Variability and Evolution, 1996, Vol. 5: 29–42
BETINA KUJAWA
Institute of Anthropology,
Department of Human Population Ecology, Poznan,́ Poland
ADAPTIVE ASPECTS OF HOMINISATION: LOCOMOTION,
MANIPULATION AND THERMOREGULATION*
KUJAWA B. 1996. Adaptive aspects of hominisation: locomotion, manipulation and thermoregulation. Variability
and Evolution, Vol. 5: 29–42, Figs. 4, Tab. 1, Adam Mickiewicz University, Faculty of Biology, Poznan´
Abstract. Certain geological, climatic and material traces researched by paleontology allow for
the reconstruction of a complex of the adaptive traits that were developed in Australopithecines.
Bone remains of the Plio-Pleistocene hominids prove that Australopithecines were undoubtedly
erect, bipedal beings, possessing certain advanced manipulative abilities. Information about their
physiology (thermoregulation, pigmentation) comes from the comparison between modern man
and philogenetically closest anthropoids, or from the specific features of Homo sapiens.
Key words: hominids, adaptation, bipedalism, manipulation, thermoregulation, pigmentation
„ ... no organism adapts in an abstract way; this phenomenon
takes place strictly within specific conditions imposed by the
environment.”
(Dobzhansky 1983)
Introduction
Looking at man, or earlier hominid forms, it is difficult to determine the kind
of specialisation they represent. The adaptations that characterise anthropoids cannot
serve as the basis for comparing them either to strong carnivores, or four-footed
ungulates, or quadrupedal savannah baboons.
* Paper
presented at IV World Archeological Congress in December 1994 in New Delhi.
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B. Kujawa
Since the australopithecine forms evolved in the same environmental conditions
as the mammals mentioned before (the African savannah becoming progressively
cooler and steppe-like), they could not remain “neutral” in the face of the changing
surroundings.
Man is unusual among other terrestrial mammals due to his bipedalism, precision
of holding and the possession of naked skin. This article provides the most accepted
explanation of the evolution of these features which seem to be the adaptations to
specific conditions imposed by the environment of the Plio-Pleistocene East Africa.
Anatomical aspect
Locomotion
The very fact of bipedalism, and hence the upright posture, of the Plio-Pleistocene hominids does not raise any doubts or reservations. However, the reason why
bipedalism had evolved remains a matter of dispute. Like Darwin (Darwin 1871),
many authors try to explain bipedalism in the context of locomotion, and consider
the possibility that bipedalism arose as a consequence rather than a cause of the
changes relating to the use of the forelimbs.
For example Kortland (1980) suggests that bipedalism might have developed as
the effect of using the forelimbs for brandishing thorn bush branches in order to
scare off attacking carnivores. It seems doubtful, however, that such an action would
be sufficient to deter a large carnivore or smaller, pack-hunting animals.
In turn Day (1986) claims that the development of bipedal posture and locomotion
can be explained in terms of improved survival advantage owing to the more efficient
picking of food from high branches, the ability to run and climb trees after an early
warning of danger, and having arms free for defending themselves or for safe carrying
of infants (considering their prolonged helplessness and dependency). However, it seems
more probable that a hominid under threat from a carnivore would have reached the
safety of a tree quicker if it made its escape in any other way but bipedal. Moreover,
an infant in an adult’s arms is a serious impediment for the carrier both during a flight
and while climbing a tree, hence an individual carrying offspring would be more vulnerable than other, even less agile, members of its group.
A different view was advanced by Washburn and Howell (1960) who claimed
that the use of tools was both the cause and effect of human bipedalism. This
hypothesis was further developed but it fell into a pattern of interpreting ’tool use’
as a manipulative and manufacturing activity.
Meanwhile, the available reports on protocultural behaviour patterns of many higher
primates have proven that the customary posture during manipulative activity is the
sitting posture, a fact which contradicts the hypothesis about a significant contribution
of such behaviour to the development of the upright posture or bipedalism.
Adaptive aspects of hominisation
31
Thus, the most important problem, namely, why the Plio-Pleistocene hominids
became bipedal while the related forms remained arboreal, vertical climbing animals, remains unsolved.
