15 Progressive evolution of man: Biological Dimension

Ms. Sangeeta Dey and Prof. A.K. Kapoor

Contents:

1. Introduction

2. Progressive evolution of man

3. Hominization Process

4. Biological dimension of progressive evolution of man

4.1 Erect Posture and Bipedal Locomotion

4.2 Remodelling of Face and Teeth

4.3 Expansion and Development of Brain

4.4 Culture and Hominization process

4.5 Language and evolution Summary

Learning Objectives:

To describe the process of evolution
To understand the Hominization process
To describe progression changes that occur in biological dimension of man
To know about factors that lead to bipedalism and erect posture
To understand impact of evolution for the biological evolution of man
To understand the importance of development of brain, speech, language in evolving human beings

1. Introduction

About 2000 million years ago, the first cellular forms of life appeared on earth. Some of these cells had the ability to release oxygen. Slowly, these single celled organisms became multi-cellular life forms. About 500 years ago, invertebrates were formed. Jawless fish evolve around 350 million years ago. Sea weeds and few plants existed around 320 million years ago. The first organisms that invaded land were plants. About 350 million years ago, the animal’s lobe fins type fishes evolved into first amphibians who lived both land and water. These were the ancestors of modern day’s frogs and salamanders. The amphibians evolved into reptiles which lay thick –shelled eggs that do not dry up in sun. The turtles, tortoise and crocodiles are the descendants of these reptiles. In the next 200 million years, reptiles of different shapes and sizes dominated on earth. Some land reptiles were the dinosaurs. About 65 million years ago, the dinosaurs suddenly disappeared from the earth. The first mammals were like shrews. Their fossils are small size. Mammals are viviparous and protected their unborn young inside the mother’s body. When the reptiles come down, mammals took over this earth, that’s how evolution occurs.

Evolution is the gradual process by which the present diversity of plants and animals arose from the earliest and most primitive organisms, which is believed to have been continuing for atleast the past 3000 million years. Until the middle of the 18th century it was generally believed that each species was divinely created and fixed in its form throughout its existence. It is believed that every species was individually created by God in the form in which it exists today and is not capable of undergoing any change. This is referred as theory of special creation. Special creation was contradicted by fossil evidence and genetic studies, and the pseudo scientific arguments of creation science cannot stand up to logical examination. It was the generally accepted explanation of the origin of life until the advent of Darwinism. However, Lamarck is the first biologist to publish a theory to explain how one species could have evolved into another. He suggested that changes in an individual are acquired during its lifetime, chiefly by increased use or disuse of organs in response to “a need that continues to make itself felt” and that these changes are inherited by its offspring. Thus the long neck and limbs of a giraffe are explained as having evolved by the animal stretching its neck to browse on the foliage of trees. This is also called as inheritance of acquired characteristics. But it was not until the publication of Darwin’s Origin of Species in 1859 that special creation was seriously challenged. Unlike Lamarck, Darwin proposed a feasible mechanism for evolution and comparative anatomy. The modern version of Darwinism which incorporates discoveries in genetics remains the most acceptable theory of evolution.

2.Progressive evolution of man

About 15 million years ago, primates called Dryopithecus and Ramapithecus were existing on the earth. They were hairy and walked like gorillas and chimpanzees. Ramapithecus was more like while Dryopithecus was more ape like. So, they were the forerunners of hominids.
Progress was made further when a skull was discovered about 5 million years ago. It had a brain size of about 500 cm3 within the range of ape brain but its jaw and teeth were human like. It was probably not taller than 4 feet but walked up right. It was named as Australopithecus africanus which lived in East Africa grass lands. Evidence shows that they hunted with stone weapons but essentially ate fruits.
From the Australopithecus evolved the Homo Habilis (the Handy man) which was characterized by having a larger brain than Australopithecus (650 – 800 c.c.), using tools and being bipedal. They probably did not eat meat.
Homo erectus appeared about 1.5 million years ago. Its brain capacity increased to about 800 – 1200 c.c. and they migrated to Asia and Europe. Fossils of Java man and Peking man belonged to Homo erectus. They probably ate meat.
Homo erectus was later replaced by Homo sapiens. There were several sub-species of H. sapiens, a wide spread one of which was Homo sapiens Neanderthals (Neanderthal man), a large brained game hunter. The Neanderthal man with a brain size of 1400 c.c. lived in near east and central Asia between 10000 – 40000 years back. They used hides to protect their body and buried their dead bodies.
The oldest remains of H. Sapiens sapiens (cro-magnon) appeared around 35000 years ago, probably having evolved from Neanderthal man. They had large-brain size (1500 c.c.) as they are today and existed as hunter gatherers in co-operative bands. They were stout, short and used hides for clothing. They built their huts for shelter and buried their dead bodies.

