**1. Introduction**

Two basic principles of animal life are essential for survival of the individual and as a species. Firstly, the animal should be motivated to obtain food, warmth, sexual gratification and

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comfort. Secondly, the animal should be motivated to escape from predators, cold, sexual competitors and other forms of adversity. To survive as an individual and a species, even our oldest ocean-dwelling ancestors living over 560 million years ago must have been able to react to the environment to feed, evade predators, defend territory and reproduce. Hence, their primitive nervous systems must have regulated the necessary behaviours and incorporated the most essential structures of all today's freely moving Animalia. However, since then the human brain passed through a long evolutionary pathway during which particularly the forebrain showed major changes. The earliest vertebrate brain almost completely lacked the forerunner of the human neocortex and the dorsal parts of the basal ganglia [1, 2]. These newer parts of the brain are believed to determine human behaviour to a high degree and consequently receive most attention in research of processes explaining the genesis of mental disorders (see, e.g. Ref. [3]). This contrasts the involvement in psychic disorders of those behavioural processes described above as also being displayed by the most primitive vertebrates. We want to suggest that these actions are still regulated in humans by brain structures derived from the primitive forebrain of the earliest vertebrates. Therefore, we described the anatomy of the forebrain of the earliest human vertebrate ancestors, which is believed to be comparable with the brain of lampreys [2]. From a comparison of the striatum of lampreys to that of anuran amphibians and younger vertebrates, it can be concluded that the striatum of lampreys is the forerunner of the human nuclear amygdala [4]. In anuran amphibians (frogs and toads), the lamprey's striatum is retrieved as central and medial amygdaloid nuclei, while a later ventral striatum for the first time appears in its direct vicinity [2, 4]. The lamprey's forebrain also contains a structure of which the connections are very well conserved in more recent human ancestors: the habenula. The habenula constitutes—together with the stria medullaris and pineal gland—the epithalamus and consists of medial and lateral parts [5]. The habenula regulates the intensity of reward-seeking and misery-fleeing behaviour probably in all our vertebrate ancestors. In lampreys the activity of the habenula is in turn regulated by a specific structure: the habenula-projecting globus pallidus. It is tempting to speculate that this structure has a similar role in humans, but a clear anatomical human equivalent with the same function has not yet been identified. Based upon the evolution of the basal ganglia in vertebrates and the mechanism of the emotional response, we postulate the existence of two systems regulating the intensity of the aforementioned behaviours [6, 7]. These two circuits include the activities of extrapyramidal and limbic basal ganglia and are collaborating in a yin-and-yang-like fashion. The two basal ganglia systems are linked together by the core and shell parts of the nucleus accumbens (NAcb), which regulates motivation to show reward-seeking and misery-fleeing behaviour, respectively.

The amygdala is believed to address the ability of learning to value sensory information (attentive salience). This capacity is essential to determine which sensory information is of vital importance to react on with reward-seeking or misery-fleeing behaviour. Aberrant salience is believed to be a crucial component of the mechanism of (schizophrenic) psychosis and the antipsychotic effects of dopamine antagonists [8]. We believe that the amygdala regulates the behavioural output by affecting the connection of the ancient forebrain (amygdala, hippocampus) with the monoaminergic centres of the midbrain through the medial and lateral habenula. Therefore, we will describe the evolution of the amygdaloid complex and its final anatomy and functioning. Subsequently, we will integrate these findings with our model of neuronal circuits regulating pleasure and happiness and give an explanation how the connection between amygdala and habenula is involved regulating these behaviours.
