Primates: an introduction

Definition of a primate; primates as vertebrates; primates as mammals; humans as primates; allometric peculiarities of primates; classification problems; dietary generalisations.

Definition of a primate

From Martin's chapter in Wood et al. 1986. (OHP of primate skeleton)

Like many definition, the definition of what makes a primate (as opposed to a rodent, or a carnivore etc.) is complex. There is little argument as to the core groups of animals today that are primates as I will be illustrating later, but as one goes back in the fossil record, there is more dissension. Still, a purely descriptive definition is needed as a starting point:

Unguiculate, claviculate, placental mammals, with orbits encircled by bone; three kinds of teeth, at least at one time of life; brain always with a posterior lobe and calcarine fissure; the innermost digit of at least one pair of extremities opposable; hallux with a flat nail or none; a well developed caecum; penis pendulous; testes scrotal; always two pectoral mammae. (Mivart 1873)

* Unguiculate - possessing nails, hooves or claws

* Claviculate - possessing a clavicle (collar bone)

This has been brought up to date with little change by Le Gros Clark 1959:

Le Gros Clark's definition

1. Preservation of generalised limb structure with primitive pentadactyly.

Describe primitive pentadactyly limb (3 girdle bones; 1 upper limb bone; 2 lower limb bones; carpals/tarsals; meta-carpals/tarsals; phalanges), and how various other mammalian orders have lost various bones (especially fusing the lower limb bones).

2. Enhancement of free mobility of the digits, especially of the pollux and hallux (both used for grasping).

Demonstration - and maybe get them to take off their shoes to try their feet.

3. Replacement of sharp, compressed claws by flat nails; development of very sensitive tactile pads on the digits.

4. Progressive shortening of the snout.

5. Elaboration of the visual apparatus, with the development of varying degrees of binocular vision.

Orbits ringed with bone.

6. Reduction of the olfactory apparatus.

These 3 are all linked and progress from prosimian through monkeys to humans.

7. Loss of certain elements of the primitive mammalian dentition. Preservation of a simple molar cusp pattern.

Tooth formula reduction: primitive mammal 3.1.4.3/3.1.4.3; prosimians and NWMs 2.1.3.3/2.1.3.3; OWMs & apes 2.1.2.3/2.1.2.3. Note, that it is the anterior 2 premolars that we have lost.

8. Progressive expansion and elaboration of the brain, especially of the cerebral cortex.

9. Progressive and increasingly efficient development of gestational processes.

And this has been further expanded by Napier and Napier 1967:

10. Prolongation of postnatal life periods.

11. Progressive development of truncal uprightness leading to a facultative bipedalism.

Problems

At first view, this seems OK (apart from the dreadful language which makes the whole thing read like a life insurance document). But there are problems with this definition:

Firstly, there is no unique characteristic that defines a primates. It is a list of shared characteristics and trends - most of which aren't even derived, but are retentions of ancestral features, which is definitely not good.

Secondly, many of these features are behavioural, or depend on soft tissue anatomy. They won't help us identify a fossil primate.

Fortunately, there are a number of very specific features that we can use, such as details of the bones of the foot and skull, but even so, it can be difficult, especially when dealing with very early mammals, to decide whether they are primates or not. There are even certain modern groups, such as the tree shrews that some authors consider to be primates - though in general, most people classify them separately. In fact bats are sometimes now considered to be the closest relative of primates!

Classification problems

As I mentioned before, there are an awful lot of different ideas about the details of primate classification. I shall largely use this simplified version (OHP of simple primate taxonomy). However, this is not very representative of the evolutionary relationships - the ones in the handout try and represent that.

Now you've recovered from the shock, remember that you don't need to learn these - but you should know how they differ from the traditional classification.

Prosimii/Anthropoidea changed to Strepsirhini/Haplorhini with Tarsiiformes moved to Haplorhini

Dwarf lemurs (Cheirogaleidae) moved into Lorisoidea.

Lots of movement among the Hominoidea to represent the fact that Homo is now considered to be a much closer relative to the great apes, and particularly Pan..

