Introduction

As members of the species Homo sapiens, we are the product of biological evolution. More precisely, we are mammals, and within the class Mammalia we are primates (figure 5.1). Among the order Primates, we belong to the Old World primates (Catarrhini). Within this group we are members of the family Hominidae, which comprises the apes (gibbons, orangutans, gorillas, chimpanzees, and man) as opposed to monkeys, the remaining Old World primates (family Cer-copithecidae) and all New World primates (suborder Platyrrhini). Among the hominids, we are members of the group great apes, or Homininae, which includes the gorilla (Gorilla gorilla), man (Homo sapiens), the two chimpanzee species (common chimpanzee, Pan troglodytes, and pygmy chimpanzee, Pan paniscus) and the orangutan (Pongo pan) and excludes the gibbon (Hylobates).

Traditionally, gibbons, orangutans, gorillas, and chimpanzees were grouped together into the family Pongidae, while man (with his extinct ancestors) was put into the separate family Hominidae. Such a distinction, though psychologically understandable, is completely unjustified in the light of modern taxonomy and evolutionary biology. Biologically, we Homo sapiens are more closely related to the two chimpanzee species than to any other living primate; we share about 99% of our genes with them. Therefore, humans and the chimpanzees should be placed together in a separate taxon, for which no name yet exists. The closest relative of this nameless group is the gorilla, and the closest relative of the African great apes is the orangutan.

The earliest primates originated at least 65 million years (My) ago, the separation between Old and New World primates took place about

40 My ago, and that between Old World monkeys (Cercopithecidae) and apes, including humans, about 30 My (or less) ago. Gibbons branched off at 19-17 My, the orangutan at 16 My, and the gorilla at 9-8 My. Humans and chimpanzees separated 6.7-6.2 My ago (Byrne 1995). Thus, given the evolutionary age of about 65 My of the order Primates, the divergence between Homo and Pan is relatively recent.

This evolutionary history shows that there can be no doubt that taxonomically and biologically we are (in a nested sense) mammals, primates, apes, and the closest relatives of chimpanzees. Do these evolutionary relationships similarly determine our "higher" cognitive abilities, including consciousness? There are probably the same number of neurobiologists, psychologists, and philosophers who accept as reject the idea that consciousness has evolved in parallel with the biological evolution of Homo sapiens, and with the evolution of the human brain in particular. Even if all of us accept the idea that consciousness is strictly bound to brain activity, it still remains undecided whether the brain centers involved in consciousness are shared with at least one other group of animals or are unique to humans. There is, of course, the possibility that some states of consciousness can be found in at least some nonhuman animals, while others are unique to humans.

To prove the presence or absence of states of consciousness directly is impossible, since we are uncertain about consciousness even in our nearest relative. There are, however, indirect ways to determine the presence of consciousness in animals as likely or unlikely. In essence, these are (1) to check in groups of animals for the presence of those cognitive functions which in humans can be exerted only consciously; (2) to examine which parts of the human brain are necessary for (and active during) the various states of consciousness; (3) to examine which of these centers

Figure 5.1

Taxonomic relationship (cladogram) of the primates. (Modified from Nieuwenhuys et al. 1998.)

Figure 5.1

Taxonomic relationship (cladogram) of the primates. (Modified from Nieuwenhuys et al. 1998.)

of the human brain are present (and active) in the brains of those animals which—based on behavioral evidence—show certain states of consciousness; (4) to compare the ontogeny of cognitive functions, including states of consciousness in humans, with the ontogeny of the human brain (in the ideal case, the first appearance of certain states of human consciousness should coincide with the maturation of certain centers in the human brain). Taken together, these pieces of evidence should give us a relatively reliable picture of the way different states of consciousness have evolved in parallel to vertebrate, mammalian, primate, and human brain evolution.

Definition of Cognition and Consciousness

In the following, I use the term "cognition" in a wide sense to designate brain functions that ex clude only primary sensory and motor functions, "autonomic" functions of the brain, reflexes, and reflexlike stereotyped behavior. Cognition, thus, includes such diverse functions as perception, learning, memory, imagination, thinking, expecting, and planning, whether accompanied by consciousness or not. From this it follows that cognition is not necessarily restricted to human beings, nor does it presuppose the existence of consciousness.

In humans, consciousness or awareness varies widely with respect to intensity and content, ranging from deep coma to the highest degree of concentration, from alertness to self-reflection. The most general form of consciousness is wakefulness or vigilance. It is characterized by a general responsiveness to sensory stimuli.

Vigilance is usually combined with subjective awareness or conscious experience of something. This "something" includes external as well as internal bodily stimuli, my own emotions, and my mental activity. From this awareness results the experience of my own presence in the world. Attention is the most characteristic state of increased awareness. A more special type of consciousness is body-identity awareness, the belief that I belong to the body which apparently surrounds me. There is autobiographic consciousness, the conviction that I am the one who existed yesterday. There is reality awareness of what was going on in the past and is happening in the world surrounding me. There is awareness of voluntary control of movements and actions, of being the author of my thoughts and deeds. Finally, there is self-awareness, the ability of self-recognition and self-reflection.

These different aspects of consciousness can dissociate, that is, they can occur independently of each other after damage of different parts of the brain (Kinsbourne 1995; Knight and Grabowecky 1995; Moscovitch 1995). Thus, there are patients who have all normal states of cognition, consciousness, and intelligence except that they deny belonging to their own body, or who do not know who or where they are.

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