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Photo of Hilary Deacon The First People

Extraordinary archaeological discoveries are helping piece together the evolution of human life in sub-Saharan Africa. Explore the past on this story with Hilary Deacon, an expert on African archaeology and the emergence of modern humans. Hilary answered viewers' questions as part of an Ask the Scientists panel following the premier of the Scientific American Frontiers Special Science Safari. Here are viewers' questions and his answers:

Q Are there similarities between the art of primitive people in Africa and the art of primitive people in other parts of the world?

We use the word primitive rather loosely in every day speech and need to keep in mind that all people in the world, no matter what the differences in their life styles, are very similar in their genes and in their minds. The wiring of the central nervous system, the brain, is the same. This is a consequence of a shared ancestry in the last 200,000 years which on geological time scales is recent. All modern people and their direct ancestors inherited the same mental senses and none would be truly primitive. Art traditions may differ in detail between different parts of the world but there is an essential unity in how images are visualized.

This can be clearly seen in the rock art, the paintings and engravings done on rock outcrops and in caves. Some of these images are very ancient, 10s of 1000s of years old, yet they have elements in common where ever found. David Lewis-Williams who discussed rock art on the program has explained the universal similarities in certain types of images found in the rock art in Africa, the Americas and Europe by the trance hypothesis. The art was made by shamans and in an altered state of consciousness, induced during trance, images are experienced (as much as seen) and are later depicted in the art. When someone gets a knock on the head in a comic book illustration, stars and wiggly lines are shown in the bubble. This is trying to show what happens during an altered state of consciousness and the stars and lines represent the kinds of bright flickering shapes, known as entoptic phenomena, that are seen in the first stage of hallucination. All people have the same experience because of the functioning of the brain. At deeper levels of trance the images may be transformed from memory into culturally significant and powerful forms that will differ from culture to culture and not be universal.

A further point to remember is that much of the traditional (rather than primitive) art is essentially religious in character. In motivation it has to do with healing the sick, communicating with ancestors and warding off evil and natural disasters. This in turn gives art a degree of universality. Your question points to an interesting field of study comparing similarities and differences in traditional art forms in different geographical areas.

Q Why do you think it is important to study human evolution?

There is a universal curiosity about where we as humans came from and how we got to be like we are. This curiosity is real because at an individual level we need to know and draw strength from knowing who are our parents, grandparents and other relatives. It is part of belonging to a group. The study of human evolution is an extension of this to more remote ancestors. So at a very general level it is important to study human evolution to understand ourselves.

I would like to emphasize the importance of curiosity as the basis of all human learning and progress. Curiosity about how all living things are related and why living things show such variety has led to the development of the theory of evolution. The essential concept is modification with descent through natural selection and is a powerful explanation for the unity and diversity of life forms. The study of human evolution is the special field that explores how natural selection has worked to make humans part of this diversity. Humans are unusual in their two legged striding movement, their large brains, their use of tools (anything from a knife to an aeroplane), their talking and many more traits. Humans have also become very successful in moving out of the tropics, expanding to all corners of the globe and beyond and becoming very numerous. The human story is a long one going back some six million years to the last common ancestor with our nearest relatives in the animal kingdom the African great apes and this story is a continuing one because we are part of that evolution. It is a very exciting story and one in which we can be justifiably curious.
I'm assuming that you've seen the math formula illustrated in the Scientific American Frontiers teaching guide for Science Safari. In order to standardize the points of measurement so that my 6th graders can perform these calculations, at what points on the humerus (top and bottom) and the radius (top and bottom) do you suggest measurements be taken? Since precision is necessary, I'd like your opinion on this. Is this formula standard for humans of all ages? Would it be the same for young middle schoolers who are undergoing rapid growth and development? Also, from an evolutionary stand point, is this formula constant for all species of Homo, or just for Homo sapiens?

The following are the questions from the teaching guide: (a) In calculating the height of a person from measuring the upper arm bones as set out in Activity 1, what reference points can be used to get reasonably precise measurements for the humerus and the radius? (b) Would the formula be standard for people of all ages including individuals undergoing rapid growth and development? (c) Is the formula constant for all species of Homo or just Homo sapiens?

