While "intelligent" animals may evolve on other planets by relational pathways similar to those seen on Earth, the phenomenon of "cognition " is quite distinct from that of intelligence and may well be quite rare.
Whether we are alone in the Universe is a perplexing problem admitting no easy solution. Unless another biological system is discovered, we have only one laboratory from which to generate hypotheses-our own biosphere. If one suggests that it is improbable that intelligent life exists elsewhere in the Universe, a common reply is "intelligent life as we know it". This is nihilistic in the sense that what does not exist cannot be discussed; our knowledge is necessarily restricted to empirical experience and our capacity to imagine. What is known or imagined can be discussed and considered; what is not imagined is obviously beyond discussion.
Since we have no data on any exobiological system, we are also limited to a discussion of process, at the molecular, organismic, or ecosystemic level. It is reasonable to assume that organic evolution, as a process, obeys in exobiological systems the same fundamental strictures that define it on Earth and that it consistently proceeds by differential reproductive success in variants of self-duplicating molecular bundles of one sort or another. In short, even if we cannot observe the structure of exobiological organisms, we can make quite reasonable inferences about the processes by which they might evolve.
 Our existence on this planet provides the de facto basis for the conclusion that the evolution of intelligent life is possible. The critical point is estimating its probability. Such estimates have been frequently offered in recent years and range from 10-7 to 10-9 (Miller and Orgel, 1974) for our Galaxy. These estimates take into account such factors as the number of stars, the rate of star formation, the average number of planets per star, and so on. Any such estimate also requires a probability for the evolution of intelligent life. It is difficult to specify a basis for determining this particular probability. Usually it is simply stated and is clearly accompanied by several unstated assumptions about the evolutionary process, including the belief that biological systems, once initiated, tend to evolve toward higher levels of complexity, and that such complexity naturally leads to intelligent life. It is with these assumptions that I wish to deal here.
Before the fairly recent development of a synthetic theory of organic evolution, it was often held that organisms existed because of an élan vital Huxley once commented that this explanation was about as effective as the invocation of an élan locomotif to account for the motion of a train. Early evolutionists believed in orthogenesis-that evolutionary progress was determined by some ultimate goal. Everything that we have learned about organic evolution contradicts such a view. Simpson (1949) concluded that only one universal trend characterizes evolution: "a tendency for life to expand, to fill in all the available spaces in the livable environments, including those created by the process of that expansion itself." Nothing that we know of the evolutionary process or of the factors that led to the appearance of man justify an élan intelligent or the existence of intellogenesis. If we wish to make estimates of the probability of intelligent life on suitable planets, then we must clearly identify the events and processes by which it appeared on this planet. This is the only method available short of pure fantasy. Are there fundamental evolutionary trends that led to the appearance of man, or is man to be viewed as a highly specific event? The question is far from trivial. If man represents the final expression of a recurrent process, then other communicating beings probably exist in the Universe. If, however, he is the historical consequence of a nonsystematic sequence of specific events, then the probability of other communicating beings depends directly on the likelihood of similar historical sequences. The question can be broken down into three specific questions:
- 1. Do biological systems naturally evolve toward more complex states?
- 2. Does increased intelligence favor survival and reproduction? That is, is it generally under positive selection?
- 3. Does increased intelligence generally evolve toward cognition?
The remainder of this paper will address each of these questions briefly.
An almost bewildering continuum of complexity exists in Earth's biosphere. It is natural to assume that this complexity, which is the product of evolution as interpreted from the fossil record, has evolved over a considerable time period and is the product of a tendency toward entropy reduction. An organic system, once established, gradually develops individual units (organisms) of greater complexity. Why should such a process occur? It is frequently assumed that complexity enhances the fitness of an organism-it increases its chances of survival and reproduction. This is, however, counterintuitive. A complex machine is inherently less reliable than a simple one. The principle of decreasing reliability with greater complexity is also evident from our knowledge of organic evolution: the survival of a mammal depends on the successful coordination and functioning of many highly sensitive organ systems.
One problem lies in a failure to distinguish between ecosystems and organisms. The principles governing these two kinds of antientropic devices are frequently quite distinct. Complex organisms are more likely to become extinct than simple ones, but complex ecosystems are more stable than simple ones. Organisms require some means of energy transformation or a direct supply of energy, and their energy resources are often limited. Efficiency in energy utilization militates against the development of backup systems, since such systems will only be required in unusual circumstances and are energy costly, reducing the chances of a particular organism's successful competition with conspecifics or other species. This process, constantly repeated in the evolutionary cycle, is known as specialization. The koala represents a pinnacle of adaptation with respect to finding, reaching, masticating, digesting, and excreting eucalyptus. It performs these functions with greater efficiency than any other mammal, but its systematic improvement of its ability in this area has reduced its capacity to respond to ecological change. Any serious interruption of eucalyptus supply could lead to its rapid extinction. Indeed, the great majority of mammal species have become or will become extinct because of specialization.
