About one-third of the world’s land surface is arid or semi-arid. The Earth’s landmasses are losing 24 billion tons of topsoil and making 12 million hectares of the Earth useless for cultivation every year. The desertification is, however, not necessarily anthropogenic. So, first, we must elucidate what are the natural conditions of the desert. Then we can identify what are the artificial causes of desertification.
- 1: Natural Conditions of Desert
- 2: Artificial Causes of Desertification
- 3: Human Contribution to Vegetation
- 4: Related Works
- 5: References
1 : Natural Conditions of Desert
There are many natural conditions of the desert. Here I enumerate just three important causes.
1.1 : Downdraft of Hadley Cell
Deserts spread around the latitudes 30° north and south. These zones are called horse latitudes or subtropical high. These mid-latitudinal belts are very dry because Hadley Cell descends there.
The Hadley cell is a circulation that dominates the tropical atmosphere. Solar radiation hits the earth near the equator most intensively and evaporates much moisture. The air, as it climbs, condenses the moisture and produces the heavy downpours. Moving away from the equator to the mid-latitudes at high altitudes, the dry air descends towards subtropical high.
Deserts in the mid-latitudinal belts are Sahara Desert in northern Africa, Arabian Desert in the Middle East, Thar-Cholistan Desert in India and Pakistan, Mojave Desert, Sonoran Desert, Chihuahuan Desert in US and Mexico, Atacama Desert in Chile, Namib Desert, Kalahari Desert in southern Africa, Great Sandy Desert, Great Victoria Desert in Australia.
It is the solar radiation that causes the circulation. So, when it gets weak, the Hadley Cell shrinks and the subtropical high moves equatorward. So, the subtropical high has not stayed at the same latitude. For example, as I described before, after the Holocene Climatic Optimum, the region north of 35° north latitude got wet, while the region south of 35° north latitude got dry. This was because the subtropical high and humid temperate climate advanced southward.
1.2 : The Western Side of Continents
If you look at a world map of climate, you will find that there are exceptions to the latitudinal explanation. Deserts occupy only the western side of continents at the latitudes 25-35° north and south, while forests cover the eastern side. UNEP explains this difference as follows.
The gravitational drag is greatest in the equator, where the centrifugal speed of the earth is fastest. Thus, as the earth turns, ocean currents and winds flow in the equator from east to west, tugged by universal gravitation, forming the equatorial currents and the easterly trade winds. As the westbound surface waters move away from the continents, they pull cold, nutrient-rich waters to the surface that generate a cool, stable coastal atmosphere, with little evaporation from the sea and very low rainfall other than morning fogs.
Simply and symmetrically drawn, the difference between the eastern and the western side of a continent would be explained like this.
1.3 : Inlands and Rain Shadows
The areas located deep within a continent tend to become deserts. Gobi Desert in Mongolia, Taklamakan Desert in China, Kara Kum and Kyzyl Kum Deserts in Central Asia, Great Basin Desert in US, etc. are outside subtropical high belts, but still arid, because they lie far from the sea and air currents, which stem from seas and traverse vast land distances, have already lost most of the moisture they originally carried.
A region that is not far from the sea but behind a mountain and on the leeward of the moisture-laden wind tends to be arid. The region is called a rain shadow. When the moisture-laden wind climbs the mountains, the air expands and cools with the water vapor falling on the windward side or top of the mountain. Having left their moisture behind, the descending wind, compressed and hot, makes the leeward part, namely the rain shadow of the mountains, arid and desert, though the windward slopes of the same mountains are wet and covered with forest. Owing to the rain shadow effect, some region is arid in spite of tropical location. The Sechura Desert in Peru and Ecuador is an example.
2 : Artificial Causes of Desertification
The natural conditions mentioned above do not tell us why deserts are rapidly spreading all over the world, because the natural conditions remained almost the same for a long time. Although they are not negligible, modern desertification owes much to human activities. I first describe direct causes of artificial desertification, then retrace them back to the root cause and lastly work out a solution to it.
