Psychobiology and Neuroscience

Paleoneurology and the origin of man

by Mauro Mancia, Milan *

In this paper I would like to present the contribution that neurophysiology has recently made to paleontological research. I will be making reference to the work of Jerison (1973, 1976), to the work of Jerison (1973, 1976), Holloway (1974), Leakey & Lewin (1977), Leroi-Gourhan (1965) and Eccles (1979).
At the basis of this article is the concept that Figure I shows the ratio between body and the development of the brain and of its plasticity has permitted adaptations to environmental changes in such a way as to be a necessary step in any process of natural selection. Therefore the history of course of evolution that led to the human species is also and particularly the history of course of evolution of its brain and of the ever more
complex operative circuits which have permitted Man to adapt to any change in his environment.
An evaluation of this great development can be deduced from a study of the encephalization quotients (EQ) elaborated by Jerison (1973). This EQ indicates the ratio of the actual brain size to the expected one as given by the average for living mammals. It is expressed by the formula E = K.P 2/3, where E indicates brain weight, P body weight and K a constant which for mammals , can be considered equal to 0.12.
Figure I shows the ratio between body and brain weight according to EQ.

FIG. I - Brain and body weights of tbe largest specimens of 198 vertebrate species graphed on log-log coordinates. RectangJe contains the full range of data on living men from their sample; four points on the borders of the rectangle are tbe heaviest and lightest individuals with respect to brain and body.
(From Jerison, 1969)

Two distinct goups are found: the higher represents mammals, primates and birds; the lower reptiles and skeleton fishes. The four linked points represent Man's EQ. All points lie along two curves whose slope is about 2/3 on the logarithmic coordinates, which means that the weight of the brain in different species varies with the body weight elevated to 2/3 according to the EQ formula. The study of the EQ has shown that mammals 60 million years ago had a EQ of 0.3 which has remained unvaried only for basal insectivores but has rapidly increased, starting 50 million years ago, for primates and dolphins. As can be seen in figure 2, dolphins began to increase their EQ, which reached the value of 6, as early as 30 million years ago, while prosimians remained at the value of 1.

FIG. 2 - Calculated encephalization quotiens (E.Q.) are plotted against the time in millions of years before the present age. Note change in time scale at 10 million years. BI, basal insectivores; Al, advanced insectivores; P, prosimians; AF, Australopithecus africanus; HE, Homo erectus. The solid square and circle are H, Neanderthalis and Homo sapiens respectively.
(From Eccles., 1979)

The brain of Homo abilis developed from that of Australopithecus about 3 million years ago with a EQ of 4.2, while Homo erectus reached its EQ of 6.5 only 1 million years ago from which an EQ of 8.5 which characterizes Homo sapiens has developed.
Figure 3 shows that from an evaluation of the EQ it is possible to distinguish three different points of the same curve for three different animals. The monkey, which has a brain four times larger (48 g.) than the presumed weight (12 gr.), has an EQ of 4. For a dog the ratio is only 1 and for a stupid animal like a tapir it is 0,5.

FIG. 3 - Q.E. for three different species of mammals according to Jerison formula. Note that a monkey whose brain is four time larger ( 48 g.) than the presumed weight ( 12 g.) has a Q.E. of 4. For a dog the ratio is 1 and for a stupid animal like a tapiro is 0,5.
(from Jerison, 1976)

If we now look at the most significant contribution that neurophysiology has given to paleontology , we can see that it derives from particular techniques permitting the study of casts of fossil hominid brains, particularly of the Australopithecus discovered by Leakey in 1959. The evaluation of brain volume, shape and degree of corticalization thus achieved made it possible to advance an hypothesis on the origin of Man (Holloway, 1974).

This hypothesis suggests that the common ancestor of man and monkey is the Driopitecus who lived in central and northern Europe, Asia and East Africa from 30 to 40 million years ago in the Cenozoic period. It is possible that Ramapithecus can already be considered to be a hominid on the basis of the study of the teeth and jaw, which are unfortunately the only fossils found. Ramapithecus had the same diffusion as Driopithecus and this seems to be a good reason for considering it to be the first link in a paleonthological chain eventually leading to Homo Sapiens.
Proof is lacking regarding the passage from sulcus Ramapithecus to Homo Sapiens. Two hypothesis can be put forward: one states that Homo Sapiens has derived directly from Ramapithecus through Homo abilis (1.9 million years ago) and Homo erectus (750.000 years ago). According to this hypothesis, Australopithecus (Africanus, Boisei, Robustus) would represent a branch in extinction (fig. 4). According to a second hypothesis, Australopithecus africanus (3 million years ago) is the essential link in the passage from Ramapithecus to Homo Sapiens .

