Lecture 3 Man’s unique features
So far we have discussed evolution, classification and the other extant primates, but we have said
very little about the one which interests us most, Homo sapiens, ourselves.
Man is a primate, related to the apes but distinguishable from them. How? What makes us different? We can usually recognise a man or woman walking down the street towards us, and we
know that an approaching figure is definitely not a gorilla or a gibbon. To do this we must define
accurately what a human being is, or what a human being is not. This is fairly easy confronted by
a living breathing animal, but becomes more difficult when we only have a piece of fossil bone, or
perhaps a tooth or two.
We start from the viewpoint that man is not alone: there are and there have been other animals on
the earth that are man-like. Some of these are recognisably other humans - our sisters: other races
of mankind are man. Some, like the great apes are near to being men - our first cousins. Others,
like the monkeys and the other primates are less like us - our distant cousins.. And some are very
dead, and exist only in the fossil record - our aunts. So we look for our relatives both in the world
around us and in the rocks at our feet.
This point of view makes two large assumptions. The first is that we can define man: we can say
what is or is not a human characteristic. The second is that evolution, or change, occurs in time
and the situation around us now is not the same situation that existed a million, or ten million
years ago.
So how do we define man? Which of our characteristics is special or important? It is fairly easy to
write down a list of primate feature which we have modified..
Perhaps the most obvious differences are brain size and tooth and locomotor specializations.
Brains
This is a simple one. Humans have relatively enormous brains: on average about 1400 ml in
volume, around three times the figures for apes.
Teeth
Our dentition is unique because both males and females have small canines that closely resemble
incisors and do not project beyond the level of our other teeth. Compare the human dentition with
another primate, say a gorilla (SLIDE). First the molars and premolars differ. In Gorillas the cusps
are rather pointed and projecting, probably an adaptation to chewing tough vegetable material.
In male baboons (and other primates) the canines (upper and lower) are long and pointed and
interlock with each other (upper behind the lower and outside the lower premolars). This means
that the upper canine hits the 1st lower premolar, which is in turn a different shape from the
second (bicuspid) premolar. The first premolar is elongated from front to back and shears against
the upper canine rather like the blades of a pair os scissors. This ‘sectorial’ or cutting arrangement
is practically universal in a male primates other than man.
In females the canines are smaller and the canine barely projects. The first premolar is therefore
less modified and less elongated. In man and woman the canines are much smaller and the first
and second premolars are both bicuspid and practically identical - homomorphic.
What are the functions of this canine/premolar unit in male hominids? Obviously useful in some
feeding behaviour, but that can’t be the answer because males and females have the same food
and feed in the same ways, and females have small canines. So the answer must lie in something
males do and females don’t - displays of aggression. The big teeth are rarely used in fighting, but
are used to intimidate rivals.
In man the morphology is different and so is the behaviour. Inter-individual behaviour in male
humans is different. If we look at hunter-gatherer societies still extant, and believed to resemble
the first human groupings, we see that although males are broadly dominant over females on
economic grounds and those of size there are no dominance relationships, no pecking order in the
way that these exist in gorillas or chimps. The contrast is probably more complete than between
apes and modern society. Service notes
‘There is no peck order based on physical dominance at all, nor is there any
superior-inferior ordering based upon power or wealth, hereditary classes,
military or political office.’
And again ‘ A man might be stronger, faster, braver and more intelligent than
any other member of the band. Will he have higher status than the others?
Prestige will be accorded him only if these qualities are put to work in the
service of the group’
This all sounds a bit much to our cynical ears, but hunter-gatherer societies will only work with a
great deal of cooperation between hunting males and food sharing females. Yet it has survived for
two million years, presumably because disruptive incidents are checked and potentially dangerous
male aggression subdued.
We still have to answer the question of mechanism- how did canines become reduced? That is
another story, but suggestions include a coincidence of tooth reduction with loss of a hairy coat,
because the latter might lead to accidental injuries during play or coincidence with the development of weapons - but why not have weapons and teeth?
Moving forward in the tooth row we note that human incisors are relatively small compared to
those of gorillas and chimps. We might regard this as an ape specialisation due to feeding patterns,
and man’s small incisors as a primitive condition retained, or we might suspect that food was
being prepared before eating.
A combination of small canines and small incisors the human dentition is relatively short from
front to back . This has affected the shape of the tooth row so that instead of being U shaped
with parallel sides it is parabolic.
The skull of modern man has a rounded braincase which surrounds and protects the brain, and
which also provides attachment for muscles, particularly the muscles of mastication. (SLIDE)
Because of the small jaws the muscles of mastication are also small, and the face relatively flat,
with the jaw tucked underneath the braincase. In apes the large, overhanging jaws demand large
jaw muscles which fill the side of the braincase and extend to a midline anterio-posterior crest.
The shape of the ape head determines that the foramen magnum is rather far back. The large jaws
and angulated skull in turn demand large neck muscles, and these also raise a ridge, transverse this
time across the occiput
.Bipedalism
Human adaptations for habitual bipedalism - walking evolving the full extension of hip and knee
and final push off from the big toe are widespread throughout the body: so much so that it is fairly
easy, given the right bone, to identify a fossil as a biped or not.