2.1 : Global Warming
Whether global warming promotes desertification or not is a matter of dispute. The rise in temperature encourages evaporation. So, it seems clear that global warming makes land drier and accounts for desertification. But the fact is not so simple.
First, since carbon dioxide is stuff for photosynthesis, the rise in atmospheric carbon dioxide concentration fertilizes plants. On average across several species and under unstressed conditions, compared to current atmospheric carbon dioxide concentrations, crop yields increase in the range of 10-20% for C3 crops and 0-10% for C4 crops at the carbon dioxide concentration of 550 ppm. Although this carbon dioxide fertilization effect does not apply to all species unconditionally, we can say plant response to elevated carbon dioxide is positive.
Second, the evaporation enhanced by global warming results in more precipitation. Precipitation, however, does not increase evenly. According to recent research, “anthropogenic forcing contributed significantly to observed increases in precipitation in the Northern Hemisphere mid-latitudes, drying in the Northern Hemisphere subtropics and tropics, and moistening in the Southern Hemisphere subtropics and deep tropics.”
Third, global warming has a positive effect on plants in cold regions. Global warming raises the temperature in high-latitude regions. The cooler a climate is, the warmer it gets. Of course, each plant has its optimum temperature and too rapid temperature rising makes adaptive vegetation shift difficult, but it is unlikely to result in desertification.
From these facts, we can judge that the impact of global warming on vegetation in mid- and high-latitude regions is positive. According to the 4th report of IPCC, however, even slight warming has a bad influence on vegetation in seasonally dry and low-latitude regions.
Modelling results for a range of sites find that, in mid- to high-latitude regions, moderate to medium local increases in temperature (1-3ºC), along with associated carbon dioxide (CO2) increase and rainfall changes, can have small beneficial impacts on crop yields. In low-latitude regions, even moderate temperature increases (1-2°C) are likely to have negative yield impacts for major cereals.
The carbon dioxide fertilization has a global positive effect, yet vegetation in subtropics and tropics suffers from the rise in temperature and drop in rainfall. Ironically advanced countries that are located in mid- to high-latitude regions and might get some temporary benefits are eager to stop global warming while developing countries that are located in low-latitude regions and are likely to suffer from immediate damage are indifferent to global warming.
Tropical rainforests have been rapidly shrinking throughout the 20th century, but global warming is not the sole or the main cause of desertification in the tropical zone. The main cause is soil erosion due to the direct human exploitation of vegetation. Precipitation in tropics decreases, to be sure, but it is the result rather than the cause of deforestation. Deforestation, especially desertification, should diminish transpiration and, therefore, local rainfall.
Now, what about the drought that has frequently occurred in northern China? Desert encroaches near Beijing and even the Yellow River often dries up. Chinese Government ascribes domestic drought and desertification to global warming. Northern China, however, is located above subtropical high and global warming should increase precipitation in mid-latitude regions. The reverse is the case in northern China. We have to ascribe desertification in China to overpopulation and the resulting over-extraction of water from aquifers and rivers.
Generally speaking, anthropogenic desertification owes much to the human direct exploitation of vegetation and consequent soil degradation. I will take up this universal artificial cause of desertification at the next division.
2.2 : Direct Exploitation
Three major direct causes of soil degradation are the following.
- Overgrazing, the exposition to grazing for too long, or without sufficient recovery periods (34.5%)
- Deforestation, the conversion of forested areas to non-forest land use without reforestation (29.5%)
- Non-sustainable agricultural practices, with too many agrochemicals, fertilizer, and irrigation (28.1%)
Overgrazing occupies the most, but we should notice that deforestation and non-sustainable agricultural practices often deteriorate the land so that it is only suitable for grazing. But we can safely say that livestock farming has a worse influence on vegetation than crop farming. Besides overgrazing, livestock farming has another cause of desertification. Tread pressure of livestock solidifies the ground so that rainfall does not percolate through the ground and evaporates quickly.