In a simple ecosystem, the loss of a single organism can be seriously disruptive. In a complex one, a loss is more likely to lead to replacement by a similarly functioning organism or minimal reactive shifts by a large number of participating organisms. In ecosystems, then, stability comes with complexity, and ecosystems tend toward complexity. These principles do not apply to organisms.
 The fossil record shows quite clearly that Earth's most complex organism, the mammal, is recent (about 100-150 million years old) and is the end result of a trend toward more highly integrated organisms. This commonly held viewpoint is probably incorrect, as pointed out by Dobzhansky (1970, p. 392):
....the transition from unicellular to multicellular organisms clearly involved a structural complication. Consider, however, the evolutionary sequence fish -> amphibian -> reptile -> mammal -> man. The sequence is usually taken to be progressive, yet "it would be a brave anatomist who would attempt to prove that recent man is more complicated than a Devonian ostracoderm " Although the development of a variety of sense organs is generally taken as progress, mammals and man lack certain kinds of senses present in other vertebrates - the lateral line organs of fishes, which perceive variations in pressure, or the directional receptors for heat radiation present in pit vipers.
If not a trend toward complexity, what then is the thematic characteristic of the sequence cited by Dobzhansky? The answer lies in the evolution of ecosystems and, more specifically, in trophic pyramids.
Once an entity is capable of energy transformation and utilization (whether by photosynthesis or perhaps by chemosynthesis), the stage is set for the development of "higher" organisms that can subsist on these autotrophs. More complex mechanisms, especially those of food location (sense organs), locomotion, and digestion, must be evolved to ensure a constant supply of autotrophs. The so-called advanced adaptations of mammals are accountable almost entirely as specializations in food procurement and utilization. Vision, olfaction, rapid locomotion, and learning are all means of food location and capture. As each level of the trophic pyramid becomes established, the occupation of a higher level becomes possible, although the number of levels is strictly limited by the loss of about 90% of absorbed energy as heat at each level (Odum, 1962, 1971). The evolution of this trophic pyramid is obvious from the fossil record, and it clearly explains the different kinds of organisms in the chain cited above. Each presents a new form of exploitation of lower trophic levels, and the novel adaptations in each are directly related to this fact. Some obvious corrolaries of trophic evolution can be stated:
1. Evolution continued in each level, but there is no trend toward complexity. (Are angiosperms more "complex" than gymnosperms?) In fact, there are clearly recognizable trends toward simplification (e.g., mollusks)
2. There is a clear limitation and end point to the process of trophic pyramidization.
 3. Diversity is reduced with each succeeding level added to the pyramid. There are about 2 million named species on Earth, including about 42,000 species of vertebrates and about 5000 species of mammals obzhansky, 1970).
The above outline is subject to much variation and exception. All levels of the ecosystem are continuously reshuffled as evolution proceeds. Many shortcuts and new routes are established (herbivorous mammals feed directly on monocotyledinous angiosperms; mammals utilize bacterial fermentation in digestion), but the rules generally hold and explain the process of organic diversification much more satisfactorily than a supposed trend toward complexity.
Huxley once likened the process of evolution to the filling of a barrel. If we first put large rocks into a barrel, some (ecological/eutrophic) space remains; it can be filled with smaller rocks, then gravel, then sand, and so on. The important point is that the barrel does not expand, and that the remaining space is continually reduced. Once the barrel is filled, the system continues to reshuffle and adjust, but there is no inherent drive toward complexity. It is probable that the placental mammals represent the last major addition (in terms of trophic level) to Earth's ecosystem, and there is no basis to assume that, had man not evolved, some still more complex form of life would have appeared on this planet at some point.
Our second question concerns the postulated tendency of complex organisms to develop progressively increased intelligence. Three orders of extant mammals-the Cetacea, Proboscidea, and Primates-demonstrate a greater degree of encephalization (brain volume allometrically adjusted for body mass) than other mammals. The question of intelligence in the Cetacea is still unresolved, but it is clear that much, if not most, of their unusual brain size is a direct consequence of their elaborate development of echolocation. They are social animals as well, and this also increases their need for association areas and pathways. It is difficult to judge the relative success of these mammals, inasmuch as they are the only major mammalian-grade fauna in the world's great ocean expanses, the pinnipeds (carnivore) being restricted to inland and coastal regions. The relatively small biomass of the cetaceans (compared to the remaining ocean biomass) is evidence of the limitations on the uppermost position of trophic pyramids.
