What exactly is our true plasticity and flexibility within the physical and cultural environments into which we are born and in which we develop? Scientists from such diverse disciplines as molecular genetics, evolutionary and developmental biology, anthropology, neuroanatomy, neurochemistry, comparative developmental psychology, and sociology have for some time now been focusing their attention on this specific subject.

The question they all pose is this: To what extent are organisms, in general, and human beings, in particular, able to adapt and possibly change their physical and behavioral characteristics during their lifespan? The processes studied by each of the above disciplines concern all elements of plasticity and flexibility. Indeed, systematic changes have been recorded in the structure and/or in the function of organisms over the course of time.

This multi-disciplinary research gives rise to important implications for possible corrective intervention aimed at improving human health during the course of various phases of the dynamic process of life. It has also been recognized that changes in one developmental process stem from and contribute to other correlated processes. Therefore, even though the potential for plasticity seems to be present almost everywhere, it has certain limits, and the various different structures and functions in which it manifests itself become more restricted with age.

Historically speaking, developmental studies gave rise to many diverse concepts of plasticity during the last century. In the 1901 Dictionary of Philosophy and Psychology (edited by James Marc Baldwin), plasticity is defined as ??that property of living substances or of an organism whereby it alters its form under changed conditions of life.?

At the turn of the 20th century, Darwin?s theory was still a recent phenomenon and as a result plasticity focused mainly on the implication of this new theory, specifically, how organisms change as a result of evolution. During the 1930s and 1940s, plasticity or flexibility/modifiability research once more took center stage. This was due to Karl Lashley?s concept of cerebral plasticity and his study of the negative effects of environmental deprivation in early infancy and the possible benefits derived from compensating programs of recovery.

In the 1970s and 1980s plasticity and its associated research once again attracted renewed interest. This time, however, it was not confined to infancy, but expanded to include gerontological psychology, as science was beginning to understand that ageing is not a fixed general process of decline and that even older organisms still have considerable potential for change.

There are many ways of interpreting why this constantly changing theme of plasticity attracts so much interest from so many different sources. In his book, On the Nature of Human Plasticity (1984) Richard Lerner claims that this is the direct expression of certain underlying tensions inherent in the disciplines that concern developmental processes.

From a certain point of view, the study of the development of plasticity is characterized by research into universal processes and the mechanisms of ontogenetic change. On the other hand, as soon as scientists claim to have developed clearly defined processes that are acceptable to all, there is a counter argument that challenges the idea of this universality.

Once we come to terms with the concept that as soon as developmental mechanisms are identified and understood, there is a gradual perception that the conditions controlling these mechanisms involve an increasing spiral of complex actions and reactions. Therefore, knowledge of developmental processes and mechanisms also provide us with information about their alterations and changes. The new systematic approaches to the network/systems of life now offer us a very stimulating perspective towards understanding or possibly even creating a new interactive holographic model of living systems that takes into account their almost infinite modulations and interactions.

Perhaps the most important reason why the field of plasticity is attracting such great interest is that behavioral development is intrinsically of great practical value. The study of developmental processes is therefore not limited only to the descriptions and explanations of observed phenomena, but also investigates modification and optimization of evolutionary processes of organisms in general, and in particular, those processes that concern the human species.

As a result, many researchers do not restrict their work to the simple study of how developmental processes arise, where they originate or where they are going. They also try to indicate what might happen if and when developmental conditions are varied in ways that are more or less predictable. In this multi-faceted context, research into real or potential plasticity of organisms represents a landmark in the quest for a deeper knowledge of developmental processes.

Leaving this historical-theoretical aspect aside, I would now like to focus on a research project in which I was involved together with colleagues of various disciplines. Some years ago we set up a study group to look into ontogenetics and plasticity, concentrating mainly on the health of human beings. Some theoretical-experimental findings of the group were published in an article that appeared in the July 2005 issue of Nature. Here I will discuss some of the key points from that article.

We have known for some time that many plants and animals are capable of developing in various ways, adopting characteristics that are best adapted to suit an environment in which they live. For example, it has been discovered that small size and slow metabolism can facilitate survival in adverse circumstances, while faster metabolism and larger size will benefit reproduction in times of abundant resources. These characteristics are often instilled at the birth of the organism or even determined by environmental signals to which their parents or predecessors have been exposed.

The individuals that have, in the course of their development, adapted to a specific environment might, however, be at risk when they are exposed to a different context or when, later in life, they encounter the inevitable processes of ageing. This is why biological evidence and scientific knowledge of these processes are extremely relevant in understanding human development and also its predisposition to certain diseases.

Striking evidence from a number of disciplines has focused attention on the interplay between the developing organism and the metabolic-environmental circumstances in which it finds itself. Fields of research as diverse as evolutionary ecology, behavioral development, life-history theory, molecular biology, and medical epidemiology have converged on the key finding that a given genotype can give rise to different phenotypes, depending on its conditions during ontogenesis. Many organisms can express specific adaptive responses to their environments. Such responses include immediate, short-term changes in physiology and behavior. Significantly, responses to the environment may be expressed in the offspring, rather than in the parents.

The freshwater crustacean Daphnia yields a classic example. Offspring whose mother had been exposed to the chemical traces of a predator are born with a defensive ?helmet? that protects them against predators. This structure, however, can be a liability in a predator-free environment, where its construction cost reduces competitive success relative to no-helmeted individuals. Such phenotypic mismatches between the offspring?s phenotype and its current environment can be costly in terms of both survival and reproductive success.

