D. HUMAN EVOLUTION

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Landmarks

Society has not readily accepted human evolution. Xenophanes of Colophone studied fossils as long ago as 540 B.C.. (Parker, 1992). Baron Georges Cuvier (1769-1832) studied fossils and knew they were very old and showed a succession of fossils through time. He also believed that fossils resulted from a series of catastrophes at various times. He believed that these destroyed entire animal and plant populations (Jenkins, 1978). Cuvier had a great reputation in his time, so when he claimed that fossil man does not exist, this became a dogma little challenged until Darwin published his works in 1856 (Parker, 1992). Landmarks of evolution are Lamarck 's work and the 'Vestiges of the Natural History of Creation' written by Robert Chalmers (1802-71) and published in 1844. Human fossil s found before this publication received little attention: a skull fragment found at Cannstadt in southern Germany at around 1700, human fossil remains found by Schlottheim in 1820 and the discoveries and publications of Paul Schmerling (1791-1836) of 1833-4, to mention a few of the earliest. Neanderthal remains found in Germany in 1856 were so similar to modern humans that they were not convincing evidence of human evolution. In 1890 older fossils, belonging to the middle Pleistocene period and eventually classified as a distinct species, Homo erectus, showed the long evolutionary history of humanity (Avers, 1989). This fossil could be close to the common ancestor of humans and chimpanzees.

At the onset of the 20th century, most scientists had accepted the great antiquity of the earth, the theory of evolution  and that humanity had evolved from an ape ancestor, but by 1908 the fossil evidence of early man was scarce (Reader, 1988). Even in 1929 in a book on "The Science of Life", under the chapter, "Origins of Homo Sapiens" the authors could only speak of "rare infrequent fossils . . . "now coming to light" (Wells, et al, 1930). Redefining humanity in the light of evolution through natural selection is a paradigm shift of such great proportions that it must take many generations for society to accept it. Scientists within such a field, have the evidence at hand. When presented with objective proof they are easily convinced on an academic level. For the non-professional however, there are entrenched cultural and religious beliefs and such a person's interests lie in other areas. I have seen that most of my relatives have no need to accord evolutionary facts with either their cultural, religious, or modern-scientific and technological outlook and approach towards life. Many religious authorities still ignore evolution, claiming that such beliefs lead to a loss of faith. For them, evolution is simply a false theory invented by fallible people.

Biologists included principles from genetics in evolutionary theory during the 1920s and 1930s. By 1940, although many hominid (see hominid books) (human ancestor) fossils had been found, there was little consensus amongst the scientists who had created 29 genera and 100 species, attributed to early man! Scientists accepted the descent of man from an extinct species of primate as a historical fact, but they could derive no clear description of the sequence of events from the available mass of data. Then, Ernst Mayr applied the systematic approach of classification to the hominid fossils, grouping fossils strictly according to the features that they have in common. Mayr reduced all the hominid (human ancestor) fossil records to three species, A. africanus (including small brained australopithecines), H. erectus (including the Java and Peking hominids - the archetypal Missing Link) and H. sapiens (including Neanderthals and Cro-Magnon). This focus proved the solution to the jigsaw puzzle of hominid fossils. It was not a perfect solution, but provided the basis for a much clearer picture and rational analysis of the available data.

In this period between the mid-1930s and mid 1940s geneticists, systematists and palaeontologists collaborated to create a united approach to evolution, called the Modern Synthesis. This formed the foundation of the neo-Darwinian evolutionary theory. Very simply, there are five basic principles to this theory:

[1] The genetic origin of heritable variation;

[2] Natural selection, gene flow and genetic drift (gene combination changes from generation to generation due to chance factors) drive evolution, resulting in a change in gene frequencies;

[3] Adaptation is genetically based and so leads to changes in phenotypes;

[4] Speciation evolves through reproductive isolation and (genetic) divergence of populations;

[5] Genetic changes through natural selection lead to new species and eventually new taxa. Darwin's theory also required a new approach. Naturalists of the 1840s and 1850s sought patterns instead of processes, describing what they saw, but not pondering the cause. Anatomists established abstract relationships between structures (Bowler, 1992) without relating these to the creature's habitat. Darwin sought to explain these patterns of relationship through some process. He found these in the effect of the environment upon a species and linked the evolution of a species to the environment. This was a revival of a principle promoted by natural theologians: adaptation. It marks the beginning of biogeography.

