Lecture 2 Primate taxonomy
Last time I introduced the general ideas of evolution and taxonomy. Now its time to see how this
relates to the evolution of humans.
To some extent the factors which shape human evolution
started back at the origin of the Universe, certainly the
origin of the earth. From there we see complex single
celled animals, protista, and much later the multicelled
organisms (metazoa), and very, veiry recently animals
with backbones, land vertebrates and man. Traditionally
the study of human evolution is confined to our own
order the primates
This lecture defines what a primate is, and introduces you
to some of our relatives, our cousins and cousins several
times removed. I shall show you some pictures and give
you some names, you certainly won1t be expected to
idertify these animals in an exam, but an appreciation of
the range of these animals is necessary when we come to
discuss how they, and consequently how we might have
evolved. A we talk abot primates we will construct a
family tree and see who is most closely related to
ourselves.
DEFINITION OF A PRIMATE
Like many definitions the definition of what makes a
primate (as opposed to a rodent or a carnivore) is
complex. There isn’t too much argument about the core
groups of today’s primates, but as we go back to the
fossil record there is less certainty. A purely descriptive
definition will do as a starting point :
Unguiculate - with nails, hooves or claws
Claviculate - with a clavicle
PROBLEMS
At first glance this seems OK (apart from
the long words) But there are problems
First there is no unique characteristic
which defines primates. It is a list of
shared characteristics and trends - most of
which are in fact retentions of ancestral
features rather than derived, new features.
Secondly many features are behavioral or
depend on sofi tissue anatomy and don’t
3
help with the classification of fossil bones.
Fortunately there is a series of very
specific anatomical details, such as particular features of the foot and skull, that we can use, but
even so it is difficult to decide whether some early mammals are primates or not. Bven some
extant groups can be difficult : we can’t quite decide whether tree shrews are primates or not, and
some authorities put us in bed with rabbits and
even bats.
The least contentious primate classification is
probably this one so we will stick to that details are not really important at this level
anyway.
This classification splits primates into three - the
prosimians, the tarsirs and the anthropoidea.
Lets look at these in turn.
THE PROSIMIANS
Prosimians are generally small, primitive animals with relatively small brains, limited intelligence
(defined here as manipulative behaviour and ability to cope with
unexpected situations). They are, in fact rather like the insectivores, primitive mammals like
shrews, and retain many primitive mammalian characteristics.
All non- human primates are adapted to an arboreal life to some extent. Even those who live on
the ground still sleep in trees and head for the woods when in trouble.
Fossil primates are usually known from their preserved fossil teeth. The first ones are seen in the
Palaeocene (SLIDE) as clawed quadrupeds looking rather like rats . Most of these came and
went, but some continued into the Eocene, when large numbers of primate species evolved and
spread to North America, Europe and Asia, and probably North Africa and South America too.
These Eocene primates were the first with known arboreal habits : probably because the first true
rodents, which appeared at the same time, had the edge on them as ground living species. Eocene
primates looked rather like modern ones (SLIDE ) with relatively large brains, and forward
directed eyes, indicating binocular vision. Snouts were also becoming shorter as smell sensitivity
decreased. Primates live in a three dimensional world, where three dimensional vision beats a
sense of smell any day.
What other arboreal adaptations did they have? The most important was climbing by grasping
rather than digging claws into bark. The primitive mammalian five digit hand was retained, but
modified with an opposable thumb which grasped branches. This in turn led to the loss of claws,
replaced by nails, and mobility of the wrist, elbow and shoulder joints, and the ability to pronate
the forearm. They all have a clavicle. The hip also became more mobile.
All Eocene Prosimians were vertical clingers and leapers. This means that the hind limbs only are
used for locomotion . The trunk is vertical between leaps, and at rest when the animal also prefers vertical support. On the rare occasions when vertical clingers and leapers do come to ground
they hop or walk bipedally, with the trunk upright.
Modern Prosimians are pretty similar to this but tell us something we cant get from fossils. We
can see adaptations of the skeleton concerned with leaping, but we couldn’t guess that fossil
diurnal Prosimians, like the modern ones, probably lived in groups.