The success of the Proboscidea is of interest because their levels of encephalization are also markedly higher than those of other mammals (Jerison, 1973). Yet they are clearly a relatively unsuccessful group, being  represented today by only two living species, the remains of a much greater radiation that began to wither in the Miocene era (Maglio and Cooke, 1978) This order shows clearly that encephalization in no way guarantees the survival of a species or grade. The Proboscidea are survived by less-encephalized but equally large occupants of similar ecological zones.
In terms of understanding the role of intelligence in the survival and evolutionary progression of mammals, however, it is best to turn to the order Primates since our knowledge of the anatomy and fossil record of this group is relatively complete. In fact, the order provides a direct test of many of the claims often made for the selective advantages of "intelligence."
The primates can be traced to the earliest stage of mammal radiation at the beginning of the Cenozoic era (Simons, 1972). The origin of the adaptive complex that defines them can be related to the occupation of an arboreal econiche and a feeding strategy heavily reliant on insects and small vertebrate prey (Cartmill, 1975). Only this particular combination of selective factors can explain the grasping, prehensile chiridia of primates in contradistinction to the more effective clawed chiridia of arboreal insectivores. Several anatomical trends can be identified by a review of the comparative anatomy and fossil record of living and extinct primates. Among these are a tendency to retain many generalized ("primitive") characters, with the addition of stereoscopic vision, olfactory reduction, and social behavior marked by intense parenting. Other trends include the bearing of single offspring, increased placental "efficiency," increased life span and maturation time, and finally an increase in the degree of specialization. In the search for trends toward complexity and intelligence, it should always be remembered that this order is distinguished by membership in the cohort unguiculata, the most primitive of the four mammalian cohorts (Colbert, 1969).
Encephalization in primates is a clear consequence of several anatomical complexes, including hand-eye coordination, keen vision, intense parenting, and complex social behavior. Ignoring for a moment the specific evolutionary history of man, it is instructive to look at the sequence of events that can be read from the primate fossil record. All of the above trends can clearly be seen in the hominoids of the Early and Middle Miocene. This group was directly ancestral to the present-day great apes (gorilla, chimpanzee, orangutan) and man and inhabited most of the Old World. Enough is known to suggest that the primate trends described above were for the most part fully expressed in this group in a fairly advanced state, including considerable encephalization (Radinsky, 1974, 1975). Most of this radiation disappeared during the Late Miocene (Andrews and van Couvering, 1975), and its few extant descendants are relict species surviving only in small areas of the Old World in which few significant environmental changes have occurred (Lovejoy, 1981).
 The Dryopithecinae are the best case of mammalian evolution in which to investigate the selective value of intelligence. Both the great apes and man are their direct descendants. Elements related to the evolution of cognition in man (prehensile forelimbs, social behavior, stereoscopic vision) were clearly present in the Dryopithecinae. Yet the descendants of this group (with the exception of man) are either extinct or relict and proceeding toward extinction. The primary reason for their disappearance is probably demographic and related to the reproductive process.
Two distinct strategies of reproductive behavior are often recognized in animal studies and referred to as "r" and "K" selection. While these terms represent ends of a continuum of reproductive adaptation, they are a useful heuristic device with which to discuss the dynamics of reproduction.
Only a portion of an organism's total energy budget can be used for reproduction. Other activities (predator avoidance, food searching, digestion, etc.) take up the remainder. An organism is faced with a "choice" of how its reproductive energy is to be employed in the most effective way. The energy may be spent in the production of a great number of offspring of which few survive for parenting; or, at the other end of the continuum, only one offspring may be produced, with the remaining energy used in its protection and care. Cooperative effort may enhance the success of the latter strategy, and this is a prime selective factor favoring social behavior. The obvious terminus of the r-K continuum is the production of single offspring, a universal characteristic of higher primates.
There remains one further variable: the length of time devoted to the production and parenting of offspring (i.e., the length of gestation and infancy) Clearly, there is also a point at which parental investment exceeds reproductive yield. The development of the central nervous system, more than any other factor, determines the length of time required for development. Prenatally, the brain requires a constant, uninterrupted supply of oxygen and nourishment, and postnatally it would not serve any useful function were it not for a prolonged period of exercise and development (it may also be pointed out that the human brain does not mature histologically for almost 10 years). Intelligence can aid environmental understanding, but its development is at the same time an extreme liability. The vast majority of Successful placental mammals display low levels of encephalization and correspondingly high levels of reproductive fitness and evolutionary success.