3. Hominization Process

Hominization process is the evolutionary transformation of hominoids into hominids. It is a process that occurred in the hominid line since its divergence from the common hominoid ancestor shared with any living ape. The term is applied to include all those aspects of structural and behavioural changes that occurred in the hominid line finally leading to the evolution of modern man. These changes essentially constitute the essential biological and cultural factors that contributed to human evolution.

4. Biological dimension of progressive evolution of man

4.1 Erect Posture and Bipedal Locomotion

The most obvious thing about human beings that differentiates them from all other members of the animal kingdom is their upright posture and their associated habit of walking (known technically as bipedal motion). Many of the distinctively human morphological traits are directly attributable to these two facts of human life.

The shape of the vertebral column in mammals is delicately adjusted to support the animal’s center of gravity. In a buffalo, this point is over the forelegs, and its backbone resembles the profile of a cantilever bridge with the weight carried on a central pillar. In Anthropoidea, the center of gravity is shifted back considerably and the vertebral column is unusually flexible for quadruped features that reflect a tree dwelling way of life, featuring agile grasping and jumping with strong rear leg thrusting. The stresses put on the backbone by brachiating resulted in its becoming somewhat stiffened, with several lumbar vertebrae becoming thoracic vertebrae. This fact suggests that humans are more closely related to brachiators than non-brachiating primates: quadrupedal monkeys have a average of 9 lumbar vertebrae and 10 thoracic vertebrae; gibbons average 6 and 13 respectively; great apes 4 and 13; and humans 5 and 12.

Characteristics of the human vertebral column that are specific adaptations to an upright posture include: enlarging of the lower vertebrae to absorb the forces of compression; relative constancy in the size of the spines protruding from each vertebra, resulting from a lack of weight bearing stress points along the spine (as opposed, for instance, to the gorilla, in which the spines of the neck vertebrae are elongated to provide a solid anchor for the heavy neck muscles that must support the forward-jutting head – unnecessary in humans, where the head is nicely balanced on top of the vertebral column); increase in size and number of bones in the sacrum to take up the transmission of weight through the pelvis and legs (humans average 5-6 sacral bones, great apes around 5, quadrupedal lower primates 2-4); and finally, a sharp backward curving of the spine in the lumbar region providing a solid platform to transfer the weight of the body onto the pelvis and giving the human spinal column its distinctive S-shape.

It is observed that these and other human features emerged as our ancestors became increasingly bipedal. True enough, many have mostly teeth and jaw fragments from Ramapithecus, but we think that these creatures spent a great deal of their time on the ground, on the fringes of the forests in the Miocene epoch; and if we remember that primates developed their vision rather than their sense of smell, standing on hind legs might well have been crucial for spotting the abundant meat-eating predators for which Ramapithecus with no specialized defensive weapons must have been an attractive and even easy meal. Thus we can speculate that there were strong selective pressures (in the form of carnivores) on Ramapithecus to develop highly cooperative social groups, an efficient communication code (depending on sounds rather than visual signals that would be difficult to see in the tall grass), and the habit of rearing up on the hind legs to scan the countryside for danger. Certainly, these features became the principle adaptive characteristics of subsequent hominid groups.