You might wonder why agriculture is a cause of soil degradation. Deforestation and overgrazing are the obvious destructions of vegetation, but agriculture seems to be a restoration of vegetation. Certainly, agriculture, if practiced properly, prevents desertification, but modern agriculture promotes it.
Modern farmers have sprayed pesticide and herbicide, which kill useful bacteria such as mycorrhizal fungi, leguminous bacteria and so on. Thanks to the mycelium of the mycorrhizal fungus plant can absorb mineral nutrients, especially phosphate ions, and water from the soil. In return, mycorrhizal fungi ingest carbohydrates produced by the plant in photosynthesis. A recent study suggests that the symbiosis between plants and mycorrhizal fungi has been in existence since plants invaded the land. Mycorrhiza is so indispensable to plants.
The modern farmers had to compensate for the loss of mycorrhiza by application of chemical fertilizer and irrigation. Excessive fertilizer induces insects and weeds to infest crops and spraying more pesticide and herbicide leads to a vicious circle. The application of chemical fertilizer and irrigation can trigger off salinization. So, modern agricultural practices can account for desertification.
Overgrazing, deforestation and non-sustainable agricultural practices make soil bare of vegetation whose roots bind the soil and whose leaves protect it from wind and sunlight, thus encouraging wind and water to erode soil. The erosion increases the proportion of fine material, reduces the percolation rate of the soil, increases the amount of runoff and encourages further erosion.
Deforestation and erosion result in salinization. Trees develop roots to trap the water. The roots often reach down deep enough to make contact with the groundwater and lower the water table. If they are cleared and annual crops are planted, the ground will be waterlogged, because their rooting is shallow and temporary.
More serious is the irrigation of arid lands. Excessive water reaches the basin under the arid land, which was once the seabed and includes salt deposits. After dissolving the salt, capillary upflow from saline water tables eventually reaches the surface layers of the soil. Because of aridity, the water evaporates, leaving the salt behind. The salinization is worse if the irrigation water itself is also saline.
Soil salinity makes it more difficult for plants to absorb water because it holds water more tightly than plants can extract a result, many plants wither for want of water. Farmers apply additional water to leach out the salinity from the plant root zone, which, in turn, causes further salinization. Here is a vicious circle. Deforestation and irrigation account for salinization and the salinization accounts for over-irrigation and further deforestation.
Salinization is reducing the world’s irrigated area by 1-2 percent every year, hitting hardest in the arid and semi-arid regions. Since the desertification narrows our living space, we must prevent it.
2.3 : The Root Cause
The larger the population becomes, the more food and fuel it demands. So, it is obvious that the modern population explosion of human species has promoted deforestation, agriculture, and overgrazing. For example, desertification is widespread in many areas of the People’s Republic of China, which cannot be explained except in terms of the population that has increased since 1949 for political reasons.
Because humans are living systems that maintain themselves by taking in low-entropy resources and throwing high-entropy into the environment, their growth means the decrease in low-entropy resources and the increase in high-entropy wastes, namely desertification for the other living systems.
Fortunately, the growth rate of the world population has declined since the 1970s, though the population itself still grows, as the figure below indicates.
This is because the worldwide inflation in the 1970s urged us to make our economy more energy saving. While the pre-modern economy was labor-intensive and the modern economy was (physical) capital-intensive, the post-modern economy is knowledge-intensive. The new frontiers of multiplication are no longer in the real world, but in the virtual information world, because the multiplication of meme is not so burdensome to the physical environment as that of a gene.
Overpopulation of human species is not desirable, but it does not mean that humans in themselves are harmful. Humans play an important role in maintaining the circulation of nutrients that prevent the land degradation. I will explain this positive side of human works at the next division.
3 : Human Contribution to Vegetation
Animals including humans are usually thought to be parasitic, depending directly or indirectly on plants’ photosynthesis. But actually, animals play important role in maintaining the ecosystem. Plants on land depend on animals’ activities.
