The comparative method

Fossils can tell us relatively little about the evolution of behaviour. For that, we need to use the comparative method and strategically compare different (but often closely related) species in order to (a) identify unique features in a species and (b) discern evolutionary trends.

As humans are a kind of primate, comparative research often focuses on our primate relatives.

What makes a primate?

• Relatively binocular vision (having two eyes and using them together, allowing for good perception of depth)
• Grasping hands
• Primitive dentition
• Relatively big brained
• Relatively long-lived
• Relatively long period of parental dependence
• Tendecy to be highly social

By using a combination of genetic comparisons, the fossil record, and carbon dating, a primate family tree can be constructed. The major groups within the order of primates include: prosimians, new world monkeys, old world monkeys, and apes.

Prosimians

• Earliest primates, prosimians meaning "pre-monkey".
• Includes lemurs, bush babies, lorises, pottos and tarsiers. Tree shrews are sometimes included, though others separate tree shrews into an order of their own
• Longer snout that monkeys and apes, ending in a moist nose, indicating a well-developed sense of smell
• Larger portion of the prosimian brain is devoted to the sense of smell rather than vision
• Less time in infancy than simians, only around 15% of their lifespan as opposed to 25-30% for monkeys and apes
• Inflexible faces compared to those of monkeys and apes. Most prosimians have 36 teeth, whilst west simians generally have 32
• The majority of living prosimians live only on Madagascar

New world monkeys

• Consists of around 53 species of monkey, commonly divided into two families - Cebidae and Callitricidae
• Callitricidae consists of marmosets and tamarins. Considered to be the most primitive of monkeys due to their anatomical and reproductive characteristics. No opposable thumbs. Claws on all digits except for their big toes. They do not have prehensile tails. They also lack the ability to change their facial expression
• The Cebidae monkey families are significantly heavier than the Callitricidae.
• Social group size varies from that of the squirrel monkey, which lives in troops of up to 500 individuals, to that of the night monkey, which lives in small nuclear family groups
• Unlike prosimians, most monkeys and all apes are diurnal (active during daylight, sleeping at nighttime). The night monkey is the only nocturnal monkey in the world

Old world monkeys

• Occupy a wide variety of environments in South and East Asia, the Middle East, Africa and even Gibraltar
• Around 78 species of Old World monkeys, separated into two subfamilies - the Cercopithecinae and the Colobinae. Monkeys in both groups are relatively large, being about the size of small to medium-size dogs
• Cercopithecines have two identifiable anatomical characterstics - "ischial callosities": hairless, callused areas on either side of the rump, and cheek pouches.
• The colobines lack cheek pouches. They have unusually long intenstines that increase the absoption of nutrients. Their stomachs contain more acid than those of other monkeys which speeds up digestion but results in delicate stomachs

Apes

• Apes and humans differ from all the other primates in that they lack external tails. They are also more intelligent and more dependent for survival on learned behaviour patterns
• The lower molar teeth of apes and humans have five raised points (cusps) on their grinding surfaces. Monkeys only have 4 cusps on their lower molars
• The shoulder anatomy of apes and humans also differs from other primates
• Apes and humans are members of the same superfamily Hominoidea - subsequently referred to as hominoids. There are two families within the hominoid group - Hylobatidae (gibbons) and hominidae (humans, orangutans, gorillas, chimpanzees, and bonobos).
• Gibbons sometimes walk bipedally on top of branches, however they are more efficient at brachiation (moving around in trees by swinging under branches with a hand over hand motion)
• All of the great apes have the anatomical characteristics that allow brachiation, but most rarely use this mode of locomotion because they're too heavy to be supported by small branches. Humans have also retained upper body traits that allow us to brachiate, though our legs grew significantly longer and more muscular as we became habitually bipedal. As a consequence of weakened arm muscles and added weight to the legs, we're inefficent brachiators at best.

No living primate species is our ancestor. Chimpanzees, baboons and lemurs are modern species just like us. Instead, the logic is that we shared common ancestors at some time in the past, with each species of primate.

It is possible to compare the genetic similarities between species by DNA hybridisation. When Sibley & Alquist (1984) conducted such DNA comparisons betweens groups of primates, our similarity to the other great apes, especially chimpanzees, turned out to be astonishing

Chimpanzees share a 1.6% difference in DNA to humans. They are more genetically similar to humans than they are to gorillas (2.3% difference). Humans share more DNA in common with a chimpanzee than is found between two species of gibbon (2.2% difference between common gibbon and siamang gibbon). Passingham (1982) - "many characteristics that might be taken to separate man from the rest of his fellow primates - such as large brains, upright walking, tool-use, communication and language, and intense sociality - are in fact merely extensions of typical primate features, not discontinuities from them." However, that 1.6% difference in DNA has a massive effect and still means we are very different.

Bipedalism refers to an organism that moves by means of its two rear limbs or legs. The evolution of human bipedalism begain in primates about 4 million years ago. Human beings are the only mammals that habitually walk on two legs. All other primates walk on four legs. Chimpanzees and gorillas knuckle walk.

Other primates can walk on two legs, but the action is awkward and difficult to sustain. Human hips have swiveled forward to allow a free flowing stride. There are also adaptations to the foot. In humans, the big toe is aligned with the other toes. In other apes, the toe is opposed rather like a thumb.

Most monkeys run along the top of branches. The spine enters the skull in a more horizontal angle. Apes hang from the underside of branches, the spine enters the skull at a more vertical angle. This is a useful preadaptation for bipedalism.

One hypothesis for human bipedalism is that it evolved as a result of differentially successful survival from carrying food to share with group members, although there are alternative hypotheses. Sockol et al (2007) found that bipedal walking was less energetically costly than knuckle-walking.

Bipedalism freed up the hands for fine dexterous manipulation. Humans are capable of precision grips, while the other great apes more often use semi-precision grips and find precision grips difficult (Napier & Napier 1990)

Despite the fact that great apes don't have dexterous hands as humans, they do use and make tools. The species that uses tools more than any other apart from humans are our closest genetic relative - the chimpanzee. Examples of their tool-use includes nut cracking, termite fishing, ant-dipping, and bush-baby spearing.

Habitual tool-using species: chimpanzees, orangutans, capuchin monkeys, and caledonian crows.

A human's brain is three times the size of a chimpanzee. Humans don't have a big brain in absolute terms; whales, elephants and dolphins have bigger brains than humans. However, our brain is 7.5 times bigger than an average mammal of our body size (Jerrison, 1973). Our brain is 3.1 times as big as one would expect for an average primate of our body size.

In most species, their brain is almost at adult capacity at birth. If we waited until then to produce our babies, we wouldn't be able to push them out. The proportion of adult brain weight at birth for Rhesus macaques is 60-70%. For chimpanzees it is 46% and for humans it is 23%.

Our bones do not fully calcify until one year after birth. Rhesus macaques have a gestation period of 24 weeks, their bones have fully calcified by the 18th week of gestation. The sutures between our brain plates do not fuse until about 2 years of age in humans. In most species, this occurs before birth. At the points where the plates of the skull meet are little gaps called fontanelles. It allows the bones to slide over each other, slightly reducing the dimensions of the cranium during birth. The length of a species' gestation is  correlated with adult brain size. Humans give birth when their babies are still foetal-like, because if we waited, their brain would be too large to fit through the birth canal.