These are three interesting questions. To reply in general, height or better stature is influenced by age, sex and the ethnic population. Populations adapted to cold regions like northern Canada tend to have shorter appendages (arms and legs) than populations living in the tropical savannas of Africa where body heat needs to be lost rather than conserved. Different formulas would be needed for such populations. In the activity 1 sheet different formulas are given for males and females so the sex of the individual is another variable. Age is a further variable because the limb proportions are only stabilized in the adult skeleton. Hormones control growth and the rate at which the skeleton matures. It is the onset of the hormonal spurt at the time of puberty (which again may differ among populations and be affected by nutritional history) that is responsible for the sexual differences in the skeleton.

To be more specific to the (a) question. Stature can be calculated from the length of the major bones of either the upper and lower limbs with the latter giving the more reliable estimates. The measurements are the total length of the shaft bone, known as the diaphyseal length. Without an X-ray the measurement is going to be less precise. The head of the humerus is a ball that fits into the socket of the shoulder blade, the scapula. The bone you can feel at the end of the shoulder is part of the shoulder blade (called the acromion) below which is the socket for the humerus. Rotating the arm flexes a muscle and holding the hand on this muscle you can almost feel the head of the humerus. I suggest you take this as the top reference point. The lower or distal point of the humerus at the elbow can be approximated as below the flaired part of the humerus. In a sitting position, rest right hand on the knee palm up. With the left hand using the third finger and thumb the parts of the humerus making up the flair, the lateral and medial condyles, can be felt as jutting out bone. It is below this that two bones of the lower humerus known as the capitum and trochlea form the surface against which the bones of the lower arm move. Moving the third finger down and rotating the lower arm makes it possible to feel the radial head of the radius. This is the best I can suggest for obtaining a reference point for the distal end of the humerus and the proximal end of the radius. The bottom or distal end of the radius is at the wrist at the beginning of the thumb. It is possible to feel where the radius ends by moving the hand at the wrist. Hope this helps and I hope it encourages you and your class to find out more about the anatomy of the arm.

There are some good text books on human anatomy that you could consult: two examples are T.D. White (1991) Human osteology. Academic Press, San Diego, California and P. Shipman, A. Walker and D. Bichell (1985) The human skeleton. Harvard Univ Press, Cambridge, Mass.

In the study of fossil remains and in the remains of victims of natural and accident disasters there are practical applications for the formula.

(b) From what has been said above the formula will be more precise when applied to an adult.

(c) The formula, based on the study of a large number of individuals, is a best approximation. There are tables that can be consulted for more precise estimates for some genetically different ethnic populations. We only have good data for some modern populations and these data have to be used to supplement the very limited data for prehistoric populations in earlier time ranges. Limb proportions have changed in the course of human evolution. The major change was related to the appearance of full bipedalism, the striding gait, about 1.8 million years ago. The earliest forms of Homo seem to have retained the ancestral long upper limbs possibly because they were more arboreal.

Michael DiSpezio, who originally created this activity, replies: As you have uncovered, the exact measurement ratio will depend upon many factors including age, nutrition, and genetic predisposition. With that in mind, the ratios offer at best an approximate mathematical relationship between lengths in question. Still, the validity of approximate bone length relationships is an important foundation to anthropological investigation. It offers a basis from which relative size can be inferred.

When measuring bone lengths, I would stress consistency in the measurement operation. If lengths are taken from the top of wrist bone -be consistent. If lengths are taken from the bottom of the elbow - be consistent. The deviation that students uncover can be explored in terms of "best fit" for the given equation. Challenge students to determine which measurement scheme produced data that generated the most accurate height estimates. Classroom experience can also be used to establish a ratio that truly represents the uniqueness of each class's particular age and socioeconomic composition.

Q How do scientists know where to dig for the bones?

