Homo sapiens evolved about 195,000 (160,000 – 350,000) years ago.  We are a relatively recent addition to earth’s biodiversity.  The earliest life forms evolved about 3.5 Billion Years Ago.  Multi-cellular life evolved about 650 Million Years Ago.  We are