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 and letting others in. The cell's metabolic reaction involve a variety of ions, and the membrane, by being semipermeable, controls their proportions and keeps them in balance. Another critical activity of the membrane is to continually pump out excessive calcium waste, so that the calcium remaining within the cell is kept at the precise, very low level required for its metabolic functions. All these activities help to maintain the cell as a distinct entity and protect it from harmful environmental influences. Indeed, the first thing a bacterium does when it is attacked by another organism is to make membranes.
All nucleated cells, and even most bacteria, also have internal membranes. In textbooks, a plant or animal cell is usually pictured as a large disk, surrounded by the cell membrane and containing a number of smaller disks (the organelles) each surrounded by its own membrane. This picture is not really accurate. The cell does not contain several distinct membranes, but rather has one single, interconnected membrane system. This so-called "endomembrane system" is always in motion, wrapping itself around all the organelles and going out to the edge of the cell. It is a moving "conveyor belt" that is continually produced, broken down and produced again.
Through its various activities the cellular membrane regulates the cell's molecular composition and thus preserves its identity. There is an interesting parallel here to recent thinking in immunology. Some immunologists now believe that the central role of the immune system is to control and regulate the molecular repertoire throughout the organism, thus maintaining the organism's "molecular identity." At the cellular level, the cell membrane plays a similar role. It regulates molecular compositions and, in doing so, maintains the cellular identity.
The cell membrane is the first defining characteristic of cellular life. The second characteristic is the nature of the metabolism that takes place within the cell boundary. In the words of microbiologist Lynn Margulis: "Metabolism, the incessant chemistry of self-maintenance, is an essential feature of life . . . Through ceaseless metabolism, through chemical and energy flow, life continuously produces, repairs, and perpetuates itself. Only cells, and organisms composed of cells, metabolize."
When we take a closer look at the processes of metabolism, we notice that they form a chemical network. This is another fundamental feature of life. As ecosystems are understood in terms of food webs (networks of organisms), so organisms are viewed as networks of cells, organs and organ systems, and cells as networks of molecules. One of the key insights of the systems approach has been the realization that the network is a pattern that is common to all life. Wherever we see life, we see networks.
The metabolic network of a cell involves very special dynamics that differ strikingly from the cell's nonliving environment. Taking in nutrients from the outside world, the cell sustains itself by means of a network of chemical reactions that take place inside the boundary and produce all of the cell's components, including those of the boundary itself.
The function of each component in this network is to transform or replace other components, so that the entire network continually generates itself. This is the key to the systemic definition of life: living networks continually create, or re-create, themselves by transforming or replacing their components. In this way they undergo continual structural changes while preserving their weblike patterns of organization.
The dynamic of self-generation was identified as a key characteristic of life by biologists Humberto Maturana and Francisco Varela, who gave it the name "autopoiesis" (literally, "self-making"). The concept of autopoiesis combines the two defining characteristics of cellular life mentioned above, the physical boundary and the metabolic network. Unlike the surfaces of crystals or large molecules, the boundary of an autopoietic system is chemically distinct from the rest of the system, and it participates in metabolic processes by assembling itself and by selectively filtering incoming and outgoing molecules.
The definition of a living system as an autopoietic network means that the phenomenon of life has to be understood as a property of the system as a whole. In the words of Pier Luigi Luisi, "Life cannot be ascribed to any single molecular component (not even DNA or RNA!) but only to the entire bounded metabolic network."
Autopoiesis provides a clear and powerful criterion for distinguishing between living and nonliving systems. For example, it tells us that viruses are not alive, because they lack their own metabolism. Outside living cells, viruses are inert molecular structures consisting of proteins and nucleic acids. A virus is essentially a chemical message that needs the metabolisof a living host cell to produce new virus particles, according to the instructions encoded in its DNA or RNA. The new particles are not built within the boundary of the virus itself, but outside in the host cell.
Similarly, a robot that assembles other robots out of parts that are built by some other machines cannot be considered living. In recent years, it has often been suggested that computers and other automata may constitute future life-forms. However, unless they were able to synthesize their components from "food molecules" in their environment, they could not be considered to be alive according to our definition of life.
The Cellular Network
As soon as we begin to describe the metabolic network of a cell in detail, we see that it is very complex indeed, even for the simplest bacteria. Most metabolic processes are facilitated (catalyzed) by enzymes and receive energy through special phosphate molecules known as ATP. The enzymes alone form an intricate network of catalytic reactions, and the ATP molecules form a corresponding energy network. Through the messenger RNA, both of these networks are linked to the genome (the cell's DNA molecules), which is itself a complex interconnected web, rich in feedback loops, in which genes directly and indirectly regulate each other's activity.
Excerpted from The Hidden Connections by Fritjof Capra. Copyright © 2002 by Fritjof Capra. Excerpted by permission of Anchor, a division of Random House LLC. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.