THE NATURE OF LIFE
Before introducing the new unified framework for the understanding of biological and social phenomena, I would like to revisit the age-old question "What is life?" and look at it with fresh eyes. I should emphasize right from the start that I will not address this question in its full human depth, but will approach it from a strictly scientific perspective; and even then, my focus will at first be narrowed down to life as a biological phenomenon. Within this restricted framework, the question may be rephrased as: "What are the defining characteristics of living systems?"
Social scientists might prefer to proceed in the opposite order--first identifying the defining characteristics of social reality, and then extending into the biological domain and integrating it with corresponding concepts in the natural sciences. This would no doubt be possible, but having been trained in the natural sciences and having previously developed a synthesis of the new conception of life in these disciplines, it is natural for me to begin there.
I could also argue that, after all, social reality evolved out of the biological world between two and four million years ago, when a species of "Southern apes" (Australopithecus afarensis) stood up and began to walk on two legs. At that time, the early hominids developed complex brains, toolmaking skills and language, while the helplessness of their prematurely born infants led to the formation of the supportive families and communities that became the foundation of human social life. Hence, it makes sense to ground the understanding of social phenomena in a unified conception of the evolution of life and consciousness.
Focus on Cells
When we look at the enormous variety of living organisms--animals, plants, people, microorganisms--we immediately make an important discovery: all biological life consists of cells. Without cells, there is no life on this Earth. This may not always have been so--and I shall come back to this question--but today we can say confidently that all life involves cells.
This discovery allows us to adopt a strategy that is typical of the scientific method. To identify the defining characteristics of life, we look for and then study the simplest system that displays these characteristics. This reductionist strategy has proved very effective in science--provided that one does not fall into the trap of thinking that complex entities are nothing but the sum of their simpler parts.
Since we know that all living organisms are either single cells or multicellular, we know that the simplest living system is
the cell. More precisely, it is a bacterial cell. We know today that all higher forms of life have evolved from bacterial cells. The simplest of these belong to a family of tiny spherical bacteria known as mycoplasm, with diameters less than a thousandth of a millimeter and genomes consisting of a single closed loop of double-stranded DNA. Yet even in these minimal cells, a complex network of metabolic processes is ceaselessly at work, transporting nutrients in and waste out of the cell, and continually using food molecules to build proteins and other cell components.
Although mycoplasm are minimal cells in terms of their internal simplicity, they can only survive in a precise and rather complex chemical environment. As biologist Harold Morowitz points out, this means that we need to distinguish between two kinds of cellular simplicity.6 Internal simplicity means that the biochemistry of the organism's internal environment is simple, while ecological simplicity means that the organism makes few chemical demands on its external environment.
From the ecological point of view, the simplest bacteria are the cyanobacteria, the ancestors of blue-green algae, which are also among the oldest bacteria, their chemical traces being present in the earliest fossils. Some of these blue-green bacteria are able to build up their organic compounds entirely from carbon dioxide, water, nitrogen and pure minerals. Interestingly, their great ecological simplicity seems to require a certain amount of internal biochemical complexity.
The Ecological Perspective
The relationship between internal and ecological simplicity is still poorly understood, partly because most biologists are not used to the ecological perspective. As Morowitz explains:
Sustained life is a property of an ecological system rather than a single organism or species. Traditional biology has ended to concentrate attention on individual organisms rather than on the biological continuum. The origin of life is thus looked for as a unique event in which an organism arises from the surrounding milieu. A more ecologically balanced point of view would examine the proto-ecological cycles and subsequent chemical systems that must have developed and flourished while objects resembling organisms appeared.
No individual organism can exist in isolation. Animals depend on the photosynthesis of plants for their energy needs; plants depend on the carbon dioxide produced by animals, as well as on the nitrogen fixed by the bacteria at their roots; and together plants, animals and microorganisms regulate the entire biosphere and maintain the conditions conducive to life. According to the Gaia theory of James Lovelock and Lynn Margulis, the evolution of the first living organisms went hand in hand with the transformation of the planetary surface from an inorganic environment to a self-regulating biosphere. "In that sense," writes Harold Morowitz, "life is a property of planets rather than of individual organisms."
Life Defined in Terms of DNA
Let us now return to the question "What is life?" and ask: How does a bacterial cell work? What are its defining characteristics? When we look at a cell under an electron microscope, we notice that its metabolic processes involve special macromolecules--very large molecules consisting of long chains of hundreds of atoms. Two kinds of these macromolecules are found in all cells: proteins and nucleic acids (DNA and RNA).
In the bacterial cell, there are essentially two types of proteins--enzymes, which act as catalysts of various metabolic processes, and structural proteins, which are part of the cell structure. In higher organisms, there are also many other types of proteins with specialized functions, such as the antibodies of the immune system or the hormones.
Since most metabolic processes are catalyzed by enzymes and enzymes are specified by genes, the cellular processes are genetically controlled, which gives them great stability. The RNA molecules serve as messengers, delivering coded information for the synthesis of enzymes from the DNA, thus establishing the critical link between the cell's genetic and metabolic features.
DNA is also responsible for the cell's self-replication, which is a crucial characteristic of life. Without it, any accidentally formed structures would have decayed and disappeared, and life could never have evolved. This overriding importance of DNA might suggest that it should be identified as the single defining characteristic of life. We might simply say: "Living systems are chemical systems that contain DNA."
The problem with this definition is that dead cells also contain DNA. Indeed, DNA molecules may be preserved for hundreds, even thousands, of years after the organism dies. A spectacular example of such a case was reported a few years ago, when scientists in Germany succeeded in identifying the precise gene sequence in DNA from a Neanderthal skull--bones that had been dead for over 100,000 years! Thus, the presence of DNA alone is not sufficient to define life. At the very least, our definition would have to be modified to: "Living systems are chemical systems that contain DNA, and which are not dead." But then we would be saying, essentially, "a living system is a system that is alive"--a mere tautology.
This little exercise shows us that the molecular structures of the cell are not sufficient for the definition of life. We also need to describe the cell's metabolic processes--in other words, the patterns of relationships between the macromolecules. In this approach, we focus on the cell as a whole rather than on its parts. According to biochemist Pier Luigi Luisi, whose special field of research is molecular evolution and the origin of life, these two approaches--the "DNA-centered" view and the "cell-centered" view--represent two main philosophical and experimental streams in life sciences today.
Membranes--The Foundation of Cellular Identity
Let us now look at the cell as a whole. A cell is characterized, first of all, by a boundary (the cell membrane) which discriminates between the system--the "self," as it were--and its environment. Within this boundary, there is a network of chemical reactions (the cell's metabolism) by which the system sustains itself.
Most cells have other boundaries besides membranes, such as rigid cell walls or capsules. These are common features in many kinds of cells, but only membranes are a universal feature of cellular life. Since its beginning, life on Earth has been associated with water. Bacteria move in water, and the metabolism inside their membranes takes place in a watery environment. In such fluid surroundings, a cell could never persist as a distinct entity without a physical barrier against free diffusion. The existence of membranes is therefore an essential condition for cellular life. Membranes are not only a universal characteristic of life, but also display the same type of structure throughout the living world. We shall see that the molecular details of this universal membrane structure hold important clues about the origin of life.
A membrane is very different from a cell wall. Whereas cell walls are rigid structures, membranes are always active, opening and closing continually, keeping certain substances out a...
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