Australopithecus are already clearly bipedal and erect, although certain human features had not yet completely evolved: the pelvis is not yet broadened and rounded as much as in modern humans (for support of the internal organs as well as the enhancement of the passing of a large brained infant at birth), although it is already much broader and rounder than in modern great apes; the lumbar curve is not yet fully developed; and the foramen magnum (the hole through which the spinal cord passes out of the skull and into the vertebral column) into the rear of the center of the base of the skull, meaning the head was not yet fully balanced at the top of the vertebral column. On the other hand, their feet were nearly modern; they had arches front – to rear and sideways and a full heel to provide good walking leverage, as we do. It appears that Australopithecus developed persistence hunting – pursuing game until the game was exhausted – as a major survival technique that put a premium on long distance walking. In any event, walking must have been of extreme importance to the Australopithecus because by the time of Homo erectus, the skeletal structure (with the exception of the head) was essentially fully modern, completely adapted to walking. Even the skull shows adaptation to erect posture, with the shifting forward of the foramen magnum closer to the center of the base of the skull. Thus, by the time Homo erectus trekked out across the continents of the old world, virtually all of the uniquely human traits associated with erect posture and bipedal locomotion had evolved.

4.2 Remodelling of Face and Teeth

When an infant primate is born, it passes out of its mother’s womb through the birth canal, through the vagina, and into the realities of social life. The birth canal is surrounded by that forms the lower front of the mother’s pelvis. This bone sets a limit on the size that a new born infant can be at the moment of its birth: if any part of it is too large, it will stuck with the result that both the infant and the mother are quite likely to die – which means that, they will not contribute their genes to the ongoing evolutionary process.

It was noted that a major trend of human evolutionary development was a dramatic increase in the size of the brain, which we deduce from the fossil evidence showing a rapid increase in the size of the brain case since the Australopithecus. Since the size of the head as a whole should not keep getting larger beyond limits allowed by the process of birth, there was a strong adaptive pressure to shorten the snout and reduce the size of the face to make space for cranial expansion in the course of human evolution. Fortunately, other processes came into play that made this possible.

Firstly we have to identify what changes took place in the face and teeth among our hominid ancestors. Already with the arrival of Ramapithecus we can see some hominid facial trends. The upper and lower canines interlock only slightly, and there is no diastema next to the canines to make room for such interlocking. Australopithecus continue this trend, although the face and jaw are still massive by modern standards. Heavy ridges of bone arch over the eye sockets, probably affording a measure of protection as well as shelf against which some of the large jaw muscles could be anchored. Indeed, in robust forms, the sagittal crest running from front to back on the top of the skull provides more space for such attachments. For all practical purposes, Australopithecus display little in the way of foreheads, although the gracile line shows some movement in that direction. Neither Ramapithecus nor the Australopithecus, nor even Homo erectus has any chin whatever; in fact, many still have a ridge of bone running part-way back which forces the front teeth to protrude outward. All three groupings have very large teeth (in proportion to the whole head) by modern standards.

The face of Homo erectus is by far its most primitive aspect. It is still massively jawed, with a minimal forehead sloping sharply back from heavy bone brow ridges. The skull narrows sharply behind the eye sockets, providing a groove along which the large cheek muscles that operate the jaws can run. Its front teeth still “buck” outwards and its nasal cavity, palate and tongue are long and flat compared to those of modern humans, while its larynx sits much higher than in its throat than ours. And of course, its head hangs slightly forward, somewhat awkwardly balanced on its vertebral column, with a foramen magnum that is still somewhat behind the center of the skull’s base. But in virtually every other respect Homo erectus has essentially modern skeletal features.

Even Neanderthal – the earliest member of our own species Homo sapiens – retained a heavy looking face and large teeth by modern standards, especially, the “classical” Neanderthal of south Western Europe. However, by this time we can observe the emergence of a real forehead (although some specimens still retain the heavy brow ridging of Homo erectus); the front teeth are rooted vertically, and the lower jaw that supports them juts out beyond them to give them a solid base; and a fully modern chin has developed.