Skull and vertebral column.
The human skull, because of its small jaws and reduced neck muscles sits balanced nicely on top
of the spine. The foramen magnum is positioned a long way forward, which helped the process.
The vertebral column is also different
. The apes have a roughly C shaped vertebral
column: in man there are flexions above the
hip and in the neck which bring the head
upright. Is this a bipedalism adaptation? Yes
because in infants, who are quadrupedal, the
column is C shaped. The lumbar curve is
produced by a change in shape of both intervertebral discs and wedge shaped vertebral
bodies. The pelvis has become bowl shaped,
because it now has to support abdominal and
pelvic viscera. The perineum is partly closed
by the coccyx, the former tail which is plastered forward between the legs in the position
adopted by an ashamed dog and held there by muscles. Many abdominal organs, which can hang
from their dorsal mesenteries in a quadruped, become retroperitoneal -their weight is taken by the
abdominal wall.
The pelvis, especially in the female has another
set of adaptations concerned not with
bipedalism but with the provision of a passage
for a relatively enormous brained fetus. This
creates a well marked and characteristic dimorphism in the human pelvis.
Below the pelvis the heads of the femora are
wide apart while their lower ends are close
together. (SLIDE) This carrying angle ensures
that with each step the axis of weight transmission in lower leg and foot remains close to the
central axis and hence the centre of gravity.
Weight still tends to be transmitted through the
outer side of the femur, and the
outer condyle is the larger: in apes the reverse is true.
Broad hips and the carrying angle mean that the femora
form an inverted triangle. This greatly improves stability
as at each step the pelvis, with the rest of the body can
be rotated about the axis of the fixed tibia and foot.,
with footsteps more or less in a
straight line.
The upright stance means that the
process of standing and walking is
quite different in man and the modern
apes.
In the bipedal foot the toes are short
and the tarsus relatively long. The
metatarsals are parallel and the first
and second articulate at their bases. In
all other primates the great toe diverges and can be moved to and fro for grasping. The tarsal and metatarsal bones form two
arches, one running from front to back, the other from side to side. The importance of these
arches to gait is only realised when we have flat feet, because the arches have collapsed. A flat
footed person is an inefficient walker with knees and feet turned in. Weight is transmitted through
the arches, through the metatarsals and eventually to the great toe. The last segment of the great
toe is characteristic - you can tell whether an animal is a biped or not from this single bone.
Bipedalism and the hand
Bipedalism, of course is classically said to free the hands to do other things. Once established
(probably about two million years ago) it became very successful. The reasons for this are easy to
see. A biped looks bigger, is more intimidating. One can carry objects, weapons or babies. Bipedal
locomotion, although not fast is extremely efficient. A cat or dog can outrun a man easily, but has
to rest much sooner: we can plod on for 40 or more miles in a day. Tool use is another factor, but
which tools? Tools for digging roots, cutting meat or smashing bone are mostly used sitting
down. But weapons, for hunting or fighting are generally used standing up.
Unfortunately human bipedalism is unique amongst mammals, so we have nothing to compare it
with. However all primates are bipedal at times. Clingers and leapers do so with their trunks
erect: when they come to ground they do so as bipedal hoppers. It seems unlikely that we evolved
straight from this to bipedalism without a quadrupedal phase however. Some quadrupedal monkeys have particularly well developed hind limbs for leaping, and these often feed and sleep with
trunk erect. They also stand erect whilst carrying objects or scanning the countryside for predators.
Amongst hominids gibbons are erect, and often walk bipedally along branches. The other apes
again carry things bipedally. They are also bipedal during aggression: you can persuade chimps in
zoos to charge bipedally and throw branches at leopards, or a handful of faeces at visitors. In the
wild Orangs, chimps and gorillas throw branches, stones or earth when exited.
Of course the hand is not isolated, and their are human adaptations of arm, shoulder and chest
too. Our chests are broad and shallow, the clavicle long and the shoulder blades on the back
rather than lateral. All these have been taken as being secondary to brachiation. This is not necessarily so. A broad shallow chest is easier to balance when upright. And the set of adaptations seen
in man, embracing shoulder mobility with well developed abductor muscles and weak propulsive
muscles is quite different to the brachiators set up: it could be a set of adaptations for throwing
for instance.
The hand (SLIDE) is also functionally different from that of other primates. Ours is a fairly
typical terrestrial primate hand, with five rather straight and short digits. But the thumb is relatively long compared to the other fingers and very mobile. This makes the hand capable of both
power grip (largely the last three fingers) and precision manipulation between thumb and forefinger. No other primate can oppose the thumb pulp with all other fingers.
But even more important is what the hand is wired up to. We have a relatively large cerebellum,
which controls posture and the placement of limbs in space. We have a large cerebrum too. We
also have very good eyes. This eye hand brain combination is really what makes us tick.