An interesting proposition for solving this question, supported by appealing arguments, is the so called “throwing hypothesis” put forward by Fifer (1987). This
hypothesis proposes that both the fore- and hind limbs of man were, and still are,
integral parts of a single functional mechanism, hence the main evolutionary driving
force of human bipedalism was the development of a muscular system which would
be efficient and effective during “missile-launching”. This mechanism, activating
the morphology and anatomy of the whole body, explains the development of many
specific structural features of the lower half of the skeletal and muscular systems
in a manner superior to the mechanism of bipedal locomotion itself.
According to Fifer’s concept (1987), the factor which initiated the separation of
evolutionary lines of the hominids and chimpanzees from a common ancestor was,
as the author proposes, behavioural in its nature with “(...) the protohominid preferring to stand its ground and hurl stones at the attacker and with the proto-chimpanzee preferring to threaten by brandishing branches (a threat backed by powerful
jaws and large canine teeth) and, if this failed, to take to its heels and make a fast
get-away to the nearest tree – on all fours of course” (1987, p. 139).
The regions of East Africa had been separated by a developing system of rifts,
thus isolating two populations of hominids which thereafter evolved in different
environmental conditions (Okołowicz 1969; Ksia˛żkiewicz 1979; Foster 1983; Andel
1985; Coppens 1989). It appears, therefore, that it was the environment that created
the behavioural preference suggested by Fifer. The use of stones, being as it were
“at hand”, might have created such behavioural preference in a group of hominids
because of its effectiveness, and subsequently this preference would be incorporated
into the range of behavioural patterns of the group. Natural selection, in turn, would
favour those individuals who were better adapted to the action of missile launching.
The anatomical features which developed at that time are also characteristic of modern man, and the specific as well as atypical nature of these features seems to be
explainable precisely by Fifer’s “throwing hypothesis”. The author himself calls
these features “anomalies” and attempts to justify their existence by an evolutionary
pressure in the times of the Plio-Pleistocene to perfect the mechanics of a throw.
Fifer (1987) includes the following among these features: (Fig. 1)
– massiveness of the lower limbs in relation to the upper parts of the body,
a feature that impedes rapid acceleration but enables a greater speed of upper limbs
movement necessary for effective stone throwing,
– a strongly developed largest gluteal muscle (musculus gluteus maximum)
which performs most effectively when the knee joint is fully extended, and the
least in the walking gait, a muscle which takes the role of a “stabiliser” or counterbalance when a stone is thrown forward and causes the thrower to lose his
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B. Kujawa
Fig. 1. Morphological (long and mobile torso, double-spiral arrangement of the body musculature) and
anatomical (muscles) features which perfect the mechanics of a throw
balance, a relatively long and mobile torso which, in order to maintain a stable posture
during walking or running, requires the activated muscles to outlay additional energy,
but which makes the missile-launching mechanism more effective through a “doublespiral arrangement” of the human body musculature as described by Dart (1950). The
“stone throwing” hypothesis is very interesting and it contributes new elements to the
anthropogenetic debate, although it does not take into consideration other important
features of the human anatomy, features whose presence suggests in an obvious way
a strong selection favouring individuals better adapted to bipedal walking.
The discussion presented above confirms that Australopithecines commonly used large
stones, at least in defence or attack. However, because of the canines size reduction,
gathering and processing vegetable food as well as fragmenting captured or found animals
required of Australopithecines the ability to use stones for tools with precision.
33
Adaptive aspects of hominisation
Manipulation
Evidence supporting the thesis that the Plio-Pleistocene Australopithecines used
stone tools (and possibly made them) can be found in the anatomy of these creatures
which, after all (besides appropriate nervous centres), determines the manipulative
abilities of the hand.
As it turns out, the structure of distal phalanges in the primate hand is considered
to be one of the crucial factors enabling precision holding. According to Shrewsbury
(1986, p. 234), precision holding can be defined as “combinations of the proximal
or distal ungual pulp of the pollex, or its non-pulp surface, in apposition to the
proximal or distal ungual pulp of another digit(s), or its non-pulp surface(s), for the
purposes of differential prehension.”
According to Johanson (1982), the structural differentiation of the ungual pulp
into proximal and distal regions occurs only in humans. This structural differentiation
determines certain contrasting functional properties of two areas as shown in Table 1.
Table 1
Comparison of the functional properties of the ungual pulp in Humans
Property
sensory area
Proximal area
larger
Distal area
smaller
tactile discrimination
higher
lower
friction ridges
longitudinal
lateral
mobility
greater
lesser
The structural and functional differentiation of the ungual pulp regions enables
performing several clearly distinct varieties of precision holding which have been
called „types” and have been given numbers from I to IV (Fig. 2).
