in a gently sloping valley some 40 minutes driving time from Johannesburg. The sound of hammer cracking against rock sets it apart from the quieter agricultural pursuits of its neighbors.
Even among archeological excavation sites, Sterkfontein is noisier than most. In many sites a toothbrush and dental pick are all that is necessary for scraping crusted dirt from fossils, but at Sterkfontein the fossils are contained in solid rock. Dynamite and drills are the tools of the trade here.
In addition to the problems of working the rock-bound Sterkfontein site, it has been exceptionally difficult to date. But considerable progress has been made in determining the ages of the cave deposits, now divided into six time periods, or ''members.''
The member that has proved so rich in australopithecines is now dated at 2.5 -to-3 million years ago -- far older than ever imagined by Dr. Robert Broom, who made the 1936 discovery.
The stratum containing australopith-ecines is roughly in the middle of the six layers of deposits. It is No. 4, with member 6 being closest to the surface and most recent.
Work has been continuing both above and beneath layer 4. Stone tools were found in the upper levels of Sterkfontein in 1956. It was a somewhat puzzling find, since tools were generally regarded as too advanced a development for australopithecines.
In 1976 the user of the tools was found, and determined by its skull and teeth to be a step closer to modern homo sapien than A. Africanus. This toolmaking creature is called homo habilis and has also been found in East Africa. Its age at Sterkfontein is placed at about 2 million years.
It is the digging at deeper levels at Sterkfontein, representing perhaps an age of 3.6 million years, that excites Professor Tobias. At the bottom of the Sterkfontein cave site may lie fossils of hominids (humans and their upright-walking ancestors) as old as any found in East Africa.
''The big question now is what is in members 3, 2, and 1. We haven't yet got hominids out, but we're sure he's there waiting,'' Professor Tobias says.
His confidence stems from the discovery of baboon remains in the deeper levels of Sterkfontein. In the past such finds have always preceded the discovery of hominids.