The scientist follows up clues. There are chance discoveries, someone recognizes a bone uncovered during quarrying, road building, ploughing or any such activity and reports it. The investigation of such chance finds can lead to uncovering of further finds. Again from experience the scientist may be able to predict that bones may be preserved in particular layers and places and consciously search for them.

The bones of only a small number of animals that die are preserved as fossils in the ground. It takes unusual circumstances for bone to be preserved. Lying on the surface bone cracks, breaks into splinters and eventually disintegrates in a matter of a few years. Bone will preserve if buried so there need to be conditions that will effect burial. Under natural conditions it is usually running water, a river overflowing its banks or a stream that may bury things like bones. Animals and even people may bury bones. In ground that is acid bone will not survive and the chances for preservation are much higher in ground that is alkaline, that is rich in lime. The mineral matter that gives bone strength, calcium phosphate, is rapidly lost through solution in acid ground.

Recently we had an example of a chance discovery of animal bones at a quarry near Saldanha Bay, on the coast north of Cape Town (South Africa). The quarry is producing stone for making roads and in clearing the overburden of lime cemented sands several bones were found and reported to a local museum. The bones were obviously old because they were partly mineralized, they had taken up lime from the sands. The next step was to visit the quarry and find out why the bones occurred there. It turned out that the bones were concentrated in patches. This was an important clue as to how the bones got there and why they were buried. Hyaenas make their dens in the ground and bring back food for their pups, bits and pieces of animals they have scavenged. In this way patches of bones mark their dens. James Brink who is an archeozoologist, an archaeologist who has specialized in the study of animal remains, excavated some of the burrows and has identified the animals that were collected by the hyaenas. There is a wide variety of animals, different kinds of antelope, represented by different body parts as you would expect to find at the den of a scavenger. The kinds of animals suggest a date for the bone accumulations and say something about the environment. In this case the animals are not those found there today but ones that lived in the area perhaps 80,000 years ago. It was chance that the finds were made but the scientist has to follow up the report and use his or her knowledge to explain why the bones occur there. In this case the agent of the accumulation of the bones was the hyaena but in different circumstances it can be other animals (porcupines, leopards, eagles), humans or nature through running water.

There are scientists who carry out surveys of areas. One of the best examples are the surveys that have been carried out in parts of northern Kenya and Ethiopia in areas of eroded old lake and river beds of the rift valley. The sediments are alkaline so they preserve bone and by conscious search it is possible to find concentrations of bone partly exposed by natural erosion and uncover further specimens by excavation. Some of those localities are scientifically very important because they have yielded finds of the early human ancestors known apart from the bones of many extinct types of animals.

Where preserved, fossil bones are important information about the past. One needs knowledge of anatomy to identify bone and field experience to explain why the bone occurs where it does. The scientist will want to establish not only what animal or animals are represented and in what numbers but also when the animals lived and why they died. The scientists who study bones are called paleontologists and there are many special fields of study within paleontology - for example some study the bones of dinosaurs that lived 100 million years ago and others the bones of mammals that lived in the last few million years and more recently.

Q What made you choose to be an archaeologist in Africa?

I was born and grew up in South Africa and have lived in East and West Africa for periods of my life so through circumstance my geographical interest is very much on Africa. My interests have been in earth history and I find it fascinating to be able to turn back the clock and find out how things were in the past. In the old landscapes of Africa often lying on the surface are Stone Age artifacts, flaked stones made and used by people before metals became available. These are almost permanent markers of where people lived in the past, sometimes many thousands of years ago. I got hooked on trying to piece together the past, how environments have changed and how the lifeways of people have changed. This is how my interests, particularly the Stone Age archaeology and ecology of Africa developed. As the continent of Africa has been the central stage for much of the development of humankind it has been a challenge and privilege to follow a career in archaeology.

Programs like this give you exposure to subjects like archaeology which fall outside the normal class room education and you can develop your interests further through visits to museums and the like.

Q What is the most stunning discovery you have made in your study of archaeology?