Comparative analysis of the distal phalanx structure in primates has shown that
the functional differentiation (compartmentalisation) of the human ungual pulp is
determined by a different shape of the phalanx. In the human hand the distal phalanx
is ended in a significantly broadened ungual tuft with a rough volar surface (Shrewsbury 1986), or in other words, ungual tuberosity (Bochenek 1992). In apes this
part of the phalanx is shaped into an obtuse cone with a smooth volar aspect (Fig. 3).
This explains the absence of radially extending ligaments in apes which in man are
attached precisely to that rough tuberosity (Aiello 1990).
Further, the distal phalanx in man reveals spiny bone structures to which intraosseous ligaments are attached from the sides, restraining and supporting the proximal part of the ungual pulp. Similar structures are not encountered in other primates. For reasons presented above, the ungual pulp in apes does not exhibit diverse
(regional) mobility and deformability.
What were the manipulative abilities of early hominids?
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B. Kujawa
Fig. 2. Four types of precision holding are represented for the human hand
The studies included well preserved distal phalanges of the Plio-Pleistocene hominids: two specimens from Hadar, representing the species Australopithecus afarensis, one specimen from Kromdraai of the species Australopithecus robustus, two
from Olduvai, ascribed to the species Homo habilis.
It is worth recalling that one of the major features of the human hand is the
broadened, rugged ungual tuft, unique among primates. It turns out that the development of this feature of the distal phalanx in the Plio-Pleistocene Australopithecines varies. The early Hadar forms (dated at 2.9–3.3 million years), according to
Stern and Susman (1983), exhibited only a moderate lateral broadening of the distal
Fig. 3. Palmar view of the distal phalanx of a human and a chimpanzee. Note the area of insertion for
flexor pollicis longus on the human bone
Adaptive aspects of hominisation
35
phalanx, comparable with a smooth, conical ending of the phalanx in pongids. On
the other hand, the ungual tuft in the Olduvai forms (dated at 1.8 million years) is
already finally developed and comparable to that of the Homo sapiens. It supports
the view that the Hadar Australopithecines were capable of types I, II and III precision holding, and the Olduvai forms additionally, though still imperfectly, of type
IV precision grip. Hence, the morphology of their phalanges can be defined as
“transitional” between the ape and human forms.
In particular the early, Pliocen, hominids, even though already bipedal, partially
exhibited the traits of arboreal creatures in the way that their forelimbs had not yet
been completely released from the locomotor functions. Hence, their manipulative
abilities could not have developed to the same extent as in later forms.
It has to be presumed that the appearance of first stone tools coincided with the
development of ungual tuberosity. It would confirm the early productive activity of
Australopithecines, an activity the development of which could only be successful in
specific terrain conditions. This is because the materials used for tools, or their prototypes, do not occur with equal frequency in different areas of the African continent.
Stones with sharp edges can be found most readily in the middle and upper sections
of rapid-flowing rivers, or in volcanic terrain. Basalt and flint, occurring in abundance
in the latter areas (and incidentally the oldest tool-making material), are, due to their
structure, perfect for secondary processing. No wonder, then, that the populations of
Australopithecines showed such a preference for the volcanic plateau of East Africa.
Physiological aspect
Thermoregulation
The East African Plateau was exceptionally favourable to the Australopithecines
for another reason. The Plateau, because of its equatorial position, enjoyed intensive
sunshine which undoubtedly led to the evaporation of many shallow lakes, leaving
strongly saline water at the bottom. Additionally, owing to a strong volcanic activity,
the region abounded in volcanic ashes, rich in minerals.
The tectonic activity connected with the formation of the East African Rift created
favourable conditions for the emergence of shallow salt lakes. Water was not in
short supply in this region either. The rainy season provided even surplus water,
while in the dry season at least several rivers flowing from the mountains supplied
adequate amount of water. Of course those water sources were not (and still are
not) distributed evenly over the whole area of the East African Plateau, but they
were reasonably close to each other to enable ready use of those sources during the
dry season (Newman 1970; Weiss 1981; Tobias 1991).
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B. Kujawa
The environmental conditions described above turned out to be necessary for the
evolution of certain physiological mechanisms in the early hominids, traits which
facilitated their survival in the savannah environment of the African tropics.