Prior to Darwin, naturalists such as Johann Reinhold Forster (1729-98) and Karl Willdenow (1765-1812) recognised that the environment defines the association of plants and animals of a region (Bowler, 1992). However, this adaptation discussed by natural theologians was at the hand of God, perfectly stable and never changing. It was proof of God, for His design led to creatures well suited to their environment. Even in 1844, in a book, "Vestiges of the "Natural History of Creation", written by Robert Chalmers (1802-71), he described transmutation as caused by Divine influence instead of the environment. Chalmers proposed that God had imposed a preordained sequence of development, so that organisms passed through a sequence of developmental stages to eventually attain the human form.

In the 1850's evolution became an acceptable process among some liberal theologians such as Baden Powell, as a purposeful process in which humanity was the goal toward which it was aimed. Darwin turned this idea around and said the adaptation was a continuous response to the environment and that species were changeable. In 1844 he described an animal's form or "organisation" as "slightly plastic". This adaptation of creatures to the environment is the ecological view.

In contrast to the slow acceptance evolution, the knowledge of vitamins is already a part of common nutritional knowledge. Even a pre-school child has an inkling of this knowledge. Vitamin C or ascorbic acid was first isolated in pure crystalline form by two biochemists from lemon juice in 1932 (Lehninger, 1977). Scientists generally receive and accept new ideas well before they form part of public consciousness and society quickly adopts this type of knowledge. Computing tools advanced even faster - the first commercial computer, UNIVAC, hit the market in 1951 (Hickman & Silva, 1988). Today it is an essential part of every business. It is a strange quirk of human nature and a lack of foresight as to our role on earth that we can so eagerly embrace technological tools such as aircraft and motor vehicles to change the world and our lifestyles, but we are unable to accept evolution and the immense time scale that it encompasses, nor restrain ourselves in the interest of long-term future good.

Approaching the end of the 20th century, the idea of humanity as an evolved species and its implications are hardly accepted let alone considered as necessary. A stable or content culture does not easily allow major changes, especially if they do not recognise the importance of those ideas. If the idea threatens an entrenched status quo within a society, it is accepted very slowly. Change is not deemed necessary. Evolution appears like unimportant history, irrelevant to the modern technological and industrial world; yet today we are still involved in the very same evolutionary process! If scientists and society had accorded the idea of evolution greater emphasis, perhaps our technological advance would not have been so destructive. Greater emphasis would have been placed upon the emerging ecological science due to respect for the nature of which we are a part. This would have led to a more rapid development of the concept of holism. Less emphasis would have been placed on material wealth, establishing a competitive advantage and war and more on education and social evolution. It is an unacknowledged Christian shame that so much destruction of nature and human cultures has taken place in the name of God and Jesus or within Christian societies. Missionaries forged the destruction of cultures such as that of the Eskimo (or Inuit). Traders and missionaries systematically eroded Eskimo traditions and converted the people to a lifestyle better suited to Western needs (Singer & Woodhead, 1988) and the Western economy.

There is a contradiction in the above paragraph. In a world that has seen so much change in the past two hundred years, how can people see change as unnecessary? We have scorned the need for mother nature in a moment of arrogance, seeing in progress, only the scientific reality. Seeing only the light of the day, we have forgotten that the stars of the night are also real. Why do we so readily plough up a natural wilderness and accept the latest technological toy? The answer seems to lie in the social paradigm that science is the only source of knowledge. If they see science as infallible, how can one question progress that is based on it? Tracing back we find the unquestioned support for the progress of science and reason from logical positivism of the 1920's to Bacon 's "The Advancement of Learning" in 1605. Bacon set the basis for modern science, engineering and technology  with his ideas that only the inductive method of investigation could be used to acquire knowledge. After Bacon, various philosophic outlooks dominated: rationalism in the 1600's, empiricism in the early 1700's, and the age of reason until the late 1700's.