These developments are further elaborated in the transition to fully modern Homo sapiens; the brow ridges diminish; the forehead approaches being vertical; the chin is prominent; the face (with its small vertical teeth) is flat, with a nose that protrudes. The foramen magnum is now exactly at the center of the skull’s base, the head balancing nicely on the vertebral column. The nasal cavity and palate are shortened and arched, the tongue thickened and shortened, and the larynx sunken down into the throat, which for the first time joints the mouth pretty much at a right angle rather than obliquely.

Many factors brought about these changes. First, the trend toward upright posture resulted in the foramen magnum being moved forward, tilting the head upright into a vertical position and possibly exerting some “squeeze” against the face. Second, the long, flat mouth, tongue, palate and nasal cavity with a high larynx and the wide angle at which the throat joined the mouth prior to Homo sapiens, drastically limited the number of sounds the vocal apparatus could produce and the speed with which it could produce them. If, the evolution of speech were of crucial importance to the overall process of human evolution, then there would have been selective pressures for moulding the face in the form it finally assumed. In fact, there is evidence that the mouth and tongue of Neanderthal are still too flat to produce the modern range of sounds, and also probably lack enough quickness of tongue to produce sounds at a modern rate. Finally, it appears that human teeth only really were free to become small when people gave up using the mouth as a “tool” – a “fifth hand” used for holding, tearing and even chewing items to soften them. This happened in the last 10,000 years with the development of food production, when the agricultural revolution replaced hunting and foraging as the principal means of subsistence for most of the peoples in the world.

4.3 Expansion and Development of Brain

There is a dramatic increase of the brain case as we approach specimens of Homo sapiens. This is indeed worth noting, and physical anthropologists have spent a great deal of time studying the rapid increase in brain size in the course of post-australopithecine evolution.

The reasons are complicated. First, in general, the overall size of our ancestors increased at each evolutionary stage. We concluded that there were strong selective pressures for this increase in size, probably because a larger body size made it easier to hold and use tools, and also increased the amount of muscle available to hunters and foragers on long treks.

But the brain grew larger, proportionately did the body. The most likely reason is that an increase in brain size tremendously increases the possibility – indeed the probability – of an increase in both the number and kinds of connections between the brain cells. It is an increase in the kinds of connections between brain cells that is apparently responsible for the emergence of new kinds of mental operations such as thinking and using language, operations that are fundamental to human existence.

Scientists have been able to document that the tremendous growth in brain size, by carefully studying the contours of the insides of fossil brain cases, between the Australopithecus and Homo erectus was accompanied by an increase in size and complexity of the outside surface of the brain called the Cerebral Cortex. This expansion of the cortex is the most recent evolutionary development of the brain, and it is the cortex that is primarily associated with thinking and language use.

Each of the senses is represented in a specific area of the cortex, a part of the outer layer of the brain, where the fibers carrying messages from the surfaces of the body concentrate according to the sense. Thus, for example, stimulation of the retina in the eye ball results in messages flashing along the optic nerve fibers and flooding into one area of the cortex; messages from the fingers (touch) arrive in another area; taste messages in yet a third area, and so forth. The areas of the brain that are specialized to process these incoming messages are called the nuclear zones of the cortex; and each sense has its own.

Around each nuclear zone anatomists have found that specialized bundles of fibers are concentrated into what are called association areas; and the clear differentiation of separated association areas increases as we move up the order of the primates towards human beings. Now the job that each association area performs is very complicated, but it amounts to modifying the operation of its nuclear zone. For example, damage to the visual association area in the human brain results in the person losing his or her ability to identify the objects, even though the objects are still seen (the messages are still represented in the visual nuclear zone). It appears, then, that the association areas of the brain are central to the process of recognition. They are where information about the outside world is stored, where incoming stimuli can be matched against stored experience and decisions can be made: upon seeing a saber-toothed tiger, one can decide to issue a warning to one’s neighbours and take to one’s heels – or alternatively, face the predator and try to convert into a mouth of meals. In humans large bundles of nerve fibers connect the association areas of the cortex. This provides us with the ability to pass messages back and forth directly between these association areas, to compare and contrast there different recognitions – in other words to think.