FIG. 4 - Diagrams of alternative views of man's evolution. In the lower, the Australopithecus Africanus is on the evolzctionary line that led to Homo sapiens, in the other it was on a side branch that died out. Libassi, P.T.: Early man,nearly man.
(From Eccles., 1979).

Back in 1959, L.B.S. Leakey discovered in Tanganyika an Australopithecus which he called Zinianthropus Bosei, and which walked in an erect position, used tools and whose brain weighed 530 g. Successively, the discovery of another fossil in the coast of Lake Rodolfo, named E.R. 1470, whose skull dated at 2.8 million years ago, permitted the hypothesis that this hominid might represent Homo abilis whose brain weighed from 500 to 700 gr.
The evidence suggesting that Australopithecus is the direct ancestor of Man is of a different nature: a) direct, based on the examination of casts of fossil hominid brains; b) indirect, based on a comparative study of brain measurements and of various fossils such as bones, vertebrae,jaws and teeth; c) deductive, including the study of tools made and used by hominids. All these findings suggest that our ancestors had good sensory-motor performance and therefore had reached a relatively high level of cultural development.

Examination of the various fossil remains confirms the presence of manlike attributes in the Australopithecus and suggests an erect posture:

a) particular form of the tarsus, of the skeleton of the leg and of the pelvis;
b) vertical spinal column;
c) conformation of hand suited to human-like activities ;
d) head balanced at top of spinal column;
e) parallel toes.

Confirmation of this last feature is also found in Leakey's discovery (1976) of Australopithecus footprints fossilized in a layer of ash ejected 3.6 million years ago from the crater of a volcano
The seelningly best preserved print shows a higharched foot, a rounded heel and a forward-pointing great toe as is to be found in erect walkers.
But above all a study of the cranium of the Australopithecus discovered by Leakey has revealed the essentially human neurological organization of the brain of these three million years old hominids.
The brain, though of relatively small weight (around 500 g..) is of human morphology with considerable expansion of the parietal, temporal and frontal lobes, human position of the sulcus lunatus which bounds the occipital lobe, and a more arcuate brain (i,e., a brain more developed vertically than in other primates) due to a greater development of the parietal lobe. It is on the basis of these typically human attributes that some authors such as Holloway (1947) declare these brains to have an essentially human organization, and that Leroi-Gourhan (1965) uses the term Australanthropi.
In the most modern hypothesis advanced and discussed by paleonthologists the acquisition of the erect posture represents the primary and fundamental element of all stages in the evolutionary development of Man. This acquisition in fact gives rise to a series of processes of liberation of functions and, in a parallel fashion, of structures such as to determine the progressive transformation of prehistoric Man into Homo sapiens.
Acquisition of the erect posture has as a first consequence the foreshortening of the face, since the mouth no longer being used in the search for food or in instinctive function's (defense-offense), the anteroposterior diameter of the face can be reduced and hence its structure be totally changed. The mouth, now no longer having its older functionscan develop along the lines of new requirements, amongst which those relatirig to language and symbolic functions are of primary importance. The hand no longer occupied in walking can now begin to fashion and use the objects which are to be the tools of primitive man, and can also be used for communication as an adjunct to language.

The considerable development of these completely new functions of the hand, the mouth and the organs of phonation now gives rise to development of the brain which becomes essentially human. Its volume increase progressively, and a widening of the medial fronto-temporoparietal region and a curvature of the cerebral vault are to be found. This latter develops side-by-side with a marked widening of the cranial vault which progressively opens out "like a fan" In Australopithecus this is incomparably greater than in the anthropomorphic ape and is destined to increase still further up to Home sapiens.
The aperture of the bony fan which, as Fig, 5 clearly shows, remains at 20" and 40" respectively in the ape and gorilla, whilst it already reaches 60" in Paleanthropus and 70" in Home sapiens, in such as to permit development of those cortical areas which constitute the parietal and, to some extent, the frontal and temporal lobes. In Fig. 6 it can be seen that in Australopithecus the parietal (areas 39 and 40), frontal (Broca's area 44) and temporal (area 22) cortex is already well developed. Some of these (in particular 39) show ample polysensorial convergence, have close connections with other home and contralateral associative areas and, in Man, are so extraordinarily developed that they can be considered to be, par excellence, the "associative areas of the associative cortex" (Geschwind, 1965; see also Vignolo, 1972).