E. BEHAVIOUR AND MORALITY:

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A conflict between religion and evolution arises only with the insistence upon literal interpretations. God created Adam, the first man from mud. We can comprehend this within the evolutionary context. If we were to watch Adam's creation today, we would not see him delivered to earth fully formed and life breathed into him - it would be an evolutionary process. Does this mean a denial or demeaning of God the Creator? No, we are subject to time, while God is independent of time. A Muslim academic, Tahir-ul-Qadri describes clearly the creation of Adam, the beginning of the human race. Created from the earth, God first completed the physical and biological constitution of humanity. Only after this, did God breathe his Spirit into Adam. There are thus two aspects to the human personality, the physical constitution and the spiritual constitution. Our "externality" is the physical form that has evolved, our "internality" is the Spirit, Soul, Light or Noor that God placed in humanity after his physical formation was complete. (In this context, Father Richard P. McBrien of the University of Notre Dame observes that "No Scripture scholar today would say we are literally descended from two people." (Collins, 1996))

Social or cultural forces appear to have directed some of the evolution of humanity. Campbell (1984) noted that:

"Man's emergence was precipitated by the development of a breeding system that transferred an overwhelming influence over survival to the hands of members of his own species. Deliberate parental breeding and social behaviour replaced the external environment as the main determinants of fitness of phenotypic (expressed) traits. Conscious behaviour became the driver for primitive man's evolution" (Campbell, 1984).

This type of reasoning has led many to claim that humanity is no longer subject to natural selection. This is not so. It may be true that we have managed to remove some aspects of natural selection through our cultural evolution, but we are still subject to natural selection in other areas. A white-ant individual living within the mud home that it has created as a colonial animal is similarly subject to different selective factors to an ant species that is more exposed and solitary in nature. Similarly, humanity as a highly social species in a technological system, is subject to new selective forces.

A measure of birth weight and survival of children at a London hospital showed that 8 pounds was the optimal birth weight. Children above and below this weight at birth showed increased mortality in the first month. Total mortality was 4.1% while the mortality of the 8-pound class was only 1.2%! Geneticists call this difference selection pressure or intensity favouring an optimum birth weight (Strichberger, 1985).

Some social or cultural behaviour found in early human societies must have enhanced survival and thus been perpetuated. Humanity began to be shaped by an interaction of cultural change and biological change. This increased the rate of evolution, leading to trends such as increased brain volume. People transmit traditions through language as myths that educate and guide the next generation. Culture has to be learned. Today, some of these myths are perhaps refined as our moral law and reflect divine law such as the Quranic "just measure" imparted to humanity to guide him throughout time! Others myths possessed scientific 'wisdom'. As an example, scientists recognise the importance of maintaining genetic diversity in cultivated crops in the hope the other strains and especially the ancestral or wild types may possess genes resistant to diseases that afflict modern strains. Through the selection of highly productive modern crops, many beneficial genes are often lost, so primitive varieties serve as reservoirs of potentially useful (beneficial) genes. South American Indians in some localities preserved a primitive "rogue" or "freak" variety of corn, called pod corn, in the belief that it has magical properties (Mangelsdorf, 1958). In these plants, unlike modern corn, a pod of chaff encloses each seed of corn. In preserving this, through superstitious beliefs and myth, they are preserving ancient genetic diversity!

However this simplistic speculation does not prove that human ethics is nothing but an evolutionary accident specific to our species. Creation being by God, the Divine Law (Revelation) is according to his Will. Prophets from God, such as Moses, Jesus and Muhammad (SAW), by defining God's Intentions and Law, have undeniably had great influence upon the behaviour and culture of societies over the last 20,000 to 40,000 years. Records of traditions and religious artefacts do not extend much earlier than this. These great leaders left their influence upon our societies, culture and behaviour and all claimed that the Creator guided them.

