9 Emergences of Modern Human and Dispersal

Dr. Gopal Chandra Mondal

Contents of this unit

1. Introduction

     2. Trends of human evolution

Bipedalism

Increased brain size

Refinement of hand structure

Reduction of face, jaw and tooth size

Decreased sexual dimorphism

Refinement of tool design

Power and precision grip

3. Dispersal of modern humans

Complete replacement model

Regional continuity model

Partial replacement model

Learning objectives

• To understand the emergence of modern human
• To understand the general trends of human evolution
• To study the different propositions of human dispersal

Emergence of Modern Humans and their Dispersal

Introduction

  One of the challenges of Physical/Biological Anthropology is to provide insight into the origins of humankind. The fossil record preserves traces of the past life on earth, clearly charting a progression of simple one-celled organisms to increasingly diverse forms.

  The evolution is a slow but continuous process through which the organisms transform from simpler to complex creatures. The theories, concerning the evolution of life date back to the ancient Greeks, but it was only during the nineteenth century that the first comprehensive theories of evolution were developed. Before the mid 1800s, many thinkers had suggested evolutionary theories. But those theories were not acceptable because they lack understanding of the age of the earth and explanation for the evolutionary process. One such early theory of evolution was posited by Jean Baptiste Lamarck (1809). He proposed that species change and adapt to their environment through physical characteristics acquired in the course of their lifetime and the characters so acquired are transmitted to the next generation (inheritance of acquired characteristics).

  The process of evolution was not fully understood when Darwin proposed the theory of Natural Selection (1859), because genetics and the origin of variability were unknown. Variation within the species and reproductive success are the basis of natural selection. Since the 1920s, when evolution through natural selection was tied to genetic concepts, evolution has been understood as the combined effect of four ‘forces’: natural selection, gene mutation, gene flow and genetic drift. Gene mutation and gene flow provide the raw materials, the genetic variation, for evolutionary change. Natural selection and genetic drift are then the conservative forces that limit variation and guide the direction of evolution. Evolutionary theories hold that existing species of plants and animals have emerged over millions of years from simple organisms.

  Human evolution is characterized by a number of morphological, developmental, physiological and behavioral changes that have taken place since the split between the last common ancestor of humans and chimpanzees. The most significant of these adaptations are bipedalism, increased brain size (encephalization), refinements of hand structure, reduction in face, jaw and tooth size, decreased sexual dimorphism, and refinement of tool design. Other significant morphological changes included the evolution of power and precision grip.

Bipedalism: The evolution of the primates in the Eocene, Oligocene, and Miocene epochs serves as a backdrop for the emergence of early human ancestors. By the Miocene epoch (25 million to 5 million years ago), primates in various form- the precursors of modern Prosimian, monkeys, and apes – proliferated in many geographic regions. But sometime in the late Miocene or early Pliocene, new and distinct forms of primates emerged. Classified as the family Hominidae, or hominids, these primates have subtle similarities in their teeth, jaws, and brains. However, the primary characteristic that identifies them as a distinct genus is the structural anatomy needed for Bipedalism, the ability to walk upright on two legs.

  Bipedalism is the basic adaption of the Hominin line and is considered the main cause behind a suite of skeletal changes shared by all bipedal hominins. The earliest bipedal Hominin is considered to be either Sahelanthropus with Ardipithecus, a full bipedal, coming somewhat later. The knuckle-walkers, the gorilla and chimpanzee, diverged around the same time, and either Sahelanthropus or Orrorin may be our last shared ancestor. The early bipedal eventually evolved into the australopithecines and later the genus Homo. There are several theories of the adaptation value of Bipedalism. It is possible that Bipedalism was favored because it freed the hands for reaching and carrying food, saved energy during locomotion, enabled long distance running and hunting, enhanced field of vision and helped avoid hyperthermia by reducing the surface area exposed to direct sun; all this mainly for thriving in the new grassland type environment rather than the previous forest type. A new study provides support for the hypothesis that walking on two legs, or Bipedalism, evolved because it used less energy than quadrupedal knuckle-walking. Anatomically the evolution of Bipedalism has been accompanied by a large number of skeletal changes, not just to the legs and pelvis, but also to the skull, vertebral column, pelvis, femur, feet and ankles. The foramen magnum, the opening in the base situated more or less centrally than the quadrupeds. The developed mastoid processes help to hold the skull in balancing position. To support the increased weight on each vertebra in the upright position, the human vertebral column became doubly S-shaped. The pelvic region became shorter and wider having large acetabulum cavities. The femur evolved into a slightly more angular position to move the center of gravity toward the geometric center of the body and also having developed linea aspera. The knee and ankle joints became increasingly robust to better support increased weight. The human foot has shifted from the nonhuman primate pattern of a grasping organ to a weight-bearing platform.

