When did modern-day humans first appear?

On the biggest steps in early human evolution scientists are in agreement. The first human ancestors appeared between five million and seven million years ago, probably when some apelike creatures in Africa began to walk habitually on two legs.

They were flaking crude stone tools by 2.5 million years ago. Then some of them spread from Africa into Asia and Europe after two million years ago.

With somewhat less certainty, most scientists think that people who look like us -- anatomically modern Homo sapiens -- evolved by at least 130,000 years ago from ancestors who had remained in Africa. Their brain had reached today's size. They, too, moved out of Africa and eventually replaced nonmodern human species, notably the Neanderthals in Europe and parts of Asia, and Homo erectus, typified by Java Man and Peking Man fossils in the Far East.

But agreement breaks down completely on the question of when, where and how these anatomically modern humans began to manifest creative and symbolic thinking. That is, when did they become fully human in behavior as well as body? When, and where, was human culture born?

''It's the hot issue, and we all have different positions,'' said Dr. John E. Yellen, an archaeologist with the National Science Foundation.

For much of the last century, archaeologists thought that modern behavior flowered relatively recently, 40,000 years ago, and only after Homo sapiens had pushed into Europe. They based their theory of a ''creative explosion'' on evidence like the magnificent cave paintings in Lascaux and Chauvet.

But some rebellious researchers suspected that this theory was a relic of a time when their discipline was ruled by Eurocentrism. Archaeologists, the rebels contended, were simply not looking for earlier creativity in the right places.

Several recent discoveries in Africa and the Middle East are providing the first physical evidence to support an older, more gradual evolution of modern behavior, one not centered in Europe. But other scientists, beyond acknowledging a few early sparks in Africa, remain unswayed. One prominent researcher is putting forward a new hypothesis of genetic change to explain a more recent and abrupt appearance of creativity.

The debate has never been so intense over what archaeologists see as the dawn of human culture.

''Europe is a little peninsula that happens to have a large amount of spectacular archaeology,'' said Dr. Clive Gamble, director of the Center for the Archaeology of Human Origins at the University of Southampton in England. ''But the European grip of having all the evidence is beginning to slip. We're finding wonderful new evidence in Africa and other places. And in the last two or three years, this has changed and widened the debate over modern human behavior.''

The uncertainty and confusion over the origin of modern cultural behavior stem from what appears to be a great time lag between the point when the species first looked modern and when it acted modern. Perhaps the first modern Homo sapiens emerged with a capacity for modern creativity, but it remained latent until needed for survival.

''The earliest Homo sapiens probably had the cognitive capability to invent Sputnik,'' said Dr. Sally McBrearty, an anthropologist at the University of Connecticut. ''But they didn't yet have the history of invention or a need for those things.''

Perhaps the need arose gradually in response to stresses of new social conditions, environmental change or competition from nonmodern human species. Or perhaps the capacity for modern behavior came late, a result of some as yet undetected genetic transformation.

Dr. Mary C. Stiner, an archaeologist at the University of Arizona, said those contrasting views, or variations of them, could be reduced to this single question: ''Was there some fundamental shift in brain wiring or some change in conditions of life?''

Sudden Genetic Advance

The foremost proponent of the traditional theory that human creativity appeared suddenly and mainly in Europe is Dr. Richard G. Klein, a Stanford archaeologist. He describes his reasoning in a new book, ''The Dawn of Creativity,'' written with Blake Edgar and being published next month by John Wiley.

''Arguably, the 'dawn' was the most significant prehistoric event that archaeologists will ever detect,'' the authors write. ''Before it, human anatomical and behavioral change proceeded very slowly, more or less hand in hand. Afterward, the human form remained remarkably stable, while behavioral change accelerated dramatically. In the space of less than 40,000 years, ever more closely packed cultural 'revolutions' have taken humanity from the status of a relatively rare large mammal to something more like a geologic force.''

In that view, 40,000 years ago was the turning point in human creativity, when modern Homo sapiens arrived in Europe and left the first unambiguous artifacts of abstract and symbolic thought. They were making more advanced tools, burying their dead with ceremony and expressing a new kind of self-awareness with beads and pendants for body ornamentation and in finely wrought figurines of the female form. As time passed, they projected on cave walls something of their lives and minds in splendid paintings of deer, horses and wild bulls.