"Primordial man's first manipulations of his reproduction must have been rudimentary, just sufficient to develop a more incisive ability to tamper further. They probably corresponded to a specialised form of sexual or social behaviour"
- a form of autodomestication (Campbell, 1984).

Leakey (1993) suggested the same when he said,
"the emergence of fully modern humans may have been accelerated through the effects of culture" - that "through culture, humans effectively domesticated themselves" - which "leads to rapid evolutionary change."

Ingold (1993) claims that the modern consequence of culture is that the abilities, knowledge and equipment that enable us to live one kind of life are not natural or biological, but cultural. They are the outcome of history instead of evolutionary processes and found rather than innate.

Yet, they are evolutionary processes for only successful traditions persist: "Tradition is a change that has succeeded" (Lacroix et al., 1994). We are very much a culturally determined species.

"Our self is real and it is given to us by the whole history of language, which means by the whole history of culture and history (including religion). We are defined by not less than the entire culture" (Appleyard, 1992).

Our niche includes interactions as social animals and the associated culture that prevails. Instead of insisting on the instantaneous creation of humanity, we should ponder with wonder our evolution.

F. EARLY PRIMATE EVOLUTION :

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An important term used in evolution, and biogeography that we need to understand is  adaptive radiation  (see books). (Biogeography is the study of the geographical distribution of organisms and requires a study of ecological and historical processes. Forster, as an early biogeographer, accompanied Cook on his second voyage around the globe and then published his "Observations made during a voyage around the World" (1778) (Bowler, 1992)). Evolution leads to the formation of new species, a process called speciation.

Avers (1989) defines adaptive radiation as
"the evolution from a generalised ancestral stock of diverse descendent species that comes to occupy a variety of living zones in a relatively brief interval of geologic time." (see books)

Mass extinction events and organisms' unique adaptations mark evolutionary history. Both may lead to adaptive radiation. Many such adaptive radiations often follow an animal's or plant's entry into a new adaptive zone, not inhabited by other forms of life (Myers & Giller, 1988). A unique adaptation usually precedes the entry into a new niche. Tool use by our early ancestors is one such adaptation. Once multicellular organisation became possible as atmospheric oxygen levels rose, organisms then rapidly diverged into many adaptive forms. Later unique events of adaptive radiation such as the evolution of land plants in the Phanerozoic followed. Nature's creation of adaptive innovations seems to be a contradiction. An adaptation must be to prevailing conditions, while an innovation is something new, not necessarily related to the environment. A Creationist will argue that as these innovations become the new adaptations, it is not natural selection, but God's Hand that is introducing any change that occurs. In biology, the creation of innovations is based on genetics. For each useful innovation there are many useless innovations that natural selection rapidly eliminates. I list sources of genetic variation in the section on genetics.

Mass extinctions redirect the evolutionary process in unpredictable ways, as randomly as the roll of a die. The reduction in species opens the way for the emergence of new life forms. During the 530 million years of multicellular life, there have been at least five major and many minor mass extinctions (Gould, 1994). Gould (1994) suggested that mammals and subsequently humanity may not have become dominant on earth had it not been for the global catastrophe that led to the extinction of the dinosaurs, pterosaurs and large marine reptiles at the end of the Cretaceous period 65 million years ago (Whitfield, 1993). Mammals and dinosaurs coexisted for 100 million years. Mammals remained rat-sized or smaller during the dominance of the dinosaurs. Early mammals of 50 million years ago, were probably similar to modern tree shrews  (Chiras, 1994). With the extinction of the dominant dinosaurs, mammals rapidly evolved to fill many ecological niches.

A classic example of adaptive radiation is that of Darwin's finches in the Galapagos Archipelago and on Cocos Island. An ancestral finch species arrived on these islands about five million years ago and is today represented by 14 species that evolved through adaptive radiation. This ancestor still lives in Panama in a relatively unchanged form (Avers, 1989)! Adaptive radiation has allowed Darwin's finches to occupy many different niches, using many different food-types and living in many habitats. Ecologists divide them broadly into insect eaters and seed eaters and they occupy either trees, cacti or the ground. Some species consist of subspecies, so this process of speciation is continuing even today (Begon et al., 1990).