It seems that the biggest problem faced by mammals living in the tropics is the
dissipation of body heat, since overheating of the organism is inevitably fatal (Szarski 1990). There are many ways to cope with the surplus heat generated by the
body or absorbed from the external environment (Schmidt-Nielsen 1983). It may
be an increased blood flow through the almost hairless skin of external ears (auricles), or antlers, or the cooling of venous blood through evaporation of water from
the mucous membrane of the nasal cavity and sinuses in certain parts of the cranium
(thus the blood is cooled before it flows to the cranium, a fact which allows maintaining brain temperature at several degrees lower than the temperature of blood in
the arteries of neck and trunk). It can also be achieved by way of cooling of the
nasal cavity, fauces and tongue through rapid shallow inhalations (Szarski 1990).
In the case of the Hominidae family it appears that the adaptive response to
unfavourable tropical savannah conditions included, simultaneously, several morphological and physiological aspects of the organism, forming together an integrated
functional system. This system, covering hair reduction, eccrine sweat glands, and
dark-pigmented skin probably evolved concurrently with bipedal locomotion (Zihlman 1988).
These morphological and physiological adaptations can be explained in the context of specific ecological conditions of the Plio-Pleistocene East Africa. An intensive
insolation of the open equatorial terrains during the day created completely different
climatic conditions from those prevailing in a humid tropical forest. Excluding the
periods of intensive rain, the tropical forest maintains the near-ground temperature
of about 28–32oC, which accompanied by slow air movement, high humidity and
reduced sunshine creates conditions in which heat dissipation from the organism is
not a major problem. In the open savannah an individual is exposed to direct solar
radiation and additionally absorbs heat radiating from nearby objects. Any physical
activity (and with Australopithecines such activity was probably increased due to
the dispersal of water sources and food) requires additional energy outlay, hence
the amount of body heat increases, in this case generated by the working muscles.
Even moderate physical effort doubles the Basal Metabolic Rate (Zihlman 1988).
Such combination of external and internal (organic) heat, if not dissipated, would
dangerously raise the temperature of the brain, leading to death. It appears, however,
that Morgan was not right when she claimed that the evolution of the upright posture
and bipedal locomotion ought to be explained by a smaller surface area of the body
exposed to solar radiation and to easier loss of body heat compared to a horizontal
posture of quadrupeds (Fig. 4) (Morgan 1982).
The energy cost of maintaining such a posture was, after all, high, and this fact was
not compensated for by any solution to the problems connected with excess body heat.
Adaptive aspects of hominisation
37
Fig. 4. Estimates of the total body surface area exposed to direct solar radiation by a hominid, in bipedal
and quadrupedal position, as a function of solar elevation
According to Schmidt-Nielsen (1983), similarly to other savannah animals (particularly ungulates), the system of draining venous blood from the scull in man has
been “designed” to cool the brain under conditions of hypothermia. Changes in air
temperatures induce changes in the tension of the smooth muscles of facial blood
vessels, and that in turn changes the direction of venous blood drainage in the skull.
The venous blood thus flows from the face through the ophthalmic veins and their
astomoses down the skull, so that the arterial blood can dissipate heat in the cavernous sinuses and, cooled, proceed to the brain (Schmidt-Nielsen 1983).
A very important role in this cooling mechanism is played by the nasal cavity
which houses a system of arteriovenous vascular plexus, facilitating efficient heat
and moisture exchange. As Beals (1986) suggests, the evolution of this vascular
system of thermoregulation could cause changes in the shape of the base of the
skull. Conroy (1980) observed the relation between the patterns of cerebral veins
reticulation and the sizes and shapes of openings in the base of the skull. It is
important because there is ample evidence that the size and shape of the base of
the skull determines the shape of the cranial vault and face (Beals 1986). Perharps
further research could prove that the expansion of cavernous sinuses and the increased amount of cranial fluid, which led to a greater volume of craniums and caused
their transformation, was a “prelude” to the expansion of the brain during subsequent
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B. Kujawa
stages of evolution. Close to that view is Fiałkowski’s hypothesis (Fiałkowski 1978;
Bielicki 1987) about the increasing volume of the Plio-Pleistocene hominid brains
being an adaptation of this organ to a reliable functioning under the hypothermia
of the body due to a great physical effort during “endurance hunting”. A strong heat
shock put the functioning of cortical neurons at risk, which would be fatal to an
individual. As Fiałkowski claims, the adaptation to that stress could lie in the creation
of additional cortical neurons and an additional number of connections between
them: then the brain as a whole would function properly even if some of the neurons
were momentarily blocked. It seems doubtful, however, that this hypothesis is correct
if we consider that the brain is metabolically a very costly organ. According to
Amstrong (1983), the brain, representing about 2 percent of the total body mass,
continuously uses nearly 20 percent of the body’s energy (Beals 1986). It is difficult
to see that an organ consuming so huge amounts of energy could passively increase
its size by way of natural selection.