The development of the cortical fan thus involves the areas of the associative brain, in other words that brain which neurophysiological research has related in the dominant hemisphere to language, gnosic and praxic functions, in other words to those superior functions which call for some degree of consciousness and permit some essential symbolic operations (Luria, 1962; Benton, 1965; see Bisiach ond others, 1978).

FIG. 5 - Cranium of a) hyena, b) dog, c) ape, d) gorilla, e) paleanthropus, and f) Homo sapiens. Note progressive widening of cranial fan from dog through Homo sapiens. Note also the significant difference in aperture between gorilla (40°) and Homo Sapiens (70°).
(From Leroi-Gourhan, 1964).

FIG. 6 - Brain of a) cat, b) macaque, c) chimpanzee, d) australopithecus, e) sinianthropus, f) Neanderthal Man, and g) Homo sapiens. Numbers on the various circumvolutions indicate central areas according to Brodmann. Nothe the considerable development of the parieto - occipital cortex in Australanthropus and the large cranial fan aperture which bring it very close to Neanderthal Man. In Homo Sapiens we find a further widening of the fan and an increase in the cortical areas of the "liaison brain". The drawing goes not show areas 39 and 40, which are to be considered in the space above area 43.
(From Leroi-Gourhan, 1964).

The contribution of paleoneurophysiology is thus to be identified in the discovery that new sensorimotor functions of adaptation to the environment and to locomotion correspond to significant modifications of cortical functions. Since such modifications have basically involved areas of the associative brain and their subcortical and inter-hemispheric connections, this means that symbolic and language functions, and hence all operations underlying creative processes have been conditioned over the millennia by evolutionary processes which primarily involved other functions and only in a secondary manner the specific development of determined cerebral areas.

Neurophysiological interpretation of paleonthological discoveries in fact bears evidence in favour that the change to erect locomotion goes hand in hand with an "avalanche" of events leading to a maximum of corticalization and a maximal development of the intellectual and creative capacities of Man.
But if we accept the evolutionist presupposition that, once it has begun, the process of evolution involves a system of growing complexity which permits, through the constant exercise of specific functions, modification of those very structures which make the functions possible, then the intellectual and creative process in Man, based on its corticalization and on the enrichment of the operative systems of the brain, then this process may be taken to be endless, unforeseeable in its development and tending substantially to modify those cerebral structures, which are involved in the superior functions.

Contingent to this paleontological consideration is the hypothesis (Popper and Eccles, 1977) that the liaison brain of the dominant hemisphere plays an essential part in the process of integrating information and activity "pattern" coming from the sensory world, as also in the operative mect~anisms underlying language functions.
The great development of the liaison brain, in the neuropaleontological hypothesis we are discussing here, becomes the dominant differentiating element in that evolution of the species which leads to the genus Homo rather than to the Prosimians.
In the ten million years gap from Ramapithecus to the appearance of the Australopithecus is probably hidden that evolutionary secret which has allowed only our own species to reach such a level of qualitative perfection in the cerebral circuits of the cortical liaison areas as to permit the development of language and the integration of those superior functions which lead to self-awareness, in symbolic thought and to artistic and scientific creativity.

It is in these functions that we may find the foundations of a mind-brain theory which , though remaining dualistic in its formulation, aims nonetheless to identify a level of interaction in specialized areas of the dominant emisphere. I am referring here to the dualistic-interactionist theory put forward by Popper and Eccles (1977) and recently furhered by Eccles (1979). In this article we could hardly undertake a detailed analysis of this theorie which makes it possible to view the mind-brain relationship on an experimental footing. In another publication (Mancia, 1982) I have examined certain aspects on this theory. It will suffice here to recall that , according to the Eccles - Popper hypothesis the "liaison brain" is the seat of interaction between World 1 of physical objects and World 2 in which the self-conscious mind is able to "scan" and "read" the activity of each modular unit of the cortex (Szentagothai, 1978). By this operation the self-conscious mind can organize, select and integrate in a unitary operation the information which filters into the conscious mind through the interface represented by the liaison areas of the dominant hemisphere.

This seems true also in ontogenetic development: the observation of a close correspondance and synchronization of the movements of the newborn with thelanguage of the adult (Condon & Sander,1974) suggests in fact the existence of a highly complex interactional system in which the motory behaviour of the newborn is guided andsynchronized by the adult who is speaking. Such a correlation indicates that the newborn child participates with a somethetic cerebral function (besides the acoustic and visual ones) in these basic neurobiological processees which underly the organization of its language. The synapses which the sensori-motor activity will form and activate in this process of maturation will involve those neurones of the liaison brain which make it possible which are later to govern the integration of sensori- motor functions proper to language.