Increase in brain size: There has been a trend toward increased brain size and complexity during human evolution; however, the trend was neither consistent nor steady. The size increase was slight during approximately 3 million years of the evolutionary history of the Australopithecus and early Homo lineages but rather rapid during the middle Pleistocene in Homo erectus and Homo sapiens. Increased brain size and the complexity were probably related to tool manufacture and use, increasing environmental challenges, and more complex social groups, among other factors. This may also be related to the infant’s slower maturation rate, which required extended parental investment. Learning may have been enhanced by increasing brain sizes and complexity.

	Ranges (cc)	Average (cc)
Great apes
Chimpanzee	282-500	383
Gorilla	340-752	505
Hominidae
Australopithecus Afarensis	380-500	413
Australopithecus africanus	435-350	441
Homo habilis	590-752	640
Homo erectus	750-1250	937
Homo sapiens neanderthalensis	1000-2200	1650
Homo sapiens sapiens	1000-2000	1330

   The trend in brain enlargement continued in Africa with larger-bodied H. rudolfensis and especially H. ergaster. Relative to the estimated body mass, H. habilis was actually “brainier” than H. rudolfensis and H. ergaster. A similar interpretive challenge is presented by Neanderthals versus modern humans. Neanderthals had larger brains than earlier Homo species, indeed rivaling those of modern humans. Relative to body mass, however, Neanderthals are less brainy than anatomically modern humans. Relative brain size of Homo did not change from 1.8 to 0.6 mya. After about 600 kya it increased until about 35,000 years ago, when it began to decrease. Worldwide, average body size also decreased in Homo sapiens from 35,000 years ago until very recently, when economically advanced people began to grow larger while less-privileged people did not.

Refinement of hand structure: Primates are hand-to-mouth feeders that pluck and catch items selectively by hand before ingesting them. Without tools, emergent hominins would have relied on the versatility and strength of their hands to collect food and on their teeth and jaws alone to process it. Unless they used tools to fashion carrying devices such as bags from animal skins, they would have needed a reliable source of water nearby, and they would also have been limited in the types and number of objects that they could transport through their range. In addition to transporting objects and water, there is the more obvious utility of animal skins in protecting against night chills, rain, and strong sunshine. The features of human hands are easily distinguishable from those of the great apes, and they underpin our refined manipulatory abilities. The most complex adaptations of the human hand involve the thumb, which is fully opposable, wherein a unique, fully independent muscle gives this digit remarkable strength in pinch and power grips. The fingertips are broad and equipped with highly sensitive pads of skin. The proportional lengths of the thumb and other fingers give us an opposable thumb with precise, firm contact between its tip and the ends of each of the other fingers. Special configurations of joints the bases of the fifth, fourth, and second fingers facilitate tip-to-tip precision grips with the thumb. Finally, numerous modifications of the small muscles in the hand are associated with fine control of the thumb and fingers.

Comparison of the external (A) and skeletal (B) proportions of the hands of chimpanzees (at left) and humans. From Morgan and Carrier, J.Exp.Bio. (2013).

 Australopithecus afarensis is the earliest hominin species for which there are sufficient fossil hand bones to assess manipulatory capabilities. They were capable of gripping sticks and stones firmly for vigorous pounding and throwing, but they lacked a fully developed human power grip that would allow cylindrical objects to be held between the partly flexed fingers and the palm, with counter pressure being applied by the thumb. Hand bones assigned to a 1.8-million-year-old specimen of h. Habilis from Olduvai Gorge in northern Tanzania represent an advance over those of a. Afarensis in features related to tool use. Because of an absence of fossils, it is not possible to track certain refinements in hand structure that must have evolved in conjunction with innovations in tool manufacture and use during the heydays of H. Rudolfensis, H. Ergaster (1.9–1.5 mya), and H. Erectus (1.7–0.2 mya), as well as H. Antecessor (1.0–0.8 mya) and H. Heidelbergensis (600–200 mya). Only prehistoric and modern Homo Sapiens and H. Neanderthalensis are fully represented by hand skeletons.