It has to be noted that the first distinct changes in the intracranial vascularisation
appear on the casts of the craniums of the Plio-Pleistocene Australopithecines (Zihlman 1988; Coppens 1991).
The cranial thermoregulatory system was not sufficient to dissipate large surpluses of body heat. As we mentioned before, a whole system of adaptive features of
the skin evolved, a system that enabled the survival and efficient functioning of the
hominids subjected to the stress inducting heat conditions of the savannah.
In humans the skin is an organ participating in the mechanisms responsible for
dissipating excess body heat, hence, no doubt, it also played a significant role in
the early hominids. One method of stabilising the outflow of heat from the body
was the initiation of the surface blood flow. When blood vessels in the skin dilate
the warmer blood from inside the body flows to the skin surface raising its temperature and thus enabling heat loss through conduction, convection (the physical phenomenon whereby the warmer air moves up and the cooler air – down) and radiation.
In this context important becomes the fact that the large muscles fulfilling the locomotor function are located not on the torso but on the legs (the thigh and shin),
hence the heat generated by these muscles only to a limited extent ends up in the
circulatory system. As Reichholf notes, such “structural design” obviously prevents
excessive, and dangerous, overheating of the body (Reichholf 1990).
The high efficiency of thermoregulatory processes, however, is only possible
thanks to a large number of eccrine sweat glands (about 2 million) distributed over
the whole surface of the body. They produce sweat which is carried to the skin
surface through ducts opening in the skin pores. Evaporation of sweat in higher
temperatures can lead to a loss of up to 2 litres of water in one hour, with every
gram of evaporated water dissipating 0.56 cal of heat energy (Zihlman 1988).
Such a mechanism of heat regulation requires the supply of adequate amounts
of water and salt in order to compensate for the losses incurred in sweating. As the
Adaptive aspects of hominisation
39
topographic description of the African Plateau shows, during the evolution of the
skin thermoregulatory mechanisms this condition was satisfied. At the same time
the evolution of hormonal control mechanisms took place, mechanisms which limited the loss of elements valuable to the organism through the secretion of such
hormones as the antidiuretic hormone (ADH) causing secondary absorption of water
from primary urine, or aldosterone which increases the reabsorption of sodium ions
in kidneys and eccrine glands.
Despite earlier results obtained by Montagna (1963), a research carried out by
Johnson (1974) in the late nineteen seventies on perspiration in primates has shown
that a loss of heat in the process of evaporation occurs in certain species of baboons,
rhesus monkeys and macaques, albeit to a much lesser extent than in man. In this
process the eccrine glands are engaged which prevail over the apocrine glands in
these primates. The closest to man in respect of the amount of sweat secretion are
savannah red talapoins (Erythrocebus patas). They sweat in particular after an intensive run in high temperatures, but also during a rest, which explains their increased tolerance to heat compared with forest primates. Compared with rhesus monkeys
they sweat 2 to 6 times more profusely and their eccrine glands are also distinctly
larger (Zihlman 1988). The primates inhabiting savannah areas are also more dependent on water sources (Newman 1970).
The above data shows that the development of an efficient thermoregulatory
system played a decisive role in survival and efficient functioning in the conditions
of the tropical savannah.
The similarities of evaporation process involving numerous eccrine glands in the
hominids and savannah talapoins highlights to an even greater extent the role of the
environment in the development of appropriate adaptive mechanisms.
In the early hominids, however, the functioning of eccrine sweat glands was
further enhanced through the loss of hair (in the sense of reduction of the length
and thickness of hair). As Amaral (1989) suggests, body hair performed numerous,
important to a hominid, functions: heat retention in low temperatures at night, protection against harmful solar radiation, shock absorption of falls and hits, protection
against sharp fangs or claws of attacking animals, establishing hierarchy by way of
hair bristling, enabling an infant or a growing child to cling to its mother. The losses
due to hair reduction must have been smaller than the gained advantages. The only
explanation of the hair loss phenomenon is the increased effectiveness of human
thermoregulation through the loss of heat during sweat evaporation. Naked skin
allows the overheated body to dissipate heat quickly, whereas thick body hair retains
heat through an insulating layer of trapped air.