Living orders of placental mammals, be it bats, humanity, whales or camels, have as their common ancestor a small insectivore creature that went through a major phase of adaptive radiation during the Early Cenozoic Era (Avers, 1989). We humans are first vertebrates, then mammals, then primates. As a primate we belong to a certain order of mammals having features such as an enlarged brain in proportion to body size and a well developed visual system. A fossil called Purgatorius that lived in Africa 60 million years ago is the oldest primate representative (Groves, 1994).

Anthropoids :

Prosimian primates of the Early Cenozoic (Palaeocene Epoch) were the earliest of the mammal groups that lead to monkeys and Homo sapiens. Grasping hands and feet as adaptive innovations distinguish them from other primitive mammals. These primitive mammals had tactile hairs, movable ears and their sense of smell was dominant. Most prosimians are now extinct, but lemuroids, lorisoids and tarsiers still exist. With its long geological isolation, many prosimian species survive in Madagascar. One may say that the primate niche arose because of a locomotor adaptation (Washburn, 1950).

Around 40 to 50 million years ago (four to five pages ago: Eocene or early Oligocene) anthropoid primates (Suborder Anthropoidae) diverged from the prosimian lineage (Suborder Prosimii). This radiation as anthropoid primates represented a reorganisation of the special senses enabling these creatures to be successful during the day. Monkeys developed reduced external ears and ear muscles, a reduced sense of smell and stereoscopic colour vision (Washburn, 1950). Associated with this were changes in the brain, such as the expansion of the visual and auditory areas of the cerebrum. New World monkeys (Infraorder Platyrrhine: E.g.. Marmoset) and Old World monkeys (Infraorder Catarrhine: e.g.. Colobids) diverged about 35 to 40 million years ago (Avers, 1989) (Groves, 1994). Catarrhines are the lineage that led to Old World Monkeys, apes and humans. An ancient new world monkey Branisella lived about 26 million years ago (Groves 1994).

Apes and our hominoid ancestor diverged from the Old World monkeys (Cercopithecoidae) no earlier than 31 to 32 million years ago (three pages ago). Hominoid primates include humans (Hominidae), the great apes (Pongidae) and the lesser apes (Hylobatidae) as the superfamily Hominoidae (Andrews and Stringer, 1993) (Burenhult, 1994). There is scant fossil evidence between the mid-Oligocene, 31 Mya and the early Miocene, 22 Mya. By 22 million years ago, apes had diversified
throughout the Old World, as many a 40 genera of fossil apes have been identified. Hominoid fossils appear in the early Miocene about 20 million years ago, with about 14 genera present in Africa between 22 and 17 million years ago (Begun 2003).

Lesser apes such as gibbons and siamangs (Family Hylobatidae) split from the line that led to humans about 20 million years ago. A tailless apelike creature evolved from early monkeys about 15 to 20 million years ago (Chiras, 1994). These were primitive forest-dwelling apes, found in Africa ( Kenyapithecus ) (dry woodland forests (Wynn and Retallack, 2001)) and Europe ( Dryopithecus ) (view jaw of Drypoithecus fontani ) ©1 during the middle and late Miocene. Another lineage of tailless hominoid, the ramipithecines such as Sivapithecus, lived in open woodlands in India, Pakistan and Turkey about 12 to 14 Mya ago. These were a varied group consisting of species ranging from 20kg to 70kg in weight, inhabiting woodlands and eating tough plant foods. All were extinct by 8 Mya ago, with only the orangutan surviving. Orangutans ("man of the woods", Pongo pygmaeus ) are now only found in the rain forests of Sumatra and Borneo. Some researchers put the split between the orang line and the pongids at between 12 and 10 million years ago (Burenhult, 1994). Scientists presume their ancestor to be similar to the ramipithecines, but no fossils leading to early pongids have yet been found.

