Lecture2

The Path to Homo sapiens sapiens

Evidence of Connexions Amongst the Primates and Hominim Ancestors

Primate Phylogeny● Humans are primates and we are more

closely related, both genetically and anatomically, to chimpanzees than to any other species

● It is therefore assumed that we share behavioural and conceptual similarities with the primates

● This assumption has been highly significant in psychological studies, and also in philosophical inquiries—the similarities in brain structure and perception, an inheritance of our distant evolutionary past, may bring inherited patterns of thought and perception

● The work of Carl Jung and Sigmund Freud returned to this point regularly, but even phenomenological studies have flirted with the existence of common thoughts and perceptions governed by our senses and physiology

The Oldest Primates● The oldest anthropoid fossil known

dates from ca. 35 million years ago and derives from Thailand and is called Siamopithecus eocaenus

● It weighed roughly 7 kg, being roughly the same size as a modern gibbon

● This species shares many similarities with ancient primates in Africa, suggesting a broad distribution of primates across southern Asia

● In Africa, the oldest primate is that of Aegyptopithecus, which resembled a modern lemur in size

● Surviving teeth from both species suggests that they were primarily fruit eating animals living in the trees

● Proconsul is one of the best represented Miocene hominims in the fossil record

● It lived from approximately 23 to 14 million years ago

● From the full arrray of remains it has been shown that there was considerable variation in this genus Body sizes ranged from that of a small monkey (10 pounds) to that of a female gorilla (150 pounds)

● They also inhabited a wide range of environmental niches: this included open woodlands as well as dense rain forests and it is believed that perhaps some were partially terrestrial

● This is regarded as crucial for later evolution, insofar that they could cope with a broad spectrum of environmental conditions

Proconsul africanus

● They exhibit the typical Old World anthropoid dental pattern of 2-1-2-3. Proconsul also has the typical 5-Y pattern of cusps seen in the lower molars of hominims

● Their teeth also consist of a thin layer of enamel which suggests that they were probably fruit eaters

● Brain size estimates show that their brains were as large or larger than contemporary Old World monkeys though probably not as large as contemporary hominims

● Collection of fruit, roots and nuts on the ground was a probably aspect of their subsistence

● It is perhaps this trend towards spending time on the ground that triggered a movement towards bipedalism

Sivapithecus● Sivapithecus is a genus of extinct

primates, dated from 12.5 million to 8.5 million years old

● They are known from India and Pakistan, the first specimens having been found in the 19th century

● Sivapithecus was about 1.5 metres in body length, similar in size to a modern orangutan

● Some specimens were considered to represent Ramapithecus, another species, but they are now regarded as Sivapithecus

● The shape of its wrists and general body proportions suggest that it may have spent a significant amount of its time on the ground, as well as in trees

● It had large canine teeth, and heavy molars, suggesting a diet of relatively tough food, such as seeds and savannah grasses

The Genetic Evidence

Inferences of Chimpanzee-Human Divergence

● Approximately six percent of human and chimpanzee genes are unique to those species, but we share over 99% of our genetic material with them

● Most of the big differences between human and chimpanzee DNA lie in regions that do not code for genes

● Instead, they may contain DNA sequences that control how gene-coding regions are activated and read

● Studies looking for stretches of DNA that were highly conserved amongst chimpanzees, mice and rats were then compared those sequences to the human genome sequence, to find pieces of DNA that had undergone the most rapid change since the ancestors of chimps and humans diverged about five million years ago

Genetic Divergence and Brain Development

● This resulted in the discovery of 202 'highly accelerated regions' or HARs, which showed a high rate of evolution between humans and chimpanzees

● Only three of those regions contain genes that are likely to encode proteins

● The most dramatically accelerated region, HAR1, appears to make a piece of RNA that may have a function in brain development

● The other highly accelerated regions do not appear to code for genes at all, but many are located close to genes involved in controlling when other genes get made, or in growth and development

● A genetic change, therefore, possibly occurred that triggered brain development in hominids leading to our line which was not shared with chimpanzees

Dating the Divergence● The evolutionary split between human

and chimpanzee may be much more recent, and much more complicated, than has previously been thought

● The results of some genetic studies show that the two species split no more than 6.3 million years ago and probably less than 5.4 million years ago

● Moreover, the speciation process was unusual, possibly involving an initial split followed by later hybridisation before a final separation

● The time of from the beginning to the completion of divergence between the two species ranges over more than 4 million years across different parts of the genome

● This range is much larger than had been expected according to assumptions made on the basis of the fossil evidence alone

An Earlier Divergence than Supposed ?