Unless intelligence enhances feeding strategy or locomotion, it may be expected to serve a minimal function in the survival of an organism. Intelligence may also serve in predator avoidance, but this is probably not crucial because equally successful but less costly avoidance mechanisms are normally available (coloration, olfaction, nocturnality, etc.). The encephaliztion found in primates (and cetaceans) is to a great extent directly related to  feeding strategy and locomotion. It is a highly unusual anatomical complex and it is unlikely to constitute a general trend in evolution '
To summarize this part of the discussion, an increased association capacity in the nervous system represents a reproductive liability both pre- and postnatally and may therefore be expected to undergo positive selection in only rare instances. Primates represent such an instance, because encephalization in this order can be accounted for directly by feeding strategy and locomotion at the first level and by reproductive cooperation at the second. No trend can be invoked toward encephalization in mammals using primates as an example since this order is unusually primitive in the majority of its mammalian traits. Within the evolution of the primates, a continuum may be seen in which a clearly defined limit on the degree of encephalization was reached sometime in the Miocene, and the expected endpoint of the K strategy was reached by hominoid primates, which were subsequently replaced by less encephalized, more reproductively successful cercopithecoids (Lovejoy, 1981). This clearly distinguishable end point on the r-K reproductive continuum serves as an ultimate "stop" in the evolution of "intelligent" organisms. There is no a priori advantage to intelligence, although it is a clear and unmistakable reproductive hazard.
Taber (1973) defines intelligence as "the capacity to comprehend relationships." This definition is appropriate since it is practically synonymous with the functions of the association areas and pathways of the vertebrate cerebral cortex. All animals with the ability to encode and utilize relational experience may thus be called intelligent, and a partial anatomical indicator of intelligence can be obtained from Lashley's (1949) observation that portions of the brain not directly associated with somatic function (as judged by body mass) "seem to represent the amount of brain tissue in excess of that required for transmitting impulses to and from the integrative centers
Using this logic, Jerison and others have attempted to make estimates of "surplus" or "extra" neurons in mammalian brains by considering brain volume, neuron density, and body mass. Early hominids, which had brain/body mass ratios only 1/3 those of modern man, do not yield values greater than those of some other mammals. Yet these same hominids are known to have made tools (an indication of abstract symbolizing). The brain of modern man therefore should not be regarded as the product of a simple increase in neuron number, but rather as the result of a progressively enhanced capacity of an already (at least partially) structured cerebral cortex (Holloway, 1974,  1975, 1976, 1978). A computer analogy can be used to restate the argument as follows: A computer without a compiler or input/output device is useless, no matter what the capacity of its memory. Expansion of any computer to astronomical memory capacity would serve no purpose were it not accessible to at least one language. The human brain, then, is a highly specialized organ, as much so as the kidney of the desert-dwelling kangaroo rat or the salt gland Of the albatross. Its mass is a product of selection for improved capacity but is not of itself responsible for that capacity (otherwise selection could not have acted). The critical structure of the human brain can thus be traced to an earlier evolutionary state in which it was not distinguished by its size, but rather by its structure. The antecedent selectional pathways of this structure are thus critical to an understanding of human evolution.
With reference to the computer analogy, a complex brain would not be accessible to selection in early hominids. Much of our ability to utilize symbols, formulate abstractions, and recognize the self depends on the phenomenon we call speech. The ability to speak is a neurophysiological character. It can be localized to specific areas of a single hemisphere and is associated with specialized histological structures in that hemisphere (Penfield and Roberts, 1959; Geschwind and Levitsky, 1968; Sperry, 1970). In short, the brain is preprogrammed for speech, including the necessary motocortical pathways to various peripheral anatomical structures such as the larynx, tongue, and lips. It is the capacity for complex symbolization, made possible by speech, that provides man the capacity for self-conception. Man is not an intelligent animal so much as a cognitive animal. The distinction is far from trivial. It is our ability to symbolize and not our ability to utilize relational experience that allows abstract communication. We are the only mammal with this ability, and the pathway by which this capacity evolved is critical for determining the probability of cognitive life elsewhere in the Universe.