  Reduction of face, jaw and tooth size: In hominid evolution a series of interrelated changes is noticed that are primarily associated with diet and food-processing requirements. The oldest fossil hominids have a protruding or prognathous face. In addition, their incisor and canine teeth are large compared to those of modern humans. To accommodate the larger canines, which extend beyond the other teeth, there are gaps (diastema) between the teeth of the opposite jaw. The teeth were arranged in a U-shaped pattern. Approximately 2 million years ago, these characteristics started to become less pronounced in hominids. Some australopithecines developed massive chewing muscles and extremely large molars compared to those of modern humans. Early representatives of genus Homo have smaller canines, and the gaps associated with larger teeth disappeared. In the evolution of hominid dentition, the size of the teeth has decreased and the length of that portion of the jaw that holds the cheek teeth has decreased relative to the length of the skull. The reduction in the size of the teeth may be related to the development of tool use and a more meat-oriented diet. The modern human jaw is smaller and shorter related to the skull than is the ape jaw. Since human food is usually cut up or in some way processed into smaller pieces so that it is easier to chew, humans do not need to exert much pressure when chewing as the apes do. In the course of time, fire was used to cook meat, thus tenderizing it. In modern humans, the evolution of a small jaw has resulted in the development of a chin, a product of changes in the growth and development pattern of the jaw.

  Tooth wear patterns in A. Afarensis indicate that it may have stripped vegetal foods by manually pulling them across the front teeth. The robust-skulled Paranthropus may have eaten tougher foods than did gracile-skulled Australopithecus. Additionally, some palaeoanthropologists believe that Paranthropus was vegetarian, while A. africanus had more meat in its diet. Dental morphology and wear patterns indicate that in South Africa P. robustus ate hard foods and that Kenyan P. boisei chewed whole pods and fruits with hard coatings and tough seeds, though they probably did not chew quantities of grass seed, leaves, or bone. Unlike those of Paranthropus and Australopithecus, the teeth of Homo became smaller over time. H. Rudolfensis has large rear teeth, even relative to estimated body size, but H. Ergaster approaches the modern human condition. Concomitantly, the face of H. rudolfensis is more like that of Australopithecus than H. Ergaster. One expects this trend to be related somehow to changes in diet or techniques of food preparation, but evidence to support this link is not available in the archaeological record.

Decreased sexual dimorphism : The reduced degree of sexual dimorphism is visible primarily in the reduction of the male canine tooth relative to other ape species (except gibbons) and reduced brow ridges and general robustness of males. Another important physiological change related to sexuality in humans was the evolution of hidden estrus. Humans and Bonobos are the only apes in which the female is fertile the year round and in which no special signals of fertility are produced by the body (such as genital swelling during estrus). Nonetheless, humans retain a degree of sexual dimorphism in the distribution of body hair and subcutaneous fat, and in the overall size, males being around 15% larger than females. These changes taken together have been interpreted as a result of an increased emphasis on pair bonding as a possible solution to the requirement for increased parental investment due to the prolonged infancy of offspring.

Refinements in tool design: Stone tools are first attested around 2.6 Ma, when H. habilis in Eastern Africa used so-called pebble tools, choppers made out of round pebbles that had been split by simple strikes. This marks the beginning of the Paleolithic, or Old Stone Age; In Africa the Early Paleolithic (2.5–0.2 mya) comprises several industries with the earliest man-made chipped flakes and core choppers (2.5–2.1 mya). Double-faced hand axes, cleavers, and picks (collectively known as bifaces) appeared about 1.5 mya and persisted until about 200 kya. Archaeologists have detected some improvements of technique and product during the half-million-year span of core-flake industries. Although the major bifacial industry—the Acheulian—has been characterized as basically static, it too shows evidence of refinement over time, finally resulting in elegant, symmetrical hand axes that required notable skill to make. By 1.7 mya a population of H. erectus similar to African H. ergaster lived in Eurasia at what is now Dmanisi, Georgia. The associated choppers, chopping tools, flakes, and scrapers recall the Oldowan core-flake industry of eastern Africa, but there are no bifaces among them. In Europe, Acheulian tools appear 500 kya and persist until about 250–150 kya; they also occur in South Asia. Sites in China (800 kya), Korea, and Japan contain bifaces, but they differ from Acheulian tools. No such technology has been found in tropical Southeast Asia, where bamboo tools may have sufficed.

