John Maynard Smith and Eörs Szathmary
THE ORIGINS OF LIFE
The major transitions
The easiest way of explaining what we mean by the major transitions is to list them (Table 2.2). The brief explanation of this list that now follows is in effect a synopsis of the rest of the book: if some statements seem obscure, we hope that they will be made clearer in the appropriate chapter.
1. Replicating molecules: populations of molecules in compartments. We think that the first objects with the properties of multiplication, variation, and heredity were replicating molecules, similar to RNA but perhaps simpler, capable of replication, but not informational because they did not specify other structures. If evolution was to proceed further, it was necessary that different kinds of replicating molecule should cooperate, each producing effects helping the replication of others. We argue that, if this was to happen, populations of molecules had to be enclosed within some kind of membrane, or 'compartment'.
2. Independent replicators: chromosomes. In existing organisms, replicating molecules, or genes, are linked together end to end to form chromosomes (a single chromosome per cell in most simple organisms). This has the effect that when one gene is replicated, all are. This coordinated replication prevents competition between genes within a compartment, and forces cooperation on them. They are all in the same boat. We discuss this transition in Chapter 5.
3. RNA as gene and enzyme: DNA and protein. There is today a division of labour between two classes of molecule: nucleic acids (DNA and RNA) that store and transmit information, and proteins that catalyse chemical reactions and form much of the structure of the body (for example, muscle, tendon, hair). It seems increasingly plausible that there was at first no such division of labour and that RNA molecules performed both functions. The transition from an 'RNA world' to a world of DNA and protein required the evolution of the genetic code, whereby base sequence determines protein structure.
4. Prokaryote - eukaryote: Cells can be divided into two main kinds. Prokaryotes lack a nucleus, and have (usually) a single circular chromosome. They include the bacteria and cyanobacteria (blue-green algae). Eukaryotes have a nucleus containing rod-shaped chromosomes and usually other intracellular structures called 'organelles', including the mitochondria and chloroplasts described on pp. 70-77. The eukaryotes include all other cellular organisms, from the single-celled Amoeba and Chlumydomonas up to humans. We discuss the transition from prokaryotes to eukaryotes in Chapter 6.
5. Asexual clones - sexual populations. In prokaryotes, and in some eukaryotes, new individuals arise only by the division of a. single cell into two. In most eukaryotes, in contrast, this process of multiplication by cell division is occasionally interrupted by a process in which a new individual arises by the fusion of two sex cells, or gametes, produced by different individuals. Although familiar, this transition is one of the most puzzling; we discuss it in Chapter 7.
6. Protists - animals, plants, and fungi. Animals are composed of many different kinds of cells - muscle cells, nerve cells, epithelial cells, and so on. The same is true of plants and fungi. Each individual, therefore, carries not one copy of the genetic information (two in a diploid) but many millions of copies. The problem, of course, is that although all the cells contain the same information, they are very different in shape, composition, and function. In contrast, protists exist either as single cells, or as colonies of cells of only one or a very few kinds. How do cells with the same information become different? How do different kinds of cells come to be arranged so as to form the adult structure? What problems had to be solved before animals and plants could evolve? We discuss these questions in Chapter 10.
7. Solitary individuals - colonies. Some animals, notably ants, bees, wasps, and termites, live in colonies in which ouly a few individuals reproduce. Such a colony has been likened to a superorganism, analogous to a multicellular organism. The sterile workers are analogous to the body cells of an individual, and the reproducing individuals to the cells of the germ line. The origin of such colonies is important; it has been estimated that one-third of the animal biomass of the Amazon rain forest consists of ants and termites, and much the same is probably true of other habitats. It is also interesting for the light it sheds on the origin of human societies. We discuss these origins in Chapter 11.
8. Primate societies - human societies, and the origin of language. We argue in Chapter 12 that the decisive step in the transition from ape to human society was the origin of language. We have already emphasized the similarities between human language and the genetic code. They are the two natural systems providing unlimited heredity. The nature and origin of human societies are the topic of Chapter 12, and in Chapter 13 we discuss the origin of language.
Because we are concerned with information, we should perhaps have included in our list the evolution of a nervous system capable of acquiring information about the external world, and using that information to modify behaviour. Certainly the acquisition of a nervous system was a necessary precondition for the subsequent evolution of language. Our only excuse for omitting it is one of incompetence!
Of the eight transitions that we have listed, we think that all but two were unique, occurring just once in a single lineage. The two exceptions are the origins of multicellular organisms, which happened three times, and of colonial animals with sterile castes, which has happened many times. There are interesting implications of the occurrence of six unique transitions, together with the origin of life itself, which we also think to have been a unique sequence of events. Any one of them might not have happened, and if not, we would not be here, nor any organism remotely like us.
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