The cortex also houses the nuclear zones for triggering muscle actions (motor functions). An outstanding characteristic of Anthropoidea is that they have evolved direct link ups between the association areas of sight and touch and the association areas around the major motor function centres. These direct links ups result in the ability to “fit” ones bodily movements to what ones sees and holds, while freed from other functions of the brain such as emotions that might interfere. Thus, if you think about it for a moment, you will see that these structural features of anthropoid brains amount to being a necessary precondition for using tools. In other words, regardless of why the anthropoids acquired these features, without them regular tool use – a preeminent specialty of our hominid ancestors as they evolved – could not have developed.

4.4 Culture and Hominization process

Each stage of hominid organic evolution seems to have been accompanied by the major advances in cultural evolution. Because stone tools are relatively indestructible, much of early cultural evolution is represented by the evolution of tool industries.

The importance of tools as molders of hominid evolution has been recognized for a long time. Upright posture, leaving the hands free to manipulate objects and carry things form long distances, certainly is dynamically connected with the adoption of tool use by early hominids. In order for a tool to be useful, one must have it with him when the moment arises to put it to work. This takes making the tool in advance – planning; walking on two legs to free one’s hands to carry the tool; and commitment to using the tool. To see how much of a commitment this represents, try carrying a brick around with you for a whole day. Those parts of the human brain most needed for manipulating tools are well evolved. Aside from the ones mentioned, other highly developed areas include the frontal lobes that organize behaviour into sequences and the motor association areas that control the fingers and the thumbs. The hand itself is marvellously evolved, It combines the powerful curled – fingered grip with which heavy objects can be moved, with the delicate manipulations possible when small objects are held between the fingers and the thumb (and the ability to fully oppose the thumb to all the fingers is uniquely human.

Thus, much of what we take for granted about man today is the result of natural selection operating on his ancestors, adapting them to an environment he himself has created (or began to create): tools. But culture is more than just tools. one of the most important features of culture is language, which also profoundly influenced and was influenced by human evolution.

4.5 Language and evolution

There are three areas of brain that are highly evolved in the humans and appear to be crucial for human linguistic ability. One is called Broca’s area and is located toward the front of the dominant side of the brain. This area activates, among things, the muscles of the jaw, lips, tongue, and larynx. The second is Wernicke’s area by a large bundle of nerve fibers (called the Arcuate Fasciculus) and is the brain site where verbal comprehension takes place. The third area is the Angular Gyrus, situated next to Wernicke’s area serving as a link up between parts of the brain that receives stimuli from the sense organs of touch, hearing and sight.

Man could not possibly speak without these brain areas. It is interesting to note that all three are located in the cortex – the “new” brain which, as already been emphasized is most evolved in humans and appears to have first approached its modern size and complexity in Homo erectus. The fact that all three are located in the cortex allows sensory inputs and verbal representations to be connected with each other without having to go through the “old” brain – especially the limbic system which activates such basic responses as aggression, fear, hunger and sexual arousal. Consequently, human beings can think, talk and experience the world without involving these “gut” level states. Other animals, including our primate relatives, have not developed these brain areas nearly as much as man has. Thus many important aspects of man brain’s seem to have evolved as speech specialization; and it can be reasonably supposed that verbal communication was so adaptive for the man’s ancestors, that strong selective pressures progressively moulded these changes. Certainly, language is a principle cornerstone of human existence.

These are the major progressive changes in biological dimension that characterize human evolution; erect bipedalism, reduction of face, and expansion and development of the brain. All these changes interacted with each other, and all are dynamically connected to the evolution of social groups and culture – the primary mechanism of adaptation.

Summary

Erect Posture and Bipedal Locomotion; Remodelling of Face and Teeth; Expansion and Development of Brain; Culture and Hominization process and Language and evolution are the major progressive changes in biological dimension that characterize human evolution; erect bipedalism, reduction of face, and expansion and development of the brain. All these changes interacted with each other, and all are dynamically connected to the evolution of social groups and culture – the primary mechanism of adaptation.

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