This is another primate adaptation and the groups include adult males and females and infants
associating in more or less permanent societies.
Advantages of living in groups include group protection. Group protection allows slower growth
and more time to learn. As learned behaviour becomes more important total behaviour becomes
more complex and more flexible.
So what do Prosimians look like, and what do they do for a living? The African and Asian ones
are nocturnal - bush babies and tarsiers (SLIDE ) , insectivorous clingers and leapers. Bush
babies have a group of relatives, the lorises who are arboreal slow climbing quadrupeds rather
like the sloths.
But the range of prosimians is best seen in the island of Madagascar, isolated for perhaps 35
million years and virtually free of rodents and carnivores. These range from Leptilemurs
(SLIDE) rather like bush babies, and tree living indrids to the larger types, the lemurs (SLIDE,
SLIDE) some of which have come down from the trees and become quadrupeds, with longer
arms. They also exhibit advanced behavioural traits such as grooming , albeit using specially
adapted lower incisors rather than their hands. They have also evolved dominance, or a pecking
order, but uniquely in the primates one in which the females are dominant.
We can summarise prosimian features like this:
THE TARSIRS (SLIDE) The tarsirs are rather
like bush babies but differ in some respects , in fact
so fundamentally that they are classified separately.
THE MONKEYS.
Meanwhile, away from Madagascar, Prosimians weren’t doing too well. A whole range of new
primates were appearing during the Eocene and the Oligocene which followed it (45-35 million
years ago). These new primates, the Anthropoidea evolved from Prosimians, and are recognised
by changes in brain size and form, skull structure, dentition, locomotor behaviour and inferred
changes in social behaviour, diet and ecology. The Anthropoidea are divided into two great
groups, the Old World or Catarrhines and New World or Platyrrhine tribes.
It was once thought that the new world group were primitive and the old world group evolved
from them: it now seems more likely that they arose separately from two different groups of
Prosimians, demonstrating an important trait in the primates - that a good idea, or an ecological
niche can be filled by separately evolving groups which turn out to look and behave similarly -
parallel evolution. Old world Anthropoidea are further divided into two groups, the monkeys or
Cercopithecoidea and the apes and men.
The changes which distinguish Anthropoidea from Prosimians (SLIDE) are largely based around
the head. There are changes in the cerebrum - the higher centres of the brain involved with coping
with the environment. In prosimians the relative sizes of the smell brain and sight brain are more
or less equal. In monkeys the sight centre is relatively larger, and in man the smell centre is virtually a relict. Along with an increase in the visual cortex we see increases in the areas involved in
memory, reasoning and manipulative ability. As the brain becomes bigger the skull becomes more
rounded and as the importance of a sense of smell decreases the face becomes flatter. Dentition
also changes, perhaps with face shape, perhaps with changes in diet.
The New World monkeys (SLIDE) are not specially important for our purpose. They are a group
of specialised arboreal quadrupeds with prehensile tails. The Old world monkeys are essentially
similar (SLIDE), although they are very diverse (SLIDE,SLIDE) and have developed ground
living forms as well. From the point of human evolution, however, monkeys are an irrelevance.
We think that the first monkeys were seen in Africa in the Oligocene: a possible contender,
Parapithecus, has rather monkey like teeth. But then, nothing. There is one definite monkey tooth
from the Miocene (20 mya) then nothing till the Pliocene. We don’t know why this is: in the
Miocene Hominoids of all kinds are common, and monkeys presumably shared their habitat. They
should be there but they haven’t turned up as fossils.
So let’s just salute the monkeys in passing and go on to look at the Hominoidea, the small apes,
the great apes and man. Although monkeys and apes are related there are distinct differences. The
teeth (SLIDE) are quite distinctive. Monkey molars have four cusps in two pairs linked by transverse ridges. Apes and men usually have five cusps and no ridges. It has been suggested that the
earliest monkeys were leaf eaters. But all monkeys are quadrupeds, with long trunks and flexible
vertebral columns and well developed leaping hind limbs. Basically they run about on the tops of
trees.
THE APES
The modern apes, the hominoids, have a subtropical distribution, restricted to Africa and Asia
(SLIDE). Apes are divided into lesser and
great. They share a characteristic group of
adaptations (SLIDE) which distinguish
them from the old world monkeys
(SLIDE).