● The youngest regions are unexpectedly recent, being no more than 6.3 million years old and probably no more than 5.4 million years old

● This finding implies that human-chimp speciation itself is far more recent than previously thought

● If one looks only at the X chromosome, it almost entirely falls at the lower end of the time frame

● The estimate that humans and chimpanzees probably split less than 5.4 million years ago is more recent by ~1 to 2 million years than a previous estimate of 6.5 - 7.4 million years based on the famous Toumaï hominid fossil (Sahelanthropus tchadensis), which has features thought to be distinctive to the human lineage

The cranium of Sahelanthropus tchadensis, which is commonly regarded as on the evolutionary trajectory of humans

● It is possible that the Toumaï fossil is more recent than previously thought, but if the dating is correct, the it would precede the human-chimp split

● The fact that it has human-like features suggest that human-chimp speciation may have occurred over a long period with episodes of hybridization between the emerging species

● The possibility of 'hybridization'—that is, initial separation of the two species, followed by interbreeding and then final separation—would also explain the strange phenomenon seen on chromosome X

● Interbreeding is known to place strong selective pressures on sex chromosomes, which could result in a very young age for chromosome X

Genetic Inversions● By comparing the human genome with

that of the chimpanzee, it has been discovered that chunks of similar DNA that have been flipped in orientation and reinserted into chromosomes are hundreds of times more common in primates than previously thought

● These large structural changes in the genome, called inversions, may account for much of the evolutionary difference between the two species

● They may also shed light on genetic changes that lead to human diseases

● Researchers can now readily identify the differences between the human and chimp genomes

● These differences lend insight into how primates evolved, including traits specific to humans

Inversion of DNA segments between chimpanzee and human chromosomes (blue lines). One inversion on the end of human chromosome 7 that sometimes occurs is the ancestral orientation found in primates (order of dots is yellow-green-red)

The Anatomical Evidence

Comparative Anatomy

● The anatomical similarities amongst the primates has long been recognised

● This extends to many different aspects of the body, from the disposition of the muscles to the general shape of the joints, pelvis, and so on

● It is, however, the pelvis and vertebral column that has elicited the most interest from specialists as the similarities and differences here are vital to understanding how bipedalism might have emerged

● Also, there is particular interest in the development of the hand (to grip tools) and evidence for brain structure which is important in studying higher cognitive functioning

The Pelvis

● A study of the pelvis is not only important for understanding changes in locomotion, gait and so forth, but also for the ease of birth

● In chimpanzees, the hips are narrow and long, which has the result of spreading the legs out and also having the femur project downward in an almost straight fashion

● Early hominims, such as Australopithecine, had broader hips and this made the femurs project downward at an angle towards the knees

● This was associated with the development of bipedalism, but also introduced complications in the delivery of children—especially as the cranium became larger

Feet and Hands● The feet and hands show great

similarity, but again, the greatest correspondence exists between humans and chimpanzees

● Most notable is that chimpanzees have opposable thumbs, allowing for a precision grip necessary to manipulate tools for fine work

● Humans, however, have lost the use of the big toe and the foot digits to grasp objects and to wrap around tree limbs and such

● This represents an adaptation that occurred as hominims left the trees and became bipedal—the need for such grasping limbs became less and less and it is suspected that natural selection resulted in the loss of this in favour of a foot better suited to walking and running

Stance and Locomotion

The Behavioural Evidence

Higher Primates● The distribution of the higher primates

that are most closely related to Man closely parallels that of the earliest fossil specimens of species that led towards the emergence of modern humans

● All of these primates inhabit jungle environments, but our forebears (discussed earlier) seem to have had a broader subsistence spectrum which involved the collection of food in more open land

● It is the trends towards foraging in open land, rather than remaining in trees, which seems to have favoured the development of bipedalism

● Nonetheless, as these primates are our nearest extant relatives, we can look at their behaviour and draw inferences concerning basic behaviour of our ancestors

Communication● Primates have very expressive faces

which convey a vast quantity of information and is used for communication

● In human populations, body language and facial expressions account for approximately 60% of all communication

● Almost all verbal communication is modified by body language in one capacity or another

● It is not only the facial expressions which chimpanzees use to convey information and communicate, but also body posture

● This is, of course, not unique in the mammal kingdom as most animals have specific stances and facial expressions denoting fear, aggression, submission and contentedness

Range of Expressions

● What is unique about primate facial expressions and their ability to convey information is their range

● This is possible only because of the plasticity of the face and the existence of muscles differently developed from those of, for example, canines and felines

● Moreover, it implies a sophisticated grasp of not only many visual cues but also an empathetic appreciation of the meanings

● This necessarily involves higher brain functions, to integrate so many visual signals and also to immediately recognise their meaning