As an explanation for this apparently abrupt flowering of creativity, Dr. Klein has proposed a neurological hypothesis. About 50,000 years ago, he contends, a chance genetic mutation in effect rewired the brain in some critical way, possibly allowing for a significant advance in speech. The origin of human speech is another of evolution's mysteries. Improved communications at this time, in his view, could have enabled people ''to conceive and model complex natural and social circumstances'' and thus give them ''the fully modern ability to invent and manipulate culture.''

Although this transformation, with the genetic change leading to the behavioral change, occurred in Africa, Dr. Klein writes, it allowed ''human populations to colonize new and challenging environments.''

On reaching Europe, the rewired modern humans, called the Cro-Mag nons, presumably outsmarted the resident Neanderthals, driving them to extinction by 30,000 years ago, and leaving their indelible cultural mark on the land.

Dr. Klein concedes that the idea ''fails one important measure of a proper scientific hypothesis -- it cannot be tested or falsified by experiment or by examination of relevant human fossils.''

Skulls from that time show no change in brain size and are highly unlikely to show a genetic change in the brain's functioning. Although he considers the idea the most straightforward explanation, critics object that such a concept of an abrupt ''human revolution'' is too simplistic, as well as being unprovable.

Besides, other archaeologists think it misguided to key interpretations so closely to the Cro-Magnon creative explosion, dazzling as it was. Such thinking might have been understandable, they say, when few archaeologists had investigated earlier sites elsewhere, and the little they found could not -- and still cannot -- match the artistic magnificence of Lascaux and Chauvet.

The Blombos Discoveries

But the Eurocentrism of old may have sown the seeds of its demise. As Dr. Yellen points out, the increasing research into the origins of modern behavior has been driven in part by a lively interest in explaining the source and nature of Cro-Magnon superiority in overwhelming the Neanderthals.

In the last 30 years, scientists have learned that the Cro-Magnons originated in Africa and the Neanderthals seem to have evolved exclusively in Europe. So archaeologists have begun searching more diligently in Africa for what they generally agree are attributes of early modern behavior like more complex stone technology, the introduction of tools made of bone, long-distance trade, a more varied diet, self-ornamentation and abstract designs carved on tools and ocher.

In a comprehensive study two years ago, Dr. McBrearty at UConn and Dr. Alison S. Brooks of George Washington University said the many artifacts indicative of modern behavior in Africa did ''not occur suddenly together, as predicted by the 'human revolution' model, but at sites that are widely separated in space and time.''

''This suggests,'' the scientists said, ''a gradual assembling of the package of modern human behaviors in Africa and its later export to other regions of the Old World.''

Exploring a cave at the southern tip of Africa, for example, Dr. Christopher Henshilwood of the South African Museum in Cape Town found evidence that the anatomically modern people there were turning animal bones into awls and finely polished weapon points more than 70,000 years ago.

The skill for making such bone tools is considered more advanced in concept and application than that required in producing the usual stone tools.

Three weapon points, in particular, appear to have been shaped first with a stone blade and then polished, probably with a piece of leather and a mineral powder.

''Why so finely polished?'' Dr. Henshilwood asked. ''It's actually unnecessary for projectile points to be so carefully made. It suggests to us that this is an expression of symbolic thinking. The people said, 'Let's make a really beautiful object.' ''

Symbolic thinking, scientists explain, is a form of consciousness that extends beyond the here and now to a contemplation of the past and future and a perception of the world within and beyond one individual. Thinking and communicating through abstract symbols is the foundation of all creativity, art and music, language and, more recently, mathematics, science and the written word.

Last month, Dr. Henshilwood reported details of an even more striking 77,000-year-old find at the Blombos Cave site. Two small pieces of ocher, a soft red iron oxide stone, had been inscribed with crisscrossed triangles and horizontal lines. The decoration, made by the same cave dwellers, was more evidence, the archaeologist said, that ''we're pushing back the date of symbolic thinking in modern humans -- far, far back.''

Previous excavations in the Katanda region of Congo yielded barbed harpoon points carved out of bone 80,000 to 90,000 years ago. Dr. Brooks and Dr. Yellen, her husband, found that these ancient people ''not only possessed considerable technological capabilities at this time, but also incorporated symbolic or stylistic content into their projectile forms.''

The dating of the Blombos discoveries, once suspect, is now generally accepted by other archaeologists. But a few have challenged the interpretations. If the artifacts are really that old and represent a basic change in human culture, why are they not showing up all over?

