a mechanism for the origin of the human brain: a hypothesis

A Mechanism for the Origin of the Human Brain: A HypothesisAuthor(s): Konrad R. FialkowskiSource: Current Anthropology, Vol. 27, No. 3 (Jun., 1986), pp. 288-290Published by: The University of Chicago Press on behalf of Wenner-Gren Foundation forAnthropological ResearchStable URL: http://www.jstor.org/stable/2742893 .

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A Mechanism for the Origin of the Human Brain: A Hypothesis'

by KONRAD R. FIAi+KOWSKI Iglaseegasse 68, 1190 Vienna, Austria. 9 XII 85

Increase of brain volume is one of the basic features which differentiate the australopithecines and other human ancestors from modern Homo sapiens. Compared with most evolutionary changes, this increase was rapid, amounting during the Pleistocene to an average of 50-100 cm3 per 100,000 years. The increase in hominid brain volume can be described by an S-shaped curve (see Goralski and Wierciiiski 1964).

The mechanism for this adaptation is unclear (Bielicki 1984). It is generally held that simple tool using and tool making and certain social requirements of the hunting mode of life generated powerful selection pressures for new mental abilities, such as planning, memory, more complex and more efficient communication, etc. However, tool making and tool using seem insufficient to explain the beginning of the growth. The relatively small brains of the australopithecines were sufficient for making simple stone tools. Whereas the brain more than doubled in size during the Lower and Middle Pleis- tocene and its growth was exponential, stone tools show no comparable breakthrough in sophistication. If there was positive feedback (increase in tool sophistication -> increase in brain volume -> increase in tool sophistication, and so on), it should have resulted in a more evident acceleration and perhaps qualitative change in the sophistication of tools. On the contrary, effective hunting projectiles with hafted points are apparently absent prior to the terminal Middle Paleolithic and Upper Paleolithic (Camps-Fabrer 1974; Oakley et al. 1977; Klein 1978; Binford 1981, 1982). The lack of comparable accel- erations in the two processes may indicate the lack of positive feedback between them; it may mean that although increase in brain volume produced gradual increase in tool sophistication, it was itself independent of tools and stimulated by other mechanisms.

The idea that factors other than tool making or tool using triggered brain-size increase has already been expressed, for example, by Mayr (1970:384): "Evidently the use and even the making of simple stone tools was not the decisive factor that led to the spectacular increase in brain size in the 1 million years between 1.3 and 0.3 million years ago." One of the im- mediate consequences of this approach may be the notion that increase in brain volume was a preadaptation for abstract thinking (i.e., that the human brain is a side effect of an un- known adaptation that had nothing to do with abstract thinking). Indeed, only the side-effect hypothesis can explain why, under such strong selection pressure as apparently operated in the Middle Pleistocene, the hominid brain did not develop into a simpler one just sufficient to process the information neces- sary for a pack-hunting predator (a primate analogue of the wolf or the hunting dog) to control the manufacture of simple stone tools for crushing and the use of sticks or bones for killing, intimidation, and digging. If the hunting way of life generated selection pressures for new mental abilities, such a relatively simple, specialized brain would have been efficient enough to reduce them and make the rapid further evolution of the brain essentially unnecessary. Human brain capabilities definitely exceeding those required for hunting constitute the key problem of hominid evolution. Preadaptation seems the only evolutionary pattern that might result in the observed

1 ?B 1986 by The Wenner-Gren Foundation for Anthropological Re- search, all rights reserved 0011l-3204/86/2703-0012$1.00. The author is a member of the Committee for Evolutionary and Theoretical Biology, Polish Academy of Sciences.

288

disproportion of human mental capabilities and hominid environmental requisites.

My hypothesis for the origin of human brain, first presented in 1978 and since then discussed in various fora and revised, is that the increase in size and complexity of the hominid brain (at any rate, the early stage of the process) was largely a side effect of an evolutionary response to considerably increased heat stress under conditions of primitive hunting. This hypothesis is based on certain physiological data and on von Neumann's mathematical demonstration that the reliability of a complex system can be maintained in spite of a decrease of reliability of its elements provided that the number of elements and of their interconnections is increased. According to the hypothesis, it was only later that the more parallel structures created were applied to new mental tasks such as planning, memory, and communication.