     In both Africa and Eurasia the Middle Paleolithic was long thought to have lasted from about 200 kya to as recently as 30 kya, depending upon location. While tools from the Early Paleolithic slowly changed across space and time, the Middle Paleolithic was characterized by an explosion of local and regional variations in size and shape and by frequencies of reshaped flakes, blades, scrapers, hand axes, and other tools. Projectile points began to be emphasized in some regions, with bone being used as well as stone; bone, arrow points dating to more than 60,000 years ago have been found at Sibudu Cave in South Africa.

  Although they vary across time and space, Middle Paleolithic tools as a whole are characterized by carefully prepared cores from which elegant flakes or blades were struck. Notably, tools of this type have been found at the Gademotta site in Ethiopia’s Rift Valley in stratigraphic levels that date to approximately 275 kya. Late Paleolithic industries dating to 50–10 kya comprised of diverse blade and micro-blade tools, especially in Europe. Late Paleolithic people used a variety of materials for their tools and bodily ornaments, including bone, stone, wood, antler, ivory, and shell. Stone blades were long, thin, and very effective cutting tools. Often, when they became dull, they are retouched by via pressure flaking, which required fine motor control and coordination. Micro-blades and other points were probably hafted to produce throwing and stabbing spears. Other composite tools of the period include atlatls, harpoons, fish weirs, bows and arrows. Late Paleolithic people also developed techniques for grinding and polishing, with which they made beads, pendants, and other artistic objects. They also made needles (perhaps for sewing fitted clothing), fish hooks, and fish gorges.

Power and precision grip: The hand is capable of several types of prehensile functions. In the power grip, the primate grabs an object between the palm and the fingers; in this position, much force can be applied. All primates are capable of the power grip. More important for fine manipulation of objects is the precision grip, where the primate holds an object between the thumb and the fingers. This is made possible by the presence of an opposable thumb. Humans have developed precision handling to a degree not found in other primates. In the Old world monkeys, apes and humans, the development of a saddle configuration in the joint between the carpal and metacarpal allows the thumb to be directly opposed to the other fingers. Humans differ primarily in the degree of movement possible at this joint. The human thumb is able to oppose the other fingers, and so the fleshy tip of the thumb comes into direct contact with the fleshy tips to all fingers. In the apes, the fingers are elongated, and the metacarpals and phalanges are curved; in humans these bones are straight.

Propositions of human dispersal

The questions concerning the origins and early dispersal of Homo sapiens sapiens continue to fuel much controversy among palaeoanthropologists. It is no wonder, that members of early Homo sapiens sapiens are our direct kin and are thus closely related to all contemporary humans. They are much alike us skeletally, genetically and (most likely) behaviorally as well. In fact, it is the various hypotheses relating to the behavioral capacities of our most immediate predecessors that have fired the imagination of scientists and lay people alike. In every major respect, these are the first hominids that we can confidently refer to as “fully humans”. The evolutionary story of Homo sapiens sapiens is really a biological autobiography of all humans. It is a story that still has many unanswered questions: but several theories have been proposed that seek to organize the diverse information that is presently available.

There are two major theories that attempt to organize and explain modern human origins:

(i) The complete replacement model

(ii) The regional continuity model

These two views are quite distinct and in some ways diametrically opposite to each other. Indeed, there is a third theory, the partial replacement model, a compromise hypothesis incorporating some aspects of the two major theories.

(i) The complete replacement model (Out of Africa model): The complete replacement model, developed by British paleoanthropologists Christopher Stringer and Peter Andrews (1988), is based on the origin of modern humans in Africa and migrated into Eurasia and replaced all populations which had descended from Homo erectus. In brief, this theory proposes that anatomically modern populations arose in Africa within the last 200,000 years, then migrated from Africa, completely replacing the populations of Europe and Asia. This model does not take into account any transition from archaic Homo sapiens to modern Homo sapiens anywhere in the world except Africa. According to this view there could be no admixture of migrating African modern Homo sapiens with local populations because the African modern humans were a biologically different species.