The desert locust (Schistocerca gregaria) provides another well-known example of developmental plasticity. Under low-density conditions, the locust is cryptic, shy, nocturnal, and sedentary. Under crowded conditions, the individual becomes increasingly conspicuous, gregarious, and diurnal over several generations and then migrates in enormous swarms.

Both of these examples demonstrate how the impact of the environment experienced by one generation can shape the development and behavior of the next. Therefore, as previously mentioned, depending on the stimulation and environmental changes to which a genotype is subjected, it can, in the course of its development, give rise to different phenotypes, thus demonstrating a high level of responsive plasticity and flexibility.

However, not all of the effects of the environment are adaptive, i.e. increasing the fitness of the organism. Variations within a species may be affected negatively by temperature, acidity, nutrients, water availability, population density, the presence of pathogens, predators, and exposure to toxins. Different phenotypes may reflect inevitable physical or chemical constraints. For example, reduced metabolic rates caused by low temperatures will influence growth rate and body size.

Environmental events may also disrupt developmental processes and lead to abnormalities. If conditions for development are not optimal, individuals may still be able to cope, but at a cost to their future reproductive success. The mature phenotype may often be different from one expressed under optimal conditions and not so well-adapted to adult life as would have been expected. In mice, for example, food restrictions can slow ageing by enhancing cellular maintenance and repair processes while reducing or shutting down fertility.

The varied developmental pathways triggered by environmental events may be induced during ?sensitive?, often brief periods in development. Outside these sensitive periods an environmental influence that sets the characteristics of an individual may have little or no effect. The reasons for plasticity being restricted to a particular period of life may be ascribed to the difficulties of reversing developmental processes, the costs in terms of survival or reproductive success of changing the characteristics of the adult organism. Female birds, for example, are able to alter many aspects of egg composition, including nutrients, hormones, antioxidants, immuno-globulins, and even embryo sex, in response to food availability, levels of sibling competition and the quality of their mates.

Such maternal effects can result from the influence of a single specific environmental factor that the female has experienced and which therefore also affects phenotypic development. This effect may persist over a number of generations even if the factors have changed or disappeared. For example, mammalian mothers who experience poor nutrition as fetuses often produce relatively light offspring during their breeding life spans.

These broad considerations from many fields of biology are relevant to understanding some of the critical variations in humans. The human baby responds to under-nutrition, placental dysfunction and other adverse influences by changing the trajectory of his or her development and slowing growth. Although the fetus was thought to be well-buffered against fluctuations in its mother?s conditions, a growing body of evidence suggests that the morphology and physiology of the human baby is affected by the nutritional state of the mother.

It is possible therefore that human development may involve induction of particular patterns of development by cues that prepare the developing individual for the type of environment in which he or she is likely to live. Individuals may be adversely affected if the environmental predictions provided by the mother and the conditions of early infancy prove to be incorrect. Mothers who live in habitual conditions of food scarcity, for example, produce much smaller fetuses enclosed in a more abundant placenta, as if the fetus is trying to compensate for such malnourishment by absorbing as many nutrients as possible from the mother.

Extensive epidemiological studies carried out for the most part in India have revealed that people whose birth weights approached the lower end of the minimum range, yet who grow up with high levels of nutrition due to increased affluence, run a high risk of developing coronary heart disease, type 2 diabetes, hypertension, and forms of premature obesity. Those born as heavier babies and brought up in affluent environments are much less likely to contract the above diseases. Although rapid improvements in nutrition and other social conditions appear to be beneficial to the life of the growing organism in the short term, it can have a damaging effect, as it does not follow the ‘environmental prediction’ that the mother had programmed for it.

At this point we are looking at the field of diseases caused by civilization and the recent developments of Darwinian medicine that focus on the history of our species in a hunter-gatherer environment and the onset of agriculture some 10,000 years ago, when we evolved and thus definitively adapted to a new way of life. Cultural development followed rapidly over a short period of time and did not necessarily follow the same rules of biological evolution of organisms, which actually date back to the ‘dawn of time’, thousands of millions of years ago when our planet was formed and life first appeared. Plasticity of organisms is therefore undoubtedly a powerful, but not all-powerful mechanism, especially in the short term.

I would like to conclude by citing a concrete example, which is supported by sound epidemiological data. The emergence of a new middle class in the Indian sub-continent has rapidly changed the traditionally poor diets of about 250 million Indians with a much more substantial nutritional intake. This new, improved regime may be too rich for those organisms that had historically evolved in the context of a low nutritional environment. One of the possible side effects and costs of this rapid mutation in life style is the dramatic prediction that India will soon have some 57 million diabetics!

Bruno D’Udine, graduated in Pharmacology, and worked for many years for the Italian National Research Council at the Institute of Psychobiology and Psychopharmacology in Rome, where his main interest in research was the ontogeny of behavior. For extensive periods he worked at the University of Cambridge, the University of Edinburgh and the Pavlov Institute in Leningrad. At the moment he is Visiting Professor at the Scuola Superiore di Antropologia CulturaleEpistemologia della Complessità, University of Bergamo, Italy.

In 1997 he gave the keynote address at the Rolf Institute of Structural Integration’s® annual meeting in Denver. His address was published in Rolf Lines as ‘An Evolutionary Perspective on the Body-Mind Relationship’, April 1988. In October1994 Rolf Lines published his article ‘Trends in Darwinian Medicine’. He also addressedCertified Rolfers in 1986 at the European annual meeting, with a talk entitled ‘Biological Considerations on Rolfing’, which he prepared together with his Rolfer wife Carla van Vlaanderen.