It is important to point out that man did not develop the peripheral anatomical structures requisite for speech as a consequence of selection for speech. As duBrul (1958) established, the anatomical specializations that allow the formulation of voiced and unvoiced phonemes (including the control of the nasopharyngeal passage, the separation of the laryngeal and palatal aditi, the migration of the diagastrics from the hyoid, and the spreading of the oral diaphragm) are direct functional adaptations to the habitually erect posture induced by bipedal locomotion. Furthermore, the modifications of the oral cavity that permit consonant formation were primarily brought about by early hominid dietary specializations, including a reduction in mandibular length, loss of the sectorial canine complex and anterior tooth reduction, and elevation of the temporomandibular joint. These changes were induced by greater seasonality of food sources and a proliferation of Miocene mosaic environments; they were originally completely unrelated to speech capacity.
 Speech as a communicating medium could not have arisen de novo. Pre speech mechanisms of communication were almost certainly prerequisite and probably included a unique constellation of behavioral mechanisms such as manual gesturing, plasticity of facial expression, primitive vocalization, and intense parenting (Hewes, 1973; Holloway, 1969; Falk, 1978, 1980). No other order of mammals evinces the manual dexterity, facial plasticity, or parenting mechanisms of primates, and no other primate shows evidence of the dietary, locomotor, or sexual specializations of early hominids (Lovejoy 1981). In short, man is not only a unique animal, but the end product of a completely unique evolutionary pathway, the elements of which are traceable at least to the beginnings of the Cenozoic. We find, then, that the evolution of cognition is the product of a variety of influences and preadaptive capacities, the absence of any one of which would have completely negated the process, and most of which are unique attributes of primates and/or hominids. Specific dietary shifts, bipedal locomotion, manual dexterity, control of differentiated muscles of facial expression, vocalization, intense social and parenting behavior (of specific kinds), keen stereoscopic vision, and even specialized forms of sexual behavior, all qualify as irreplaceable elements. It is evident that the evolution of cognition is neither the result of an evolutionary trend nor an event of even the lowest calculable probability, but rather the consequence of a series of highly specific evolutionary events whose ultimate cause is traceable to selection for unrelated characters such as locomotion and diet.
The evidence bearing on the evolution of cognitive life on suitable planets is necessarily derived from our knowledge and understanding of the process and participants of organic evolution as it has occurred on this planet during the last 3.5 billion years. Three conclusions seem reasonable:
1. Planets on which complex autotrophs evolved may be expected to support pyramids of eutrophic organisms in ecosystems that tend toward complexity. There is no evidence, however, to support the belief that their participating organisms would demonstrate continually increasing complexity. The strictures of increasing cell/organ unreliability and the limitations of the r-K reproductive continuum would operate as effective "stops" in such complexity, as they appear to have done on this planet.
2. Intelligence is not the product of a general trend. Increased capacity of a central nervous system to encode relational experience is only useful in the selective sense if it improves the capacity of the organism's other activities (locomotion, reproduction, feeding behavior). Intelligence also leads to  increased reproductive liability, and the most successfully adapted organisms (including mammals) on this planet exhibit lower brain/body ratios than most others. Only in rare instances where "intelligence" enhanced the locomotor or feeding strategies of mammals (echolocation, hand-eye coordination in arboreal environments, and small object feeding, etc.) was it favored on this planet.
3. While "intelligent" animals may evolve on other planets by relational pathways similar to those seen on Earth, the phenomenon of "cognition" is quite distinct from that of intelligence and may be expected to be exceedingly rare.
Thus I conclude that man is a highly specific, unique, and unduplicated species. If we wish to make probability estimates of the likelihood that cognitive (not intelligent) life has evolved on other suitable planets, the simplest and most direct question we may pose is: What is the probability that cognitive life would evolve on this planet, were not man already a constituent of its biosphere? From what we know of the human evolutionary pathway and of the critical elements that have directed it, the odds against its reexpression are indeed remote, if not astronomical. No other mammal even remotely shares the unique attribute complex that defines either man or his evolutionary pathway. Since Homo sapiens is a unique species, we may ask this same question in a slightly different way that allows greater objectivity: What is the probability that any named species, be it mammal, reptile, or mollusk, would evolve again on this planet? That is, what is the probability that the Bornean long-tailed porcupine, for example, would appear again were the evolutionary process to be reinstated on some imaginary planet identical to ours in every way save the last half billion years? I think it quite reasonable to suppose that despite the immensity of the known Universe, the specificity in the physiostructure of any organism is so great and its immensely complex pathway of progression so ancient that such probabilities are simply infinitesimal.
In any case it is reasonable to suggest that our understanding of the process of evolution responsible for the appearance of cognitive life will, for the foreseeable future, depend directly on our knowledge of the mechanisms that produced man, and that future predictions of exobiological cognition should be fully tempered by the knowledge of our own evolution.
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