I suppose most discoveries made by archaeologists are pieces of the jig-saw puzzle rather than earth shattering finds. Archaeology is about things people forgot or left behind. That's what we tend to find but it is those smaller things, pieces of the jig-saw, that build up and eventually tell a story, something new and interesting about people not recorded in history. So archaeology is less about the individual discoveries and more about how these add up to increasing our understanding. This may make archaeology sound a lot duller than treasure hunting but it really is not. There is a lot of routine but also excitement.

If I have to say what is the most stunning discovery I have made it would be finding the oldest known remains of modern people in the lowest layers at Klasies River. This has implications for origins of all living people and as such would rate as stunning. A discovery that has given me much personal satisfaction is research that led to showing how the Stone Age ancestors of the San (Bushmen) collected and used different species of plants and made it possible to get a good, even stunning, insight into their lifeways.

Q What are the key differences between archaic humans and modern humans?

This is an interesting question which I can only answer in very general terms. Archaic humans tend to be more robust in their anatomy, the limb bones are thicker in section. The vault of the braincase would be low and the facial skeleton very broad relative to modern humans. The absence of a chin is an archaic character. You can think of modern humans as the most gracile (from graceful) members of the genus Homo.

Q What is red ochre and where is it found in the world?

Red ochre is a natural pigment or coloring material. It is formed by the weathering of rocks containing iron minerals. Iron is a fairly common element in the crust of the earth and under intense chemical weathering such as now found in the tropics the iron is released to form iron oxides. Haematite, iron combined with oxygen and water, is one of the best known minerals that may be an important constituent of ochre. Minerals like haematite have the red color of rusty iron (also an iron oxide) and are soft so they can be easily powdered.

Ochre is found in many parts of the world where older deeply weathered rocks occur. There are good examples of ochre mines in Australia which were worked in precolonial times and ochre was widely traded. Ochre is used among traditional societies throughout Africa. Red is a power color and red ochre has symbolic meaning. It usually signifies life or the living because blood is red. Thus it used in "rites of passage" like initiation.

Q How do the rock paintings created with red ochre stay on the rocks so long?

Rock paintings do survive well in protected places but they do not last indefinitely. The famous Upper Paleolithic rock paintings in caves in France and elsewhere have been dated to between 32,000 and about 12,000 years. Among the oldest is the recent major discovery at Chauvet Cave and it has been possible to radiocarbon date pieces of paint from the wall. Deep underground such sites are naturally protected in a constant environment but if opened to visitors they rapidly deteriorate.

There are some 15,000 known rock paintings in southern Africa. These like those in Australia are in much more exposed positions and while the art tradition is as old or older than in Europe, most of the visible art is from 100 to 5000 years old. Rock surfaces in exposed positions flake off or the paintings fade. Unfortunately people also damage accessible paintings by scratching and touching them. This is a conservation problem and reason to make copies of the art by tracings and photographs. In northern Australia where the tradition of painting still survives paintings have been periodically repainted illustrating that the paintings do degenerate.

The paint, mineral coloring like red or yellow ochre in an organic medium, binds well to the rock. However with the decay of any organic constituent only a mineral stain will remain. The better preserved the paintings the more detail that can be seen. It is remarkable that we have rich galleries of rock art in various parts of the world and they are something to treasure.

Q How can you tell the difference between human fossils and animal fossils, when the skeleton bones are scattered around in different places?

The skeletons of all animals reflect how they move and how they live. The bony parts support the muscles and tissues that are used in moving and the bones are shaped accordingly. Although all mammals have a similar basic skeletal plan each species is built differently. The differences may be easily seen in distantly related species like zebras and pigs but it is more difficult to tell two closely related species apart like for example the mountain zebra and the plains zebra. Human anatomy is very distinctive because humans are very specialized in the way they move, walking on two legs. The large brain is cased in the thick bones of the skull and is again distinctive.

