On considering a human psychological trait -such as depression- adaptive in a Darwinian sense, a few basic tenets should previously be raised and eventually agreed upon, in order to make sure that when the term "adaptive" is mentioned we are all speaking the same language. For a human behavioural trait to have evolved as an Evolutionary Stable Strategy (ESS) (Maynard-Smith, 1975) it should prove to be endowed with a differential survival advantage in man's Environment of Evolutionary Adaptedness (Bowlby, 1969/82). Since these Darwinian conceptualizations are far from evident and can thus be hardly taken for granted, I start the article with a brief description of current evolutionary thinking further to elaborate on depression embedded in this particular setting.
The process that determines the evolutionary history of a species is Natural Selection. This is the differential reproductive success of individuals within a population that occurs because of hereditary differences among them. Charles Darwin was the first man to recognize that if some members of a group of animals have more progeny, because of their inherited makeup, than other member, then evolutionary change will take place (Darwin 1859). Darwin developed the theory of natural selection at a time when the concept of gene as the unit of heredity was unknown to him. That is why he had to deal with rather abstract principles. He actually founded his theory on 3 principles: 1. the principle of variety: individuals of the same species vary in their ability to leave more reproductive offspring; 2. the principle of heredity, whereby progeny resemble direct ancestors more than any other member of the population; and 3. the principle of natural selection. whereby indivuduals that differ in certain traits also differ in their ability to leave reproductive descendants. (Garelli 1983)
We are now in a better position to reformulate the key observations and deductions of Darwin in modern genetic terms. This will enable us to understand the evidence and logic underlying an evolutionary approach to animal behaviour. All organisms have genes; all living things have within them nucleic acids that contain in coded form information about protein synthesis. Among other things, this information regulates the development of the individual. Within a species, many genes are present in 2 or more alternative forms or alleles. Different alleles code for slightly different forms of the same protein and thereby provide somewhat different influences on the development of individuals. This contributes to variation in the characteristics of the members of a species.
Populations are limited in size. No population can grow forever. Most populations fluctuate in a fairly narrow range about an average number. The number of copies of each gene in a population is therefore limited. Consequently, it follows that there will be competition between alleles of a gene to occupy the limited number of spaces in a gene pool. If one allele over a period of time consistently makes more surviving copies of itself than another, it will eventually replace the alternative form within the population. This is natural selection. Furthermore, the living members of any modern species have within them alleles that have generally been successful at copying themselves and passing from generation to generation; that is, these alleles have been "selected for". Because the development of traits is regulated genetically, we should expect that the characteristics of organisms will tend to help an individual's genes survive. The logic of natural selection requires that over time animals will evolve behavioural adaptations that promote the survival of genes that underlie advantageous traits. Genes in individuals that are reproductive failures will tend to be eliminated -that is why genes for depressive behavioural traits are unlikely to be present in human's genome-; genes with the capacity to steer development along lines that produce reproductively successful individuals should tend to survive -that is the case of attachment behaviour. All the evolved characteristics of an individual, be they developmental mechanisms, physiological systems or behavioural capabilities should be related to the survival of the genes within the individual. Seen in this light, life is essentially an unconscious contest among different alleles to survive and replace alternative forms. E.O.Wilson has vividly expressed this view with his epigram: "An organism is DNA's way of making more DNA". Genes may survive indefinitely, whereas individuals do not; they are merely "survival machines" (to use Dawkins' metaphor (1976)), produced by a set of genes for the welfare of those genes.
An evolutionary approach to behaviour -as that adopted by Bowlby's theory of attachment- rests on the concept of natural selection. And natural selection operates on the species' environment of evolutionary adaptedness: in the case of homo sapiens, the environment in which the species emerged: long before the agricultural revolution, about 10,000 years ago, probably about a million years ago. Because genes have been selected for their ability to survive, an evolutionary approach predicts that all the evolved characteristics of an animal should tend to promote the survival of the genes that underlie these characters. Therefore, the fundamental evolutionary question about behavioural traits is: how might a behavioural action help maximize the individual's fitness? This inquiry is crucial for the understanding of attachment behaviour and some of its pathological outcomes: e.g., depression.
Animal behaviour is the outcome of a complex developmental process that is regulated by the genes. The connection between the genes and behaviour is an indirect one, with many intervening steps between chromosomes and behavioural responses. Genes regulate enzyme production, which in turn controls the development of the physiological mechanisms necessary for behaviour. Although a full understanding of these events is still in its infancy, there is an incresing body of knowledge about the links between genes, neurophysiology, behaviour, and the environment.