3.1 : Necessity of Animals
In order to live on, plants must take in carbon, hydrogen, nitrogen, oxygen, potassium and phosphorus in bulk; calcium, magnesium, and sulfur in small amounts; iron, copper, zinc, manganese, boron, chlorine and molybdenum in very small amounts.
Among these elements, they can take in carbon, hydrogen, and nitrogen from air and water by themselves. As plants cannot directly take nitrogen in, some symbioses can extract nitrogen from the air and fix it in the soil. It would, however, be very difficult for plants to ingest the other nutrient elements if it were not for animals. These nutrients in the soil are gradually washed out by rain, flow through rivers and are deposited at the bottom of seas, lakes, marshes and other water areas. Without animals, the soil would soon come short of them.
Nutrients dissolved in water are ingested by microbes such as plankton, which are eaten by fish and other aquatic animals, which are in turn eaten by birds, humans and so on, whose excrement and dead bodies are reduced to the soil. The other animals that do not eat aquatic animals also contribute to plants. They eat other plants or animals and excrete or die elsewhere, thus distributing nutrients evenly.
Without animals, nutrients would be shuffled around the earth, for landmasses subside and ocean floors rise over time. But, the subsidence and the upheaval occur so rarely that plants cannot help depending on animals for usual circulation of nutrients.
The role of humans in the ecosystem also consists in retrieving nutrients from the water. Without humans, other animals such as birds, amphibians, aquatic insects and so on would fill this role. The anadromous fish like the salmon return to the streams where they were hatched and die there and thus bring back nutrients from the sea. However, the wider human influence becomes, the narrower their habitat becomes and the more responsibility of the retrieval the human race must take.
From this point of view, we can distinguish “throwaway civilization” from “bring back civilization”. Roughly speaking, the Eastern civilization that feeds on fish and other aquatic animals is the latter and brings back human excrement to the soil, while the Western civilization that feeds on cattle and other terrestrial animals and throws human excrement into the sea is the former.
The origin of the Eastern civilization is the classical Chinese Civilization located along the Yangtze River (Chang Jiang). In contrast with the well-known civilization along the Yellow River, it fed on rice, bean, and fish. Pigs ate the human excrement and pigs’ excrement was reduced to the soil. That is why the ancient Chinese character for the toilet is the pictograph that represents a pig fenced with walls. This “bring back civilization” was succeeded by Japan.
The origin of the Western civilization is the Sumerian Civilization. They deforested cedars of Lebanon, raised cattle, irrigated arid land and ended up with salinization. Egyptian, Indus and European civilizations succeeded this “throwaway civilization” and today it is the most dominant. Even China and Japan abandoned “bring back civilization” and adopted “throwaway civilization”. In the next division, let me illustrate the traditional Japanese lifestyle as an exemplar of “bring back civilization”.
3.2 : Traditional Japanese Exemplar
Although Japan is heavily industrialized and densely populated country, 67% of the country is forested, while only 27% of the land in the world is forested. The reason is mainly natural but Japanese traditional way of life has something to do with this high rate of forestation.
In the early part of the Edo Period, Japan came to a deforestation crisis but managed to overcome it. Jared Diamond enumerated many possible reasons why Japan could avoid an Easter-Island-like catastrophe, of which some are right and others are doubtful. For example, he rightly pointed out “Japan’s lack of goats and sheep, whose grazing and browsing activities elsewhere have devastated forests of many lands.” What about this one?
Like robust Polynesian and Melanesian islands, Japan has rapid tree regrowth because of high rainfall, high fallout of volcanic ash and Asian dust restoring soil fertility, and young soils.
To be sure, Japan has heavy rainfall, because it is located on the west of the vast Pacific Ocean and above subtropical high, but volcanic ash and Asian dust do not fertilize lands. Major composition of volcanic ash and Asian dust is silicon that is not nutrient for plants. The southern part of Kyushu covered with volcanic ash is barren and unsuitable for growing a rice crop.