This process explains the child's interest in, and the meaning of, its innumerable repetitions of linguistic forms in forms proper to somestethic sensoriality, before it is able to transfer them into more-specific functions directly connected with speech. The language of the child will thus be affected by the neurophysiological organization of the liaison brain which it has achieved as a baby, an organization which can hardly fail to be influenced by the form and structure of the linguistic system, and by the culture to which it belongs and is developing. If these ontogenetic deductions are of any validity, they may then be taken to constitute a valid argument also in support of the hypothesis which attributes to the hand and to its sensori-motor to function the main responsability for the development of those cerebral areas which belong to the cortical fan, and which in the dominant hemisphere govern the symbolic and sensori-motor functions of language.

The contribution of neurophysiological thought to paleontology is essentially this, and confirms that the development of the liaison brain is central in the mystery of human paleontology. They also throw light on the importance of particular sensori-motor conditions linked with locomotionin the differentiation , over millions of years, between specific brain functions which led form Ramapithecus to Australopithecus, and from these to the function of the human brain of today.


This paper reports some neurrophysiological contributions to paleontological research. The curves of the brain-body ratio and those of tile interactional encephalization quotients (EQ, given by E = K.P. 2/3 ) plotted against time up to the present age for various species, suggest that the EQ remains at 0.3 only for basal insectivores while dolphins has already reached the value of 630 million years ago and Home Erectus, who developed from Australopithecus reached an EQ of 7 only 1 million years before the present age.

Paleontological characteristics of the Australopithecus are presented which make it possible to consider this fossil a hominid with human capabilities including language and use of tools. NeurophysiologicaI evidence supports the paleontological hypotesis that bipedism and the release of hands from locomotory functions were the primary and basic element of human evolution.
This event induced an increase of brain weight and enlargement of the angle of fan-shaped area of the parieto-temporal-occipital cortex and a development of cortical areas belonging to the liaison brain.

The role played by the liaison brain in language and symbolization is discussed together with its implications for the dualist-interatioinist hypothesis put forward by Popper and Eccles.
The hypothesis is advanced that Man's creative and intellectual processes, based on corticalization and development of operative systems in the brain, are endless processes aimed at changing those central structures whose functions are essential for Man's evolution.


In questo lavoro si riportano alcuni contributi neurofisiologici alla ricerca paleontologica.
Le curve che si riferiscono al rapporto peso del cervello/peso del corpo e quelle relative al quoziente di encefalizzazione (QE date da E = K.p2/3) contro il tempo prima dell'era attuale, per le diverse specie, suggeriscono che il QE rimane di 0,3 solo per gli insettivori di base, mentre i delfini raggiungono ii valore di 630 milioni di anni fa, e 1'Homo Erectus raggiunge un QE di 7 solo 1 milione di anni prima dell'era attuale.

Sono state presentate le caratteristiche paleontologiche dell'Australopiteco che permettono di considerare questo fossile un ominide con capacità umane, incluso il linguaggio e l'uso di utensili.
L'evidenza neurofisiologica fa da supporto alI'ipotesi paleontologica che I'acquisizione della posizione eretta e la liberazione delle mani dalle funzioni locomotorie costituiscano un elemento di base primario dell'umana evoluzione. Questo evento ha indotto un incremento del peso del cervello e un allargamento del ventaglio della corteccia parieto-temporale-occipitale e uno sviluppo delle aree corticali appartenenti al cervello associativo.

Il ruolo giocato dal cervello associativo in relazione al linguaggio e alla simbolizzazione viene in seguito discusso unitamente aile sue implicazioni con l'ipotesi dualista-interazionista elaborata da Eccles e Popper.
E' stata avanzata I'ipotesi che i processi creativi e intellettuali dell'uomo, basati sulla corticalizzazione e lo sviluppo di sistemi operativi nel cervello, possano essere considerati processi senza fine tesi alla modificazione di quelle strutture cerebrali corticali le cui funzioni sono essenziali per l'evoluzione dell'uomo.


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* (Courtesy of “Primeval Sculpture”,1984, vol.1, number 1, and of Mauro Mancia, neurophysiologist, full.Professor of Human Physiology II, University of Milan, Italy, and full-member of SPI).