Behavioural Systems Underlying Attachment
If we define attachment as a genetic bias all infants are endowed with to seek and keep proximity to their caregivers, we shall see the component, discrete behaviours, such as smiling, crying, following, approaching, clinging, as expressions of the underlying attachment behaviour system. Like all other behavioural systems, the attachment behaviour system is activated, regulated and terminated under certain circumstances. For example, infants are known to desire closer contact with the caregiver (the attachment figure) when thy are ill, tired, frightened or in certain prepotent situations such as unfamiliar environment or darkness. When the infant achieves locomotion a new behavioural system becomes activated: that of exploratory behaviour. Exploration of the environment is antithetical to attachment. It is of the utmost importance to focus the relationship of the infant to his mother as keeping a balance in the interplay between both systems. One of the most important functions of the attachment behavioural system is to intervene in the infant's excursions into the environment, in response to a variety of potentially dangerous events, thereby deactivating the exploratory system and activating the attachment system, thus seeking proximity or contact to his mother. (Bowlby 1973, Ainsworth 1982). Several studies show that the children approach their caregivers not only in response to dangerous stimuli but also they do so to check the availability and attentiveness of the caregiver, in a sort of permanent monitoring activity, after which the child wanders off to play again; after a while he returns again, and so on and so forth. This kind of behavioural pattern is referred to in the literature as the baby using his mother as a secure base (Ainsworth 1978).
Many animal species have evolved behavioural systems that lead the individuals of such species to behave cooperatively. Social communities are so widespread in nature that a taxonomic classification proves almost impossible: it ranges from species to families to phyla; e.g., invertebrates like ants or bees to vertebrates such as fish, birds and mammals. Such striking deployment of social organization calls for an evolutionary explanation. Genes that favoured cooperative behaviour, reciprocity and communication, protection and care towards other members of the species - generally close kin- have been selected for (Wilson 1975).
One of the most important evolutionary movers to cause socialization is defence against predation. Let us remember that natural selection favours individuals whose progeny reaches reproductivre age. Therefore, in order to avoid extinction, it is necessary to protect the young. The ability to avoid early death in a population represents the degree of differential mortality among the species. A major component of natural selection is high pre-reproductive mortality. In all species -with the exception of recent human history- more individuals die in each generation than survive to reproductive age. Darwin realized that high reproductive mortality was a necessity, a logical necessity. It followed from a single fact regarding nature, that is, that over long periods of time, the amount of living material on the earth does not increase, or increases very slowly. This means that for a typical species, numbers must remain relatively constant from generation to generation. Otherwise, the total biomass of living creatures would increase year after year. Once we assume that numbers remain relatively constant, then it is easy to calculate how many individuals in each generation die before reproduction. Human beings probably have the slowest natural rate of increase of any living creature, and we know that until the discovery of agriculture about 10,000 years ago, human numbers increased very slowly (Leakey 1981). If a woman who survives through adulthood has about 6 children, then we know -for numbers to have stayed constant- that 4 of these 6 must have perished. We know from studies of contemporary hunter-gatherer cultures that about 20% of the children born perish in the first year of life (Trivers 1985, Garelli 1984). Darwin saw clearly that the tremendous reproductive potential of all creatures must result in intense competition to survive, a competition that would result in non-random differential mortality. Those creatures who happen to be born with characteristics that make it easier to survive will endure while others will fall the wayside. Now what could be a deterrent to non-random mortality, that is, what could reduce the exposure to predatory animals, especially among younger member of species? Grouping as in bird flocks or fish schools, socialization, cooperation, parental care and attachment.
Can Depression be Considered Adaptive?
A postlorenzian ethologist would certainly deem a depressive condition maladaptive, since chronically proving no threat to predators only enhances the individual's chance to become their favourite prey. Predators differentially predate on defenceless animals, such as youths, juveniles. or otherwise handicapped individuals.
Harmlessness -actually harmlessness or defencelessness displays- may prove adaptive while ocassionally fighting for hierarchical ranks intraspecifically, never interspecifically. Furthermore, depression, not as a clinical condition, but as. e.g., a normal phase of mourning, may prove adaptive within the social structure of a species but highly dangerous in the environment of evolutionary adaptedness.
The reverse, denial of depression is, according to Bowlby (Attachment and Loss, vol 3: Loss, p. 72), similarly maladaptive, for individuals inclined to deny the nature of their affliction "... are more prone to suffer breakdowns in functioning when, for periods lasting weeks, months or years, they may be unable to deal effectively with their environment."
To my mind depression mainly stems from our our being an extremely social species. I think we are social as from birth -and very probably before that, too. We are deeply biased to seek and keep both physical and emotional proximity to a distinct, preferred figure: the attachment figure, in Bowlby's terms. Temporary or permanent separation or loss of the attachment figure brings about a normal process: mourning. And there is no mourning without depression. Moreover, as stated above, if no depression becomes apparent, a pathological condition ensues: denial of mourning. If, on the other hand, depression is never overcome, we face another psychopathological syndrome: chronic mourning.
Consequently, I can think of two possible outcomes: either we consider depression an inevitable consequence of loss, thereby putting the individual at risk of annihilation -cf. above about the high mortality rate in prehistoric man- or parental bonding and attachment have been selected for - among other things- to prevent depression from ocurring and thus saving the affected individual from sure extinction. In any case, selection, whether natural or artificial could hardly be expected to produce individuals prone to form attachment and affectional bonds and simultaneously spare them the pain accompanying loss.
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