THE LESSER APES
The lesser apes are the gibbons
(SLIDE,SLIDE). Gibbons are the widespread (SLIDE SLIDE) modern descendants of a very ancient lineage: the earliest
gibbons are Oligocene i.e. roughly as old as the monkeys. The earliest gibbons (from Africa) had
distinctive teeth and jaws, but their limbs were primitive, with arms and legs of nearly equal
length. Modern gibbons, now restricted to S.E. Asia, are different.
Modern gibbons are brachiators (SLIDE). This is a method of locomotion we have not met
before, where the animals progress rapidly beneath branches by means of elongated forelimbs
extended above their heads. Brachiation in gibbons alternates with running on their hindlegs along
branches - brachiation gets you from tree to tree but is inefficient and tiring over long distances.
Like the other hominoids they lack tails.
Because of this method of locomotion we find a characteristic set of skeletal and muscular modifications in brachiators. The trunk is short, the chest is wide and shallow. Flexion of the back is
unimportant if you are hanging by your arms, so the back is rigid and not too muscular. The arms
are long and mobile with long curved fingers. The thumb is well developed for manipulation and
grooming. The legs are relatively short, but locomotion on branches rather than under them is
bipedal - they are also bipedal on the ground (although they rarely come to earth in the wild).
This cluster of adaptations places the gibbons close to the great apes - but the fossil history and
the rather odd behaviour of gibbons (they mate for life, live in small family groups and are fiercely
territorial, guarding a very small area 1/4 mile square) suggests parallelism rather than common
ancestry. Once again a case of different groups of animals doing the same thing and thus looking
similar.
THE GREAT APES
Orang Utans and Gorillas
The Orang utan (SLIDE) (Orang utan = man of the woods: Orang utang = man in debt) is a
creature of tropical rain forest, and as this has shrunk so has its range (SLIDE). Once found
widely in tropical forests they became confined to China and Indochina, then to Borneo as the
forests shrank. They have also been heavily hunted by man, and the present population is very
sparse. They are therefore difficult to observe and have aberrant behaviour: once, we surmise,
they behaved rather like chimps. Orangs are fruit eaters and often hang by their arms (SLIDE):
they can brachiate, but are generally quadrimanus (like quadrupedal but with four hands rather
than four feet). Hands and feet are long and curved. Each night they build a new nest.
The gorilla (SLIDE) is a forest dweller too, but in Africa: the largest primate (SLIDE), so it
spends a great deal of time on the forest floor. Gorillas only brachiate as infants. Usually they are
knuckle walking quadrupeds, walking on the backs of the third and fourth fingers (SLIDE). The
fingers, wrist arm and arm joints are adapted for this, which they do in trees as well as on the
ground. The foot has some rather human like features. Unlike other hominoids gorillas eat leaves
not fruit. They live in small groups and are rather tolerant animals, quite unlike the Victorian
concept of large, savage beasts.
Chimpanzees
Chimpanzees (SLIDE) are again African tropical forest dwellers but also found in open land and
forest fringes (SLIDE). They spend around 80% of their lives in trees, feeding on fruits. They
hang by their arms, occasionally brachiate, climb quadrupedally and knuckle walk or move
bipedally along branches. On the ground they are knuckle walking quadrupeds, occasionally
bipedal. Diet is sometimes varied by meat (colobus monkeys) which is much prized and savoured,
eaten between mouthfuls of leaves. They have been known to be cannibalistic. They also catch
and eat termites, fishing for them with sticks. In fact they are making and using tools. They also
use chewed leaves as sponges to soak up water for drinking or washing and throw stones. Chimps
are genetically the closest relative of man, and share several very human characteristics. For
instance the chimp has well defined facial expressions (SLIDE,SLIDE) which he uses in a predictable fashion to indicate his moods and to communicate with other chimps.
Fossil apes.
If apes are related to man (as they undoubtedly are) then their fossil ancestry is of interest to us.
Modern apes are specialised, but fossil ape ancestors may have been our ancestors too. I intend
to deal with fossils, both ape, human and intermediates in a later lecture.