As for fertilizer, Diamonds also says, “Pressure on forests as a source of green fertilizer for cropland was reduced by making much more use of fish meal fertilizers.” Trees were not primarily used as fertilizer. Japanese used woods first as fuel and then ash as fertilizer. Dried sardine was sometimes used as fertilizer, but it was too expensive for ordinary farmers to apply it frequently. The common fertilizer that Japanese farmers usually used was human excrement.
Human manure is rich in nitrogen and phosphorous but short of potassium. So, Japanese farmers mixed it with potassium-rich ash. As ash is alkaline and human manure is acid, the mixture is neutralized.
In the Edo Period, Japanese ate fish and other marine animals, reducing their excrement to the ground so that it could function as manure. Human excrement was sold and bought in the market. Thus the nutrients that flow into the water area were retrieved to the ground.
On the other hand, Western people eat cattle and throw their excrement into the water area. I know that they used the excrement of cattle as manure, but this only circulates the nutrient within the land.
The Western people must eat meat, because wheat lacks lysine, methionine, and threonine among essential amino acids, while the Eastern people do not have to eat meat because rice includes all essential amino acids except lysine, which can be supplemented with beans.
This custom of fermenting human excrement and using it as manure probably came from southern China. In Japan, the custom was first written in the document in 924 and became most popular in the Edo Period. Edo was one of the largest cities those days in the world, but this size did not destroy the environment. Tokyo Bay was full of the materials of sushi. The rice fields around Edo yielded a good harvest.
In the Meiji period, Japan was modernized under the strong influence of the Western civilization. City dwellers increased and fields decreased. Human manure became superfluous. Finally, cheap chemical fertilizer has replaced human manure. Was it the right choice?
Chemical fertilizer makes agriculture unsustainable. Among three major elements of fertilizer, namely, nitrogen, potassium, and phosphorus, the latter two are mined from land. Therefore, the usage of chemical fertilizer promotes the flow of nutrients from land to water areas.
Potassium is still abundant, but phosphorous becomes scarce. If it should be exhausted, we must retrieve it from water areas. It would cost very high to retrieve directly the phosphorous dissolved in water. Fortunately phosphorous is concentrated in the bone of fish and whales. We only have to catch fish and whales and return their bone to the soil.
Another problem of chemical fertilizer is that it usually does not include all essential elements. Depending only on chemical fertilizer for a long time impoverishes the soil. That is why the application of organic fertilizer, especially human manure is so important.
The importance of returning nutrients to the soil is still not well understood. The would-be environment conservation groups that campaign for the abolition of whaling symbolize it. They might think eating beef is ecologically superior to eating whale meat. I will show the contrary is the truth.
Japan has a long history of whaling. Whales have been a source of food, oil, and various materials for Japanese. Yet they never made whales extinct, for they just hunted only those they could observe from land. At the end of the Edo period, whales near Japan rapidly decreased, because the Western (especially U.S.) fleet of whalers hunted them excessively. They hunted just for the sake of oil. So, they had to kill numbers of whale and dump them except oil at sea. The Western burned whale oil and did not retrieve nutrients from the sea. The dumped whale body sank to the seabed and their nutrients were not utilized. On the other hand, Japanese utilized all parts of a hunted whale so that they completely retrieved its nutrients from the sea and return them to land.
When cheap petroleum replaced whale oil, the Western countries withdrew from whaling, but Japanese still continued whaling, because it was so efficient and profitable. Then Western would-be environment conservation groups started bashing Japanese whaling. In 1982 the IWC (International Whaling Commission) voted to enter into a moratorium on all commercial whaling. Is this moratorium ecologically necessary?