● Other animals tend to rely on a smaller range of signals and scents

Vocal Communication● We also share similarities in vocal

communication, but again this is not unique to primates as most other mammals signal one another through sounds

● However, the chimpanzees have not only instinctual sounds that designate danger, aggression and submission, but also use sounds to communicate information in collective hunting expeditions

● Moreover, chimpanzees have distinctive 'hoots' which permit others in the group to recognise them individually

● Higher status chimpanzees 'hoot' more than junior members of the group, perhaps indicating that vocal communication is associated with status

Signals and Gestures

● Given the limited range of sounds that a chimpanzee can make, they rely equally as much (or perhaps more) on gestures and signals

● Hand and limb gestures are used, along with the 'hoots', in organised hunting expeditions

● A successful kill is signalled by a loud and excited 'hoot' and the entire group recognises this and ceases the hunting, re-grouping

● Most gestures are, however, involved in social bonding and status maintenance or assertion within the group

● Grooming is particularly important, to strengthen bonds amongst the chimpanzees in the group

'Linguistic' Capacity● Washoe was a chimpanzee who was

the first non-human to learn to use some of the signs of a human language, that of American Sign Language

● She also passed on some of her knowledge to her adopted son, Loulis

● It is reported that Washoe could reliably use about 250 signs

● This test demonstrated 'that the chimpanzee subjects could communicate information under conditions in which the only source of information available to a human observer was the signing of the chimpanzee;' 2) 'that independent observers agreed with each other;' and 3) 'that the chimpanzees used the signs to refer to natural language categories, for instance, that the sign DOG could refer to any dog, FLOWER to any flower, SHOE to any shoe'

Tool Production● One of the most significant

discoveries was in October 1960 when Jane Goodall observed the use of tools among chimpanzees

● Recent research indicates that chimpanzee stone tool use dates to at least 4,300 years ago

● Other primates have also been known to use tools, such as orang-utans and gorillas, but also non-primates such as birds

● The tools of the chimpanzees are, however, the most diverse and sophisticated

● Stone tools to open nuts and grind plant material are known, as well as the use of sticks to extract termites and other insects along with sharpened sticks used as spears

Transmission of Knowledge● The capacity to produce certain tools

was transmitted within a social group, and it appears mainly from the mother to its offspring

● Of the groups studied in West Africa, each had its own particular style of producing tools

● Existence of different traditions seems equivalent to the subtle stylistic distinctions that can be seen in the early tool-kits of hominims in East Africa

● This is clearly expressed in the stone tools and form of termite sticks

● In experiments with chimpanzees in captivity, the offspring must learn a new tool production technique before two years of age or thereafter does not seem able to acquire the skills

Implications for Early Hominim Behaviour

Interpreting Hominim Behaviour

● The behaviour of the hominims in East Africa, and those that evolved from these and expanded into Asia and Europe, has been interpreted as being similar to that of the chimpanzees but becoming more elaborate through time

● This has led many to use interpretations based on chimpanzee behaviour and simple hunter-gatherer people

● It is not to imply that the hunter-gatherers are living fossils, but the group dynamics and adaptations are thought to share broad patterns with those of the hominims

● We must, however, recognise that culture also drove adaptations and evolutionary developments

Social Dynamics● The social dynamics of chimpanzee

groups and hunter-gatherers in marginal environments is based on the extended family

● An hierarchy exists in these groups, but only insofar that there is a dominant male

● This is not, however, a closed network for individuals join and leave the group

● Such occurrences increase the chances of success in long-term survival, for a closed network would result in inbreeding and probably lead to the reproductive failure of the group

● A loose conception of territory also prevails in these small groups, where there is privileged access to resources

Tool Production

● Based on the evidence afforded by chimpanzees, and the ethnographic evidence of hunter-gatherers, tools convey social meaning and reflect social identity

● A wide range of tools were likely produced, but only those made from non-perishable material survive for archaeological analyses

● Skills to produce these tools were probably learned within the group and it may be that, like chimpanzees, early hominims acquired these skills from the mother at an early age

● A tremendous conservatism in tool production is evident in the archaeological record, and this may have much to do with the method of technological transmission

Subsistence● The occupation of the jungle margins

or savannah rendered it important for early hominims to have as flexible a subsistence regime as possible

● A fundamentally terrestrial existence would have recommended the collection of plant foods from the ground or at a relatively low height in the trees

● Hunting and scavenging were likely more important to our ancestors in East Africa than it is to chimpanzees, given the environment which they occupied

● This may perhaps imply that group hunting was also more important, and therefore, that communication was vital

● We shall return to these questions frequently in later lectures