Noting that no similar artifacts had been found in at least 30 other sites in the region of Blombos, Dr. Klein said the ''unique find'' did not justify a revision of ideas about when and where modern behavior began.

Dr. Yellen disagrees. The population of modern Homo sapiens then was small and probably widely scattered, he explained, and so ideas and cultural practices might have been slow to travel among different groups.

''Think about trying to start a fire with too little tinder,'' Dr. Yellen said. ''You make sparks. But it takes a certain density of the stuff before the fire is going to catch and go somewhere. So when you don't have other people in your face, you probably won't get or don't need the richness of behavior that came later.''

The Social Factor

Variations on this theme are offered in other attempts to explain scattered finds suggesting the presence of modern cultural behavior outside Europe before the Cro-Mag non efflorescence.

Dr. Stiner and her husband, Dr. Steven L. Kuhn, both archaeologists at the University of Arizona, said their research in Turkey and Lebanon showed that people around 43,000 years ago were making and wearing strings of beads and shell ornaments of highly repetitive designs. Some of the shells were relatively rare marine varieties, luminous white or brightly colored. The bone of an eagle or vulture was incised for suspension as a pendant.

These were presumably objects of social communication, readily conveying information about kinship, status and other aspects of identity to outsiders.

''Ornamentation is universal among all modern human foragers,'' Dr. Stiner said. Not to mention in complex societies that send social signals with wedding rings, designer clothes and hot-label sneakers.

At the Mediterranean coastal dig sites of the Ucagizli Cave in Turkey and Ksar Akil in Lebanon, in the corridor of migrations into Eurasia, the two archaeologists also found remains of animal bones, indicating a marked change in diet over time. The people there were eating fewer deer, wild cattle and other large animals. They seemed to be hunting and gathering fewer of the slow-reproducing and easy-to-catch animals like shellfish and tortoises and more of the agile animals like birds and hares.

Their living conditions had changed, Dr. Stiner and Dr. Kuhn surmised, and one cause could have been population increases that pressured their resources. Not that the region suddenly teemed with people, but where populations had been sparse, even modest increases could double or triple their numbers, forcing them to turn to lower-ranked food sources.

Families and groups would be living in closer proximity, with more occasions to interact, which could account for the creation of so many body ornaments as part of a shared system of communication, signaling from afar to outsiders one's group identity and social status

''Expressions of who you are had become much more important,'' Dr. Stiner said.

In a report in June in The Proceedings of the National Academy of Sciences, the two archaeologists noted that this ''habitual production and use of standardized ornaments first appeared at about the same time'' at two other widely separated sites, in Kenya and Bulgaria. That implied ''the existence of certain cognitive capacities and that these evolved relatively late in prehistory,'' the two researchers said, but they were probably not a consequence of a sudden genetic mutation.

''The fact that traditions of ornament making emerged almost simultaneously in the earliest Upper Paleolithic/Late Stone Age on three continents argues strongly against their corresponding to a specific event in the cognitive evolution of a single population,'' Dr. Stiner and Dr. Kuhn said.

Dr. Gamble, a visiting professor at Boston University this semester, attributed this changed behavior less to specific population pressures than to generally increasing social competition. The response was new strategies for procuring food, sharing ideas and knowledge and organizing their societies. This would have been an advantage to societies as they moved into new lands and dealt with new circumstances, including non-modern humans they came in contact with.

''Population pressure didn't get us to the moon in 1969,'' he said. ''There was social competition in the cold war. That's an extreme example, but something like that is what we are seeing in the form of an intensification of social life'' at the sites in Turkey and Lebanon.

Along the same lines, Dr. Randall White, an archaeologist at New York University who specializes in Cro-Magnon creativity, said findings of early personal adornment in Africa and the Middle East indicated that the capacity was there and latent long before modern humans reached Europe.

''Already,'' Dr. White said, ''people had a capacity for symbolic thinking. That's important. Then they invented it in response to a certain set of circumstances.''

Modern humans, in the face of competition, tapped inner resources for cultural attributes, enabling them to maintain a common identity, communicate ideas and organize societies into ''stable, enduring regional groups,'' Dr. White said. They thereby established a decisive edge over the Neanderthals in Europe and, among other advances, the start of representational art.

The debate over the origins of modern human culture is far from resolved, and with the quickening pace of excavations, the issues may grow even more complex and confused. As archaeologists remind themselves, culture today is hardly uniform from place to place, and it probably never was.