To be sure, some species of whales, for example, Blue Whales in the Antarctic are on the verge of extinction. But the moratorium has not increased them, because it has increased Antarctic Minke Whales that competed with Blue Whales for Antarctic krill in the same habitat. The World Conservation Union does not consider the Minke Whale “threatened”. We should allow regulated commercial whaling of species at “lower risk” so as to prevent desertification.
3.3 : The Original Role of Humans
Eating aquatic animals is not only good for the health of the Earth but also for ours. The unsaturated fat of fish and whale meat promote good HDL cholesterol, while saturated fats in beef, pork and poultry do the opposite. Our body is made so as to adapt to the aquatic environment.
In the 71st annual meeting of the American Association of Physical Anthropologists (Buffalo, New York; 10-14 April 2002), a report that eating marine food expanded human brain was one of the hottest topics.
Illustrations of human ancestors routinely show brawny hunters bringing home the wildebeest, butchering meat with stone tools, and scavenging carcasses on the savanna. But a more accurate image might be ancient fishermen–and fisherwomen–wading into placid lakes and quietly combing shorelines for fish, seabirds’ eggs, mollusks, and other marine food.
About 60% of human brain consists of docosahexaenoate (DHA) and arachidonate (AA). Humans cannot generate their materials, omega-3 and omega-6 fatty acids. Although we can ingest omega-6 not only from fish but also vegetable oil, we can ingest sufficient omega-3 only from fish, algae, and other marine food.
This fact might support the aquatic ape hypothesis proposed by Elaine Morgan. According to her, there are many grounds for it besides dietetic one. Let’s examine the grounds respectively.
First, she claims that the aquatic ape hypothesis can explain the nakedness of human bodies. All aquatic animals are hairless. She believes elephants are naked because they were once aquatic or semi-aquatic. But it is wrong. There is another condition for nakedness. Tropical large (more than 1 ton) animals are naked so as to radiate heat efficiently. There is, however, an intermediate case of two conditions. Tropical medium-sized semi-aquatic animals tend to be hairless. Babirusa (Babyrousa babyrussa) is as heavy as humans and littoral; that is to say, it lives near the shores of rivers and lakes. For the same reason as Babirusa, humans got naked.
Second, she claims that the aquatic ape hypothesis can explain human bipedalism. Some primate species descended from trees to Savannah without walking on their hind legs, because terrestrial bipedalism burdens vertically the lumbar vertebrae. Under conditions of head-out immersion in water, erect posture enables hominids to breathe without imposing a strain on the spine. On the other hand, few tetrapods became bipedal in the aquatic or semi-aquatic environment. So, we need another condition: brachiation as pre-adaptation to bipedalism. Here again, the coincidence of two conditions is necessary.
Third, she claims that the aquatic ape hypothesis can explain human hair tracts. The hairs on an ape’s back point downwards, while those on a human backtrace the flow of water over a swimming body holding its head above water and performing a breaststroke. This fact indicates that humans were not aquatic but semi-aquatic.
Fourth, she claims that the aquatic ape hypothesis can explain human fatness. Human infants are especially fatter than those of other apes and helpless without mother. They were not originally burdens. A newly born baby, thrown into the water, does not drown, but float because of ample fat, begins to swim instinctively. In the water, a human female can bear a baby by herself.
Fifth, she claims that the aquatic ape hypothesis can explain human ventro-ventral copulation. It is rare in land mammals, but common in aquatic mammals. She says, “Whales and dolphins, dugons and manatees, beavers, and sea otters are among the numerous aquatic species which mate face to face.” Sea otters do not mate face to face. During copulation, the male holds the female’s muzzle in his jaws so that she may not drown. It shows penetrating from behind in the water is dangerous for mammals.
Elaine Morgan thinks hominids evolved in the sea and regards Hadar in the Afar region as the possible cradle of mankind. Today Hadar is no longer considered to be the cradle of mankind. In 2002, the fossils of Sahelanthropus tchadensis, the oldest (6-7 million years ago) known human ancestor, were found near Lake Chad. The ancestor is thought to have lived close to a lake. Some 5000 years ago, the lake was called “Mega-Chad", then the size of the Caspian Sea, but it is thought to have been freshwater lake as today’s Lake Chad is. So, our ancestor should have evolved near a freshwater lake.