Humans as primates

"a hodge-podge and makeshift creature"

Apes redesigned as bipeds

Short-faced skull with large, rounded braincase, balanced upright on the spine.

Small jaws, small, thickly enamelled teeth, low-crowned molars and parabolic tooth row.

Long thumb, meeting fingers for precision grip.

Short lower back (lumbar region).

Broad, short, pelvis.

Legs longer than arms

Big toe aligned with others transmits weight when walking.

Anatomical problems

Teeth - overcrowding is a legacy of jaw shrinkage.

Slipped disk - often due to degenerative change caused by excessive loading on lumbar spine due to bipedal not quadrupedal gait.

Appendicitis - infection in a vestigial offshoot of the gut.

Hernia - intestines are less well attached in upright humans than in quadrupeds and intra-abdominal pressure is higher too.

Varicose veins - upright posture raises leg blood pressure and faulty valves allow blood to pool.

Flat feet - two feet supporting entire body weight.

Pregnancy - bipedalism reduces size of birth canal and increased brain size makes baby's head larger.

Scaling

Size is important. Especially in comparative anatomy. If we are trying to recognise adaptations then we need to control for size. This either means only comparing animals of a similar size which reduces sample size enormously, or finding some way of factoring size out.

Some features vary with body size isometrically. We can then just use a simple ratio to compare these. The size of the lungs and the heart are a fairly fixed proportion to the body size. As is the mass of the skeleton: about 6% of body mass in primates which is lower than the 7% for other mammals, reflecting the longer limbs of primates.

More commonly features do not change in this nice linear fashion, but allometrically.

Allometry

In general, most size dependent characters follow some sort of power function:

This can be made into a straight line graph by taking logs of both sides of the equation:

We can then use this line to predict expected values of a feature for a given body size and clearly see any animals that deviate from the expected values. This deviation can even be quantified by dividing the actual value of a feature by expected value, producing a quotient or index. One commonly quoted is the encephalisation quotient, which is the actual brain size divided by the expected brain size for an animal of that size. Humans have an EQ of 5, meaning that a human brain is 5 times larger than expected from their body size. An interesting observation.

Sometimes, when this sort of data is analysed, there are two or more obvious grouping that can have separate lines drawn through them. For example, eyes get bigger with increased body size, but the ratio between eye size and body size decrease with increasing body size. When logged, this produces a nice straight line graph. But when you look at all primates, then it is clear that there are two groups that could be separated: the nocturnal prosimians have bigger eyes for their body size than the largely diurnal anthropoids. Hardly unexpected.

Allometric scaling can also be used within a species to look at things like growth and development. Brain size, as a fraction of body size, is higher in younger primates. It has been suggested that neotany, the retention of infant features through to adult life, might be the developmental explanation of our high EQ.

Generalisations

The changes in proportions brought about by allometric considerations have led to some size dependent generalisations:

* Small primates tend to be arboreal and larger ones tend to be terrestrial (movement costs, branch sizes, predation).

* Small primates tend to be insectivorous and larger ones eat fruit, and the largest eat leaves. BMR, gut transition time.

* Small primates are more often nocturnal, and larger ones diurnal (predation).

* Increased body size improves thermoregulation, longevity and reduces reproductive rate.

Bibliography

Library

 TITLE           :Major topics in primate and human evolution / edited by
                  Bernard Wood, Lawrence Martin, Peter Andrews
 IMPRINT         :Cambridge : Cambridge University Press, 1986

 AUTHOR(S)       :Martin, R. D. 1942-
 TITLE           :Primate origins and evolution
 IMPRINT         :Chapman and Hall, 1990

 AUTHOR(S)       :JONES, Steve  et al.,
 TITLE           :The Cambridge encyclopedia of human evolution / edited by
                  Steve Jones, Robert Martin and David Pilbeam / executive
                  editor Sarah Bunney
 IMPRINT         :Cambridge : Cambridge University Press, 1994.
      

Other

Conroy, G. C., 1990. Primate Evolution, Norton, London.

Lambert, D., 1987. The Cambridge Guide to Prehistoric Man, Cambridge University Press, Cambridge.