ENVIRONMENTAL CONDITIONS

It is generally assumed that hominization began in the tropics. The skeletal remains of the earliest undoubted hominids (Johanson, Taieb, and Coppens 1982, Lovejoy, Johanson, and Coppens 1982), the australopithecines, show that these crea- tures were bipedal (Lovejoy, Heiple, and Burstein 1973), adapted to running and walking (Leakey and Hay 1979, White 1980), that their brain volume was very small (still within the upper limits of the range of variation of the great apes), and that in the Lower Pleistocene they were probably well on their way to adopting a very unusual (for a primate) hunting ecolog- ical niche (Wolpoff 1980). Early hominid hunting techniques are not known, but it seems logical to suppose that before such techniques developed into organized, cooperative big-game hunting they were for a long time essentially individualistic, involving long, strenuous pursuits of medium-sized mammals (primarily ungulates) in the open woodland or savannah. The persistence hunting (Krantz 1968, Watanabe 1971) still em- ployed by some modern human hunters must have been very exhausting for early hominids, a species not adapted to pro- longed physical strain in hot grasslands. It is widely assumed that the loss of fur, one of the most conspicuous evolutionary changes occurring in the hominid line, was primarily a result of adaptation to greatly increased heat stress associated with this type of behaviour.

Detailed discussion of persistence hunting and related energetic problems is given in Carrier's (1984) "The Energetic Paradox of Human Running and Hominid Evolution" and the comments on it. According to Carrier (p. 489),

analysis of the morphology and physiology of human locomotion, although incomplete and largely speculative, suggests that man is a primate that has become specialized for (among other things) endurance running. This points to a scenario concerning the origin of hominids that has been proposed in one form or another by several authors (Dart and Craig 1959, Ardrey 1961, Morris 1967, Krantz 1968). Exercising anthropoid primates can dissipate heat more rapidly than certain cursorial mammals. This suggests that the earliest hominids may have been more effective at heat loss than many other contemporary mammals. Furthermore, early australopithecines were fully bipedal. As a result, they were probably not constrained to the typical 1:1 breathing pattern of running quadrupeds. Consequently, it is possible that one of the important factors which differentiated hominids from other anthropoids, and ultimately led to the evolution of H. sapiens, was the occupation of a new predatory niche. This hy- pothetical niche was that of a diurnal predator which depended upon exceptional endurance in hot (midday) temperatures to disable swifter prey animals....

Alternatively, it has been argued on the basis of limb morphology that early australopithecines may not have been particularly skilled runners (Jungers 1982, Stern and Susman 1983). Also, analysis of the tools and animal bones from excavated Oldowan campsites (Leakey 1971), along with what is known of dental morphology, suggests that hunting may not have been an important hominid activity until the early Pleistocene (Wolpoff 1980).

CURRENT ANTHROPOLOGY



Thus, in the early Pleistocene hunting (involving long, exhausting runs) became an important hominid activity, and rapid brain-volume increase followed. What was it that triggered this development? Not sophisticated activities like planning or speech, as for them the initial small brain of early Pleistocene hominids seems too primitive.

THERMOHOMEOSTASIS AND HUNTING

Considering in detail the problem of thermohomeostasis in birds and mammals, most researchers have come to the conclu- sion that the main adaptive significance of this regulatory mechanism is the maintenance of a constant temperature of the brain (Baker 1979, Caputa 1983). The temperature associated with the highest enzymatic efficiency of brain biocatalyzers has been demonstrated to be 380 ? 20 C, varying slightly for different species within these limits (Narebski 1975). Below this limit, efficiency decreases, but in a reversible way. Above it, efficiency decreases rapidly, and a substantial decrease may cause a severe breakdown of brain functions. According to Baker (p. 120), "The brain is particularly sensitive to abnormal temperatures. A rise of only four or five degrees C above nor- mal begins to disturb brain functions. For example, high fevers in children are sometimes accompanied by convulsions; these are manifestations of the abnormal functioning of the nerve cells of the overheated brain. Indeed, it may be that the temperature of the brain is the single most important factor lim- iting the survival of man and other animals in hot environ- ments. " In some of the hunting or fast-running small mammals of the tropics, very sophisticated adaptations against brain overheating have been developed. In the case of Thompson's gazelle (Taylor and Lyman 1972), the heat exchanger (rete mirabile) between the two streams of blood allows-after a seven-minute run at 40 km/h-only a 10 C increase in the brain blood temperature, whereas the temperature of the rest of the arterial blood increases by as much as 50 C. Larger animals, such as the eland, have developed evaporative cooling mechanisms. A complete survey of brain-cooling mechanisms in mammals is presented in Baker's paper.