PierolapithecusPierolapithecus catalaunicus is a 13-million-year-old fossils of new species of ape, a fruit-eater  that weighed about 35 kilograms. The species may have been the last common ancestor of humans and all great apes living today. The great apes—which later gave rise to humans and which now include orangutans, chimpanzees, and gorillas—are thought to have diverged from the lesser apes about 11 to 16 million years ago. Today's lesser apes include the gibbons. There are several contenders for the common ancestor, including Kenyapithecus and Equatorius or the older Morotopithecus and Afropithecus.  But existing fossils indicate that these ape species were more primitive than Pierolapithecusskull of Pierolapithecus. This is the first appearance in the fossil record of the modern apelike thorax - a wide, flat ribcage, shoulder blades that lie along the back (as in modern great apes and humans), and a stiff lower spine.

Early to mid-Miocene Africa was a wetter climate than today (Coppens 1999). Equatorial tropical rainforests were massive and extended across unbroken lowlands from the Atlantic to the Indian oceans. Eight million years ago, tectonic forces began the formation of the east African rift valley. Mountains formed on the west side of the rift and created a rain shadow over east Africa by preventing the easterly flow of rain clouds. As well as this tectonic induced climate change, there was a simultaneous global cooling and drying trend (Potts 1998; Cane & Molnar 2001). East Africa became significantly drier. The common ancestors of modern chimpanzees and humans was split into two geographically separate populations by these geological events. The ape populations of the tropical rain forests of west Africa evolved into modern chimpanzee species. On the increasingly open and dry habitats of east and north central Africa the apes evolved into early ancestors of modern humans. Evolutionary adaptation to a changing climate followed by  and adaptive radiation in east African hominins led to the emergence of modern humans.

Our hominoid line (humans, gorillas, chimpanzees and lesser apes) is one amongst the superfamily Hominoidae. Extinct Hominoidae lineages including Afropithecus, Dryopithecus, Kenyapithecus and Oreopithecus all existed before our hominid (see hominid books) line split from the great apes (chimpanzees, orangutans and gorillas). Hominids diverged from the pongids no more than seven to nine million years ago (half a page ago) (Avers, 1989). Possibly, our common ancestor with chimpanzees and gorillas inhabited Africa between five and seven million years ago (Dawkins, 1993) (Burenhult, 1994). The term "hominid" refers to creatures characterised by an upright posture and biped locomotion. Humans (Hominidae) and African apes (Pongidae) differ dramatically in appearance, yet our genetic relationship is so close that our structural proteins are similar. Our alpha and beta haemoglobin protein chains, fibinoproteins and cytochrome c proteins are identical (Strickberger, 1985). This would be a strange trick by God was it not for the fact of evolution. Studies show that humans share 98.4 percent of their DNA with the common and pygmy chimpanzees. A bit further removed from both us and the chimps are gorillas with a difference of about 2.3 percent (Vines, 1993). There may have been a three-way split from a ramipithecine-like creature, leading to humans, chimpanzees and gorillas.

Australopithecines (see books) are believed to be a part of the most ancient hominid lineage leading to humans. An older possible ancestor to Australopithecus sp. has been found at dates ranging from 4.4 to 5.8 million years old and called Ardipithecus ramidus (Australopithecus ramidus (White et al., 1994)). This group is now split into two species, Ardipithecus  ramidus (4.4Myr) and Ardipithecus kadabba (5.2–5.8Myr). Another candidate is the 4.2 million year old, bipedalAustralopithecus anamensis (3.9–4.1Myr) found near Lake Turkana in Kenya (Lemonick & Dorfman, 1999). In October 2009, in a special issue of Science, a multidisciplinary international team presented Ardipithecus ramidus from Aramis, Ethiopia as the oldest known skeleton of a potential human ancestor.

