ILYA PRIGOGINE, who won the Nobel prize in 1997 in chemistry for the discovery of dissipative structures, is considered the father of "chaos theory." Prigogine died earlier this year after preparing this column for NPQ's Nobel laureate series.

Brussels -- Fernand Braudel, the economic historian, once wrote that "les evenements sont poussiere" (events are passing), insisting on the shaping importance of the "longue duree." Federico Mayor, the biochemist and former head of UNESCO, has argued the opposite -- that the discontinuity of history suggests that events play a central role.

What do we mean by "events"? We would probably agree that, in the 20th century, the Russian revolution of 1917 and the fall of the Berlin Wall were "events." Either one might, or might not, have happened. But the position of the moon in 1,000 years would not be considered an event because it can be predicted by Newton's laws of the motion of bodies.

But are there also events in nature? Social science has long questioned deterministic laws because of the free will human societies have to make choices. But, might it be possible that natural systems are far less bound by deterministic laws than physics and biology have so far been willing to admit? Nature's form of choice is what I call "bifurcation." In complex natural systems, just as in society, the future is not given. Within bounds, it can go this way or that. Events in nature correspond to novelties that we find at all levels from cosmology to microbiology. We may thus speak of the "creativity" of nature.

The diversity of species is perhaps the most obvious example of nature's creativity. For example, there are about 12,000 species of ants known today. It is very difficult to imagine that all of these species were already programmed at the time of the Big Bang.

Though the past may now seem to have been determined, it is actually one realization among many possibilities that could have occurred. Similarly, the future is not determined because there will be events, the outcome of which we cannot predict.

The more we study nature, the more we are impressed by its complexity, with even the smallest particles organizing themselves during a state of disequilibrium and apparent disorder.

Complexity is the outgrowth of the far from equilibrium situations which are preeminent in nature. A classic example of complexity in hydrodynamics is the Benard vortices.

If we heat a liquid from below, we will at first have only thermic conductivity from the lower, hotter layer to the upper colder layer. But if we increase the difference of temperatures we will see the formation of vortices that look like small tornadoes. This formation corresponds to billions of particles whose motion is correlated to the motion of surrounding particles, producing large-scale organized flows.

In these Benard vortices (named after the scientist who discovered them), we see a very simple case in which a system evolves from macroscopic disorder at a state of near equilibrium (when the temperature of the upper and lower layer is almost the same) to remarkable macroscopic organization due to molecular interactions under constraints that are far from equilibrium.

The case of the Benard vortices would seem to repeal the usual formulation of the second law of thermodynamics, which emphasizes the destructive role of entropy, or the dissipation of energy. Instead, as we have seen, the distance from equilibrium can lead to the formation of new structures.

We do not yet know the origins of the Big Bang, but we can imagine that it was preceded by what is called a "quantum vacuum" in which no particles were present, only quantum fluctuations. If particles were thus created at the moment of the Big Bang, and since particles are associated to an entropy, it means that the Big Bang would be the irreversible process "par excellence." From that moment on an "arrow of time" has kept moving ahead in which entropy doesn't just lead to dissipation, but to the possibility of events and new structures not already determined. We can now see that evolution is associated with entropy.

The classical ideal of science was to describe nature as a geometry. Now we see that nature is closer to biology and human history since there is a narrative element in nature as well -- a story of paths taken or not taken. Indeed, the more we understand the structure of the universe, the more it begins to have common elements with human societies.

Let us go back to events. Events are not isolated, but the result of the interaction of various influences.

In the 1917 revolution in Russia, the end of the czarist regime could have taken different forms. The outcome which finally emerged was the result of numerous factors, including the weakness of the czar, the unpopularity of his wife, the hesitations of Kerensky and the determination of Lenin. These "microstructures" and their fluctuating fates determined the eventual outcome of the crisis.

Modern science has led us to a better understanding of the mechanism of the event. In physics or chemistry, events are associated with bifurcations. When we follow the trajectory of a system, there may appear situations where that trajectory becomes more and more unstable and then breaks into a multiplicity of new trajectories.

Which of these branches will be chosen is a question of probability. The smallest fluctuation may determine the future of billions of particles, organizing themselves into a supermolecular order.

I have always thought that the idea of bifurcation is a useful metaphor for the social sciences. Of course, I don't mean to suggest that human sciences should be reduced to physics. But understanding the science of complexity is a far more useful metaphor than the traditional appeal to Newtonian physics.

Human history may be viewed as a succession of bifurcations, such as, for example, the transition from the Paleolithic to the Neolithic age, which happened at almost the same time all over the world. This transition appears as a bifurcation linked to a new systematic exploitation of vegetable and mineral resources. Yet, many branches emerged from this bifurcation, such as the Chinese Neolithic, the Near Eastern Neolithic or the Pre-Columbian Neolithic. Each bifurcation had beneficiaries as well as victims.

The transition to a Neolithic age gave rise to hierarchical societies as the division of labor established inequality. That, in turn, led to slavery, which continued to exist all the way through the 19th century.

I am convinced that we are approaching a bifurcation of similar magnitude that is connected to the explosion of information technology. We are coming closer to a "networked society" as people are linked more tightly together than ever before. This future holds both great promise and peril. But what might be the result of this bifurcation? To put the question in the long perspective of biological evolution: Will the networked society act more like large ant colonies or a civilization of free people?

The larger the human population becomes, the more possibilities there are for non-linear fluctuations -- individual choices -- because there are more and more players. On the other hand, as that population becomes more networked, there may be an opposite effect: the imperatives of the connected collective overwhelm the individual ability to make choices.

Let us look at biological evolution. There exist many different ant societies; some are small, including a few hundred individuals; some are large, including a few million individuals. Their social lives are very different. In small ant societies, the individual ants behave independently, going out on their own to forage and bring back their prey.

In large societies, we see collective motions. Accordingly, the role of individual activity is drastically reduced. Human societies could face the same evolution when tied together by networks.

Years ago I studied traffic flows. I found that when the flow of traffic was light, each driver behaved more or less as he wanted. I called that an "individual regime." But when the flow becomes more and more dense, the characteristics of a "collective regime" take over in which everybody is pushing the other and is pushed by the other.

These ideas do not suggest a pleasant future.

To bring science closer to human perception has been the main aim of my work. The task of this endeavor is to find that narrow way between the deterministic sciences which make of man an automaton and a world open to chance.

The future is not given. Especially in this time of globalization and the network revolution, behavior at the individual level will be the key factor in shaping the evolution of the entire human species. Just as one particle can alter macroscopic organization in nature, so the role of individuals is more important now than ever in society.