During a fast run, then, the prey of prehuman hunters was capable of maintaining its brain temperature within optimal limits. The hunters themselves began to develop a cooling system of their own involving the dispersion of heat through the skin of the cheeks (Caputa 1983), the loss of their fur, and the body-cooling evaporation mechanism described in detail by Carrier (1984). Cooling by evaporation is, however, highly wa- ter-consuming (contrary, for instance, to the gazelle's cooling system), and its efficiency could decrease after a long run at high temperature (Hammel 1968) or under conditions of high humidity, allowing blood temperature to rise to the point where the functions of brain cells would break down. Accord- ing to Caputa (1983, translation mine), "Even at 40-41? C there are malfunctions and structural changes in brain cells, while no pathological changes are observed in the body at this temperature. Although heat-stress death is usually caused by malfunctions of respiratory and cardiovascular systems, they are secondary to the primary malfunctions of the brain cells controlling these systems. " Because the number of brain cells is very large and they are spread out over an area in which temperatures (determined by very local blood temperatures) may vary slightly from one point to another, the deterioration process does not influence all the cells to exactly the same extent. In other words, the process is random in character. Its results may include convulsions, failure of cognition sufficient to lose the prey, and, sometimes, death.

RELIABLE SYSTEM FROM UNRELIABLE ELEMENTS

The mathematician and computer scientist John von Neu- mann has demonstrated mathematically that it is possible to

Vol. 27 *No. 3 *June 1986

obtain a reliable system for information processing in spite of the fact that some of its elements operate unreliably (von Neumann 1963 [1952]). He writes (p. 353), "The complete system must be organized in such a manner that a malfunction of the whole automaton cannot be caused by the malfunctioning of a single component, or of a small number of components, but only by the malfunctioning of a large number of them." According to von Neumann, a reliable system for information processing having unreliable elements can be achieved by in- creasing the number of basic elements and, more important, of the interconnections between them. This points to a kind of solution that could have been utilized in the process of evolutionary adaptation in the case of random damage to brain cells. For adaptive purposes, all that mattered was the reliability of the brain as a whole. If, under greater heat stress, the cooling system became unable to prevent the impairment of some neurons, maintenance of the effective functions of the brain as a whole might have been achieved by an increase in the number of neurons and, more important, a substantial increase in the number of interconnections between them. As the result of this adaptation, the prehuman brain would have been able to maintain its information-processing capabilities unimpaired at temperature levels slightly above those which would have been detrimental to nonadapted brains. If reliability adaptation oc- curred, then the interconnections between the elements in the human brain would be expected to be significantly more parallel than in the brains of the great apes.

The structural redundancy of the human brain is a fact. Comparing the human brain with the brain of a chimpanzee, one finds that its volume is three to four times greater but, owing to the lower packing density of the neurons, contains only one and a quarter times the neurons. The difference is approximately 1.4 billion neurons. Moreover, surgical removal of up to 1.4 billion neurons in many cases does not significantly interfere with brain functions (Holloway 1966). The main difference between the human brain and that of a chimpanzee probably lies in the number of interconnections: in the human cortex dendrites have more branches and axons are statistically shorter. Most researchers comparing these brains morphologi- cally agree that their elements are alike and the main difference between them is in their organization. Thus, the human brain has a different, more parallel organization, and this organization is specific to it. This difference between human and chimpanzee brains is basically consistent with von Neumann's prin- ciple of a reliable system from unreliable elements. That the third element of the causal link heat stress -* reliability adaptation -* more parallel organization is observed in reality and specifically in the human brain adds support to the hypothesis.