Other ancient hominid fossils complicating our origins include, Kenyanthropus platyops (3.5 Myr) ( National Geographic image ), Orrorin tugenensis (6Myr) and Sahelanthropus tchadensis (6 to 7 Myr). Sahelanthropus tchadensis (Toumaï) is unusual and may redfine human origins. Its face is "tall" with a massive brow ridge, while the mid-face is short (in the superoinferior dimension), being less prognathic than either Pan or Australopithecus (Brunet, et al. 2002).

Our human lineage, the genus Homo,  are classified as hominines. Early hominid fossils are up to five million years old. A hominid includes modern humans and our evolutionary ancestors back to the point where we diverged from the line leading to the living apes. Pongids are living apes and their ancestors. Hominid evolution has been established from several African sites such as Laetoli (3.75-3.5mya), Hadar (3.3-2.9 Mya), Omo (1-3.6 Mya), Koobi Fora (0.7-4 Mya), West Turkana (1-4 Mya), Olduvai (0.6-2.2 Mya), Sterkfontein (1.5-3.1 Mya) and Swartkrans (0.95-1.85 Mya) (Aiello & Dean, 1990). Other important sites are Taung, Kromdraai, and Makapansgat (3-2.5 Mya). Two newer sites are Gladysvale and Drimolen (Tobias, 1995).

In primate evolution, lemurs, monkeys, apes and humans represent a series of adaptive radiations. Each is less variable in form and represents some innovation added to the previous successful radiation. Lemurs show a variety of locomotor, dietary and dental patterns, while humanity is very uniform in all aspects, but is anatomically, highly specialised compared with the other primates. Our method of locomotion is extreme and odd, our foot and pelvis unique, and our brain is a recent and unparalleled adaptation. Without intelligence or tools we would be feeble creatures. Yet, we retain many primitive features. From the lemuroid radiation we retain the primitive grasping adaptation, with long digits and nails. Monkey radiation provided the complex of head, brain and special senses emphasising eyesight instead of smell and hearing (Washburn, 1950). We inherit upright walking and our unique foot from the Australopithecines. This arrangement freed the hands for tool use. From the early Homo species we inherit the use of tools, fire and clothes. At sometime language also emerged.

Diet and primate evolution:  

Based on fundamental research on the influence of diet on primate evolution, scientists propose that adaptations to a specific diet in our ancestors paved the way for the development of modern humans. Human dentition, with reduced canines and incisors and large molars and premolars reflect dietary modifications. Chimpanzees, as our closest living relatives, have long canines, so in some way our evolutionary histories are very different. Our staple diet could have been tough seeds, nuts or roots. Largely, then, we are truly what we eat (Milton, 1993).  It appears that technological and social skills and increases in brain volume, representing increased intelligence, evolved in response to a change in food sources.

The proposal that "many characteristics of modern primates, including our own species, derives from an early ancestor's practise of taking most of its food from the tropical canopy"  is founded soundly upon research into primate diets by Milton (1993). This places humanity's origins in tropical forests. I will briefly detail the reasoning behind this speculation.

Different primate species occupy different ecological niches and thus different "dietary niches". Adaptations can be structural (physical, morphological) and behavioural. These two dietary specialisations allow a primate to utilise different components of the tropical forest as food. Living examples of these two extremes of adaptation are the howler and black-handed spider monkeys. These animals are of similar size and totally arboreal (tree-living). Howler monkeys feed mostly on immature leaves, while the spider monkey eats mostly ripe fruit. When fruit is abundant howlers spend an equal amount of time eating fruits and leaves, but when fruit is scarce, eat only leaves. Spider monkeys eat only a small amount of leaves and during periods of fruit scarcity, eat more leaves but also seek out all the fruit sources in the forest.

Externally, howler and spider monkeys are alike, but their digestive systems differ. Food passes through the spider monkeys in about four hours after eating, while in howler monkeys the digestive process takes about 20 hours. Anatomically, the digestive systems differ, this accounting for the different digestive processes required to handle the different diets of the two species. Howler monkeys have wider and longer colons than spider monkeys, so food has a longer passage for digestion. Howlers can hold more bulk in the larger colon. Their digestive system has some features typical of herbivores that eat fibrous material. Howlers are adapted to the slow fermentation of fibrous leaves in the caecum and colon, allowing efficient digestion of this food source. The bacterial fermentation of fibrous leaves releases volatile fatty acids providing 31% of daily energy requirements of howler monkeys. This is a morphological (and physiological) adaptation to diet.