It is possible to identify a collection of bones from a layer which could represent the remains from the camp of prehistoric hunters, a carnivore lair or a natural accumulation on the form of the individual bones. This skill has to be learnt through studying skeletal anatomy. In some museums there are collections of animal skeletons of different (modern) species, usually including juveniles and males and females, that can be used for reference purposes. The reference material to aid in the identification of extinct species is the fossil material or cast of such material.

The more complete the material is the easier it is to identify. Fragments of rib or mid pieces of shaft bone may not be identifiable to species but even fragmentary tooth material may be very specific. The identification of different species in collections of bones is routine and distinguishing between animal and human bone is not problematic.

Q How can the archaeologists tell how old the human skeletons are, like those from the Klasies River?

In an archaeological site that consists of a number of layers stacked one on top of the other then the layering gives a relative age. Any finds from the lowest layer must be older than finds from the upper layers. Thus by knowing from which layer a particular fossil came it can be dated relative to any other from a different layer. There are a number of techniques that can be used date layers in an archaeological site. Usually one is dating the layer from which the fossil came and not the fossil itself. In a layer that is undisturbed the age of the fossil must be the same as that of the layer.

The best known technique is radiocarbon dating which is very precise but can only be used to date materials 40 000 years or younger. For older time ranges like at Klasies River other techniques have to be used. Some of these alternative techniques are relatively new and are still being developed and do not yet have the precision of radiocarbon. For this reason a number of different techniques are used to date the same layers and when these give corresponding results one can have confidence in the results. At 100 000 years the results may be in error by as much as 10 000 years but this is the precision we have to accept until there are further advances in the methods.

In the example of Klasies River which is very well layered a whole battery of techniques have been applied, uranium disequilibrium dating of stalagmites, oxygen isotope measurement of shell, amino acid dating of bone and ostrich egg shell, electron spin resonance dating of tooth enamel. There are ongoing studies involving the luminescence dating of sediments and burnt stone artifacts. These studies are not done by us but through collaboration with specialized laboratories in the USA and Europe.

The best estimates of the age of the two main layers yielding human fossils at Klasies River are 120 000 years and 90 000 years. Other important find sites of the remains of early modern humans like those in Israel have been dated similar methods because they are also too old to date by radiocarbon. For the very old sites of archaic humans like those in East Africa a further method Potassium Argon dating has been used as there are recent volcanic rocks associated with the relevant layers. All this shows the extent we have come to rely on science based dating in archaeology.

Q What is the latest news about the origin of man - Africa alone or also some other areas ?

The origins of humankind are clearly African because our nearest relatives in the animal kingdom are found in Africa and the oldest fossils of australopithecines (extinct near-humans) and true humans occur in Africa. This is not disputed in scientific circles although there is some divergence of opinion about the timing of the dispersal of people out of Africa. The conventional wisdom has been that this occurred about one million years ago but there are suggestions that it may have been earlier. This event, the dispersal, is sometimes called "Out of Africa 1".

There is more debate about "Out of Africa 2", the hypothesis that modern people originated in Africa, perhaps some 200 000 years ago and dispersed over the world replacing the archaic descendants of "Out of Africa 1". One of the debated points is whether the center of origin of modern people was in sub-Saharan Africa, restricted to Africa, or whether the center of origin of modern people was Africa and the adjoining continental areas. There are some important early modern fossils from the Near East, an area that is really an extension of the African continent, and this makes thinking of larger area than just Africa reasonable. The strong "Out of Africa 2" hypothesis would argue for the biogeographic center of modern human origins in sub-Saharan Africa and the so-called weak hypothesis for a wider continental area. Both the strong and weak hypotheses deserve consideration and the evidence is not in to decide between their relative merits. The real debate is between those who support an "Out of Africa" hypothesis in one form or another and those who promote the so-called "Regional Continuity" hypothesis that sees no need to invoke a second dispersal out of Africa.

The emergence of modern people is one of major areas of research interest currently. Geneticists have found that people scattered all over the world are remarkably similar in genetic make up. This suggests we are all the descendants of a recent ( few hundred thousand years) common ancestor and that modern human populations are the result of an even more recent (50 000-100 000 years) population explosion and expansion.


Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.