Further support may be found in studies of long-distance runners. Hamilton (1973:180-81) writes:

Body temperatures do not build up in modern (or hairless) man during short sprints, but heat continues to accumulate for several minutes depending upon the place and ambient condition-wind, temperature, radiation and humidity. Body temperatures of man as high as 41.10 C have been recorded by Robinson (1963) for world record runners after racing 14 and 30 minutes under warm (300 C), humid conditions. This same temperature, 41. 1?C, was recorded by Pugh et al. (1967) as the maximum temperature of 47 marathon (42 km) runners at the end of a race. The highest value was for the individual who won the race. Pugh et al. conclude that an ability to tolerate a high body temperature is a requirement for successful marathon competition. This temperature, 41.10 C, seems to be close to man's limit in competition, which is no surprise considering how close it is to lethal limits, somewhere between 41 and 450 C. These considerations might seem to lend credence to the contention that preman exercising a hunting trade was limited by overheating, especially if there was further restriction by a permanent fur coat.

If the hypothesis is correct, the new structures of the brain created as a result of reliability adaptation should tend to support mainly the vital functions. However, exactly the opposite

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is observed; the increase in brain size affects most the cerebral association areas and much less the systems subserving the vital functions. This contradiction can be explained only through the fact that the brain we observe now is the result of at least 100,000 years' adaptation towards abstract thinking (from the turning point of the S-curve). Therefore, it is possible that the cerebral association areas have changed their functions, originally having been mainly oriented towards the vital ones. Further, the disappearance of those original functions may not be complete. There may exist (at least potentially) a mechanism for the control of functions such as breathing, heart rhythm, etc., from those areas, and this mechanism should be specific to H. sapiens. This implication of the hypothesis cannot at present be fully verified, but observations of specially trained individuals suggest the existence of such a mechanism.

Again, if the hypothesis is correct, then the resistance of the great apes to heat stress caused by extreme effort should be sub- stantially less than that of man. This implication is testable.

The model assumed by the hypothesis implies that the turning point of the S-curve of brain-size increase (the onset of saturation) depends only on the level of brain reliability attained and inevitably appears after a sufficiently long period of selection; its appearance is not necessarily connected with a change in behaviour. In fact, however, the rate of brain expansion could have declined earlier (with less reliability attained) if, for example, improvements in the organization and technology of hunting (based on mental abilities resulting from the reliability acquired) had made hunting less strenuous in terms of the amount of heat stress. This is what probably happened, sometime during the Pleistocene, and from then on the evolution of the hominid brain was no longer a "side effect" and intellectual abilities sensu stricto (planning, memory, communication) became the principal target of selection. One may speculate that if the process of reliability adaptation had been allowed to continue, i.e., if our ancestors had not begun to use their brains in a human manner too early, their brains would have developed into even more complex ones and the mental capabilities of man would be greater.

A model according to which hominid brain expansion was an evolutionary adjustment to less reliable operation of brain elements under conditions of severe heat stress during hunting explains the disproportion between the vast range of observed and expected human mental capabilities and the real needs and the relatively low behavioural complexity of the species during much of its evolutionary history. The hypothesis could have (at least theoretically) a broader scope of application. Actually, it is a model in which encephalization is a side effect of adaptation to an environment which in some manner, with not too high a probability, damages individual brain cells. Another such damaging factor might be, for example, the lack of oxy- gen for mammals exchanging a terrestrial environment for wa- ter. The lack of evident examples of such adaptation in the history of life on Earth could be a result of the extremely strong selective pressure that perhaps is connected with this kind of adaptation, making successful adaptation through encephalization improbable.

References Cited ARDREY, R. 1961. African genesis. New York: Dell. BAKER, M. A. 1979. A brain-cooling system in mammals. Scientific

American 5:114.

BIELICKI, T. 1984. Klopoty z ewolucja czlowieka. Problemy 2:2. BINFORD, L. R. 1981. Bones. New York: Academic Press.

. 1982. Comment on: Rethinking the Middle/Upper Paleolithic transition, by R. White. CURRENT ANTHROPOLOGY 23:177-81.

CAMPs-FABRER, H. Editor. 1974. L'industrie de l'os dans la prehis- toire. Aix-en-Provence: Editions de l'Universite de Provence.