Spider monkeys are less efficient at extracting energy from a fibrous diet. In other words they are not adapted to a fibrous diet. Their solution to diet is based on behavioural adaptations. They are adapted to highly digestible, energy-rich, fruit diets. As fruit is low in protein, they supplement their fruit diet with select young low-fibre leaves that provide the required protein.

The two diets have EVOLUTIONARY CONSEQUENCES. A spider monkey has to know which fruits are available, or edible, where and when. It needs to recognise fruit bearing trees, remember their locations and seasons of production. Their temporal and spatial orientation has to be strong, a feature that proves important for the evolution of tool use. Search for fruit is done in small changeable groups. Spider monkeys would more easily adapt to changing environmental conditions by changing their diet. In turn, the need to search out high energy foods over a wide and diverse forest range places more demands on "intelligence" and the spider monkey brain weighs about twice that of howlers.

As most aspects of the howler and spider monkeys' habitat are similar, THE DIFFERENCES IN BRAIN SIZE ARE ATTRIBUTED TO THEIR DIETARY DIFFERENCES AND THE ASSOCIATED MENTAL SKILLS REQUIRED. Natural selection for brain size is thus associated with dietary adaptations. Natural selection is channelling the monkeys along two different evolutionary paths. The emergence of variations has a context, in that they have to relate to the adaptations that currently benefit the species. A spider monkey that had a mutation giving it a behavioural preference for leaf eating would not have the digestive system to match this behaviour. Throughout human evolution, we have concentrated on using high energy resources, a factor that has contributed to our large brains.

Having shown the importance of diet, I must then note that a part of human radiation is an adaptation to life on the ground and no group of New World monkeys has taken up life on the ground. This is due to the soft leaves and fruits of tropical forests not being available in plains country. Monkeys adapted to this diet have not become ground-livers, whereas baboons, patas, monkeys, vervets and macaques (Old World monkeys) have (Washburn, 1950). We can find some differences between ground-living and tree-living Old World monkeys. Ground living forms need to range farther to find food and survive and so form fewer varieties than tree-living forms. For our human ancestor survival as a ground living form required that the creature become wide ranging. This reduced the likelihood of geographical isolation and many distinct varieties or species forming. Ground-living is a fundamental and very ancient adaptation on the evolutionary line leading to humanity, as the oldest hominid (see hominid books) fossils yet discovered walked upright. These creatures, the Australopithecines, had brain volumes similar to chimpanzees and were no more than small brained wild animals, being regular prey for predators such as leopards.

A similar trend in the relationship between home range and diversification is found in the lion. Panthera leo was once found from the Southern tip of Africa, in the Cape, through to large parts of India, the Middle East and south-eastern Europe, yet formed a single species with varieties (subspecies) that interbred. Our modern lion evolved over a period of 600,000 years from the massive European cave lions, Panthera leo spelaea. This cave lion was 25% larger and heavier than living African lions (Brakefield, 1993). By 300,000 years ago, the cave lion and modern lion were almost indistinguishable. Lion skeletons from Siberia, China and Europe might be of the larger subspecies. As recently as 300 B.C., Panthera leo spelaea may have flourished in Yugoslavia and Asia Minor. In North America, until about 11,580 years ago, a separate species, Panthera atrox existed. Some say that this is another subspecies of P. leo. Over the lion's range, significant variation is found in various features, such as pelage colour, size, colour of manes and the shape of the skulls. This is similar to the variation between human races. It can only be the wide ranging and interbreeding that could have kept the lion and human as single species.

Return to: Human evolution index page. Goto to next (3nd) chapter: Australopithecus .

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by Laurence Evans 1998 - 2008

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