The amount of body hair among different human populations varies, nevertheless
in all humans the skin surface is sufficiently exposed to permit efficient dissipation
of heat by perspiration. Still, the hair on the head has been retained, as well as in
the armpit areas and around external genitals. The retention of hair on the head is
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B. Kujawa
probably important for the protection of this part of the body (containing the brain)
against short wave ultraviolet radiation, and the role of hair in temperature regulation
of this organ (be it during a sunny day or cold night) – a regulation necessary
because of the absence of protective layer of muscles or subcutaneous fat. The pubic
and axillary hair in humans is, firstly, a sign of sexual maturity (conditioned, incidentally, by hormones), and secondly, a place for the sustentation of sexual (maybe
attracting?) aromatic substances.
Pigmentation
Beside visible light and long-wave (infrared) thermal radiation, the solar spectrum
contains also ultraviolet radiation. The described perspiration mechanism does not
protect the body against harmful effects of the latter, such as sunburn of the skin
with blisters, liable to cause infection. For as long as the skin was protected by
a thick coat of hair, as in other African apes, the danger of direct ultraviolet radiation
was small. Thus, during the phase of hair reduction a conflict must have arisen,
caused by pressure factors working towards slowing the process down, unless at
the same time another adaptive trait had emerged which reduced the harmful effect
of that part of the solar spectrum.
It appears that such a defensive mechanism was provided in dark pigmentation
of the skin. The skin colour among the living human populations varies depending
on the amount of pigment (melanin) in the epidermis. The melanin production is
stimulated by solar radiation, and the produced pigment in turn absorbs the ultraviolet portion of the solar radiation, thus neutralising the harmful effect.
The hairy skin of other primates is diverse in colour: even the same specimen
can have a differently and irregularly pigmented body. It is, therefore, unlikely that
the skin colour can play any role in their survival. Then, as Zihlman (1988, p. 399)
states: “the dark-pigmented skin played a significant, even decisive, role in survival,
therefore it became a common feature among the early hominids.”
Since the skin pigmentation in African apes is highly diversified, it is most likely
that the populations of hominids emerging in the Pliocene was also diverse in respect
of this feature. Presumably, then, the hair reduction phase coincided with the selection
of dark-pigmented skin. The protective function of dark skin in the environment where
ultraviolet radiation is most intense and prolonged must have played a role in the differentiation of survival advantage and reproductive success of hominids (Zihlman 1988).
A question remains, however, of the already mentioned diverse amount of melanin in the skins of the present-day humans inhabiting the whole area of the Earth.
The only advantage of the light skin over dark is that an individual having a lighter
skin complexion is able to produce more vitamin D (calciferol) from a unit of
radiation. Taking this into account, one can wonder about a potential regress in the
selection of dark-pigmented skin, the more so that in the Plio-Pleistocene environment of African savannah the food sources rich in calciferol were probably scarce
Adaptive aspects of hominisation
41
(fish, egg yolk, milk). Assuming, however, that Australopithecines were exposed to
prolonged ultraviolet radiation, they probably were able to produce sufficient amounts
of this vitamin, even though their skins were darker.
Then, the lighter skin is in modern times characteristic of the populations inhabiting high latitude areas, thus it would compensate for the lower intensity of ultraviolet radiation and possible vitamin deficiency in the diet.
Conclusion
The evolution of the family of Hominidae is connected with the great geological
and climatic events that happened in the late Miocene. The reactivation of the East
African Rift and its consequences, as for instance the cooling of climate and the drying
of the south-eastern parts of Africa where Australopithecines were exposed to the adaptive radiation, determined the direction of the changes that affected the earliest human
forms. The changes were of tree kinds: morphological, physiological and behavioural.
The sudden retreat of tropical forests and the expansion of open savannah resulted
in the conditions that required a specific adaptation. Australopithecines made many
attempts to face the conditions and realise the process of adaptation. Undoubtedly,
the most advantageous appeared: bipedality (which arose from the development of
a defence mechanism – throwing of stones), manipulatory skills (precise holding
but not as fully perfected as it is in contemporary humans), the effective thermoregulatory system (eccrine sweat glands, reduction of body hair, dark skin pigmentation).
Probably the evolving adaptive traits were strictly connected with one another.
This kind of connection is called the positive feedback loop.
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