CAPUTA, M. 1983. Obrona mozgu przed przegrzaniem. Problemy 11:17.

CARRIER, D. R. 1984. The energetic paradox of human running and hominid evolution. CURRENT ANTHROPOLOGY 25:483-95.

DART, R. A., and D. CRAIG. 1959. Adventures with the missing link. New York: Viking.

FIA*LKOWSKI, K. R. 1978. Early hominid brain evolution and heat stress: A hypothesis. Studies in Physical Anthropology 4:87-92.

GORALSKI, A., and A. WIERCII4SKI. 1964. Report of VIth Interna- tional Congress on the Quaternary, vol. 4, pp. 259-90.

HAGEL, H. T. 1968. Regulation of internal body temperature. Annual Review of Physiology 30:641-710.

HAMILTON, W. J., III. 1973. Life's color slide. New York: McGraw- Hill.

HOLLOWAY, R. J. 1966. Cranial capacity, neural reorganization, and hominid evolution: Search for suitable parameters. American An- thropologist 68:103-21.

JOHANSON, D. C., M. TAIEB, and Y. COPPENS. 1982. Pliocene hominids from the Hadar Formation, Ethiopia (1973-1977): Stratig- raphy, chronology, and paleoenvironmental contexts, with notes on hominid morphology and systematics. American Journal of Physical Anthropology 57:373-402.

JUNGERS, W. L. 1982. Lucy's limbs: Skeletal allometry and locomotion in Australopithecus afarensis. Nature 297:676-78.

KLEIN, R. G. 1978. Stone Age predation on large African bovids. Journal of Archaeological Science 5:195-217.

KRANTZ, G. S. 1968. Brain size and hunting ability in earliest man. CURRENT ANTHROPOLOGY 9:450-51.

LEAKEY, M. D. 1971. Olduvai Gorge. Vol. 3. Cambridge: Cambridge University Press.

LEAKEY, M. D., and R. L. HAY. 1979. Pliocene footprints in the Laetoli beds at Laetoli, northern Tanzania. Nature 278:314-23.

LOVEJOY, C. O., K. G. HEIPLE, and A. H. BURSTEIN. 1973. The gait of Australopithecus. American Journal of Physical Anthropology 38:757-80.

LOVEJOY, C. O., D. C. JOHANSON, and Y. COPPENS. 1982. Hominid lower limb bones recovered from the Hadar Formation: 1974-1977 collections. American Journal of Physical Anthropology 57:373- 402.

MAYR, E. 1970. Population, species, and evolution. Cambridge: Har- vard University Press.

MORRIS, D. 1967. The naked ape. New York: Dell. NAREBSKI, I. 1975. Stalocieplnosc: Wielki kompromis przyrody. Prob-

lemy 1:2-8. OAKLEY, K. P., P. ANDREWS, L. H. KEELEY, andJ. D. CLARK. 1977.

A reappraisal of the Clacton spearpoint. Proceedings of the Prehis- toric Society 43:13-30.

PUGH, L. G. C. E., J. L. CORBETT, and R. H. JOHNSON. 1967. Rectal temperatures, weight losses, and sweat rates in marathon running. Journal of Applied Physiology 23:347-52.

ROBINSON, S. 1963. Temperature regulation in exercise. Pediatrics 32:691-702.

STERN, J. T., and R. L. SUSMAN. 1983. The locomotor anatomy of Australopithecus afarensis. American Journal of Physical An- thropology 60:279-317.

TAYLOR, C. R., and C. P. LYMAN. 1972. Heat storage in running antelopes: Independence of brain and body temperature. American Journal of Physiology 222:114-17.

VON NEUMANN, J. 1963 (1952). "Probabilistic logic and the synthesis of reliable organisms from unreliable components, in Collected works, vol. 5, pp. 329-78. London: Pergamon Press.

WATANABE, H. 1971. Running, creeping, and climbing: A new ecolog- ical and evolutionary perspective on human locomotion. Mankind 8:1-13.

WHITE, T. D. 1980. Evolutionary implications of Pliocene hominid footprints. Science 208:175-76.

WOLPOFF, M. H. 1980. Paleoanthropology New York: Knopf.

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a mechanism for the origin of the human brain: a hypothesis

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