Around 3 million years ago, it became colder and drier. Our ancestors that were adapted to aquatic environment died out. Homo erectus was the terrestrial species that survived this predicament. Homo sapiens, its descendent, today is adapted to the terrestrial environment and forgets our original role in nature. Why can’t we resume it for the health of the Earth and ourselves?
4 : Related Works
Global Deserts Outlook by United Nations (2006-06-16): Amazon.com: Books
Global Deserts Outlook by United Nations (2006-06-16) on Amazon.com. *FREE* shipping on qualifyi ...
Amazon.com: Collapse: How Societies Choose to Fail or Succeed: Revised Edition e
Amazon.com: Collapse: How Societies Choose to Fail or Succeed: Revised Edition eBook: Diamond, J ...
The Aquatic Ape Hypothesis: The Most Credible Theory of Human Evolution (Retro C
The Aquatic Ape Hypothesis: The Most Credible Theory of Human Evolution (Retro Classics) - Kindl ...
5 : References
- United Nations Environment Programme (2006) Global Deserts Outlook. p.20.
- Intergovernmental Panel on Climate Change (2007) Impacts, Adaptation and Vulnerability, Climate Change 2007: Fourth Assessment Report. p. 282. C3 plants incorporate carbon dioxide into a 3-carbon compound, while C4 plants incorporate carbon dioxide first into a 4-carbon compound. C3 plants are ordinary species, but C4 plants are special species that photosynthesize faster than C3 plants under high light intensity and high temperatures and have a better efficiency of water use.
- Xuebin Zhang et al. (2007) Detection of human influence on twentieth-century precipitation trends, Nature, advance online publication, doi:10.1038.
- Intergovernmental Panel on Climate Change (2007) Impacts, Adaptation and Vulnerability, Climate Change 2007: Fourth Assessment Report. p. 275.
- United Nations Environment Programme (1991) Status of Desertification and Implementation of the United Nations Plan of Action to Combat Desertification, 1991. p.25.
- T. N. Taylor, W. Remy, H. Hass, H. Kerp (1995) Fossil Arbuscular Mycorrhizae from the Early Devonian, Mycologia, Vol. 87, No. 4, pp. 560-573
- Population Division of the Department of Economic and Social Affairs of the United Nations Secretariat (2003-2006) World Population Prospects: The 2004 Revision; World Urbanization Prospects: The 2003 Revision; 04 December 2006
- Jared Diamond (2005) Collapse – How societies choose to fail or succeed, Penguin Books. p. 304.
- Jared Diamond (2005) Collapse – How societies choose to fail or succeed, Penguin Books. p. 304.
- Jared Diamond (2005) Collapse – How societies choose to fail or succeed, Penguin Books. p. 299
- Ann Gibbons (2002) American Association 0f Physical Anthropologists Meeting: Humans’ Head Start: New Views of Brain Evolution, Science, Vol. 296. No. 5569, pp. 835-837.
- Elaine Morgan (1997) The Aquatic Ape Hypothesis, Souvenir Press, pp.83-84.
- Elaine Morgan (1990) The Scars of Evolution: What Our Bodies Tell Us About Human Origins, Oxford University Press. p.47
- Elaine Morgan (1997) The Aquatic Ape Hypothesis, Souvenir Press. p.156
- Elaine Morgan (1990) The Scars of Evolution: What Our Bodies Tell Us About Human Origins, Oxford University Press. p.151
- Elaine Morgan (1997) The Aquatic Ape Hypothesis, Souvenir Press. p.174.
- Patrick Vignaud et al (2002) Geology and palaeontology of the Upper Miocene Toros-Menalla hominid locality, Chad, Nature, 418. p.152-155.