Critical to this model are the following tenets:

• After Homo erectus migrated out of Africa the different populations became reproductively isolated, evolving independently, and in some cases like the Neanderthals, into separate species.
• Homo sapiens arose in one place, probably Africa (geographically this includes the Middle East).
• Homo sapiens ultimately migrated out of Africa and replaced all other human populations, without interbreeding.

Modern human variation is a relatively recent phenomenon.

  Using mtDNA gathered from a number of different populations, scientists at the University of California, Barkley, constructed “trees” that, they claimed, demonstrated that the entire population of the world today descended from a single African lineage. Using the same mtDNA material, other scientists constructed many trees that differed from those of the Barkley group, and some of them are without African roots. Recently, some further genetic data have helped bolster some of the main tenets of the complete replacement model. A team of Yale, Harvard, and University of Chicago researchers has investigated variation in the Y chromosome, finding much less variation in humans than in other primates.

(ii) The regional continuity model (Multiregional): The regional continuity model is most closely

associated  with  palaeoanthropologist   Milford   Wolpoff  of   the  University   of   Michigan   and   his associates. These researchers suggest that local population (not all, of course) in Europe, Asia and Africa continued their indigenous evolutionary development from archaic Homo sapiens to anatomically modern humans.

This model contains the following components:

• some level of gene flow between geographically separated populations prevented speciation, after the dispersal.
• all living humans derived from the species Homo erectus that left Africa nearly two million-years-ago.
• natural selection in regional populations, ever since their original dispersal, is responsible for the regional variants (sometimes called races) that we see today.
• the emergence of Homo sapiens was not restricted to any one area, but was a phenomenon that occurred throughout the entire geographic range where humans lived.

   Though gene flow and local selection, according to the multiregional hypothesis, local populations would not have evolved totally independently from one another, and such mixing would have “presented speciation between the regional lineages and thus maintained human beings as a single, although obviously polytypic, species throughout the Pleistocene” (Smith et al., 1989).

Source: www.wikipedia.org

Partial replacement model: The partial replacement model also begins with African early archaic Homo sapiens. Later, also in Africa, anatomically modern Homo sapien populations first evolved. This theory, proposed by Gunter Brauer of University of Hamburg, postulates the earliest dates for the dispersal for African modern Homo sapiens at over 100,000 y.a. Brauer sees the initial dispersal of Homo sapiens sapiens out of South Africa as significantly influenced by shifting environmental conditions as a gradual process. Moving into Eurasia, modern humans hybridized, probably to a limited degree, with resident archaic groups, and eventually replaced them. The disappearance of archaic humans was therefore due to both hybridization and replacement and was a gradual and complex process. This model includes components of regional continuity, hybridization, and replacement, with the emphasis on replacement.

Summary

1. 1. Human evolutionary trend is characterized by a number of morphological, developmental, physiological and behavioural features
   2. The most significant of these trends are Bipedalism, encephalization, refinements of hand structure, reduction in face, jaw and tooth size, decreased sexual dimorphism, refinement of tool design, power and precision grip.
   3. Due to the achievement of Bipedalism, several changes have taken place in the skeletal structure.
   4. Increased brain size and the complexity were probably related to tool manufacture and use, increasing environmental challenges, and more complex social groups, among other factors.
   5. Change in food habits resulted into the reduction of prognathus face, decrease in the size of teeth.
   6. Gradual development of brain, freeness of hands, and development of precision grip helped humans to gradually prepare refine and effective tools.
   7. The complete replacement model of human dispersal denotes that anatomically modern populations arose in Africa within the last 200,000 years, and then migrated from Africa, completely replacing the populations of Europe and Asia.
   8. The regional continuity model advocates that local population (not all, of course) in Europe, Asia and Africa continued their indigenous evolutionary development from archaic Homo sapiens to anatomically modern humans.

References

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• The Illustrated History of Humankind: People of the past, the epic story of human origins and development. Vol.1; General Editor: Goran Burenhult; A Weldon Own Production. 2004.

Web Links: Accessed In the month of May- June, 2015

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• http://blogs.plos.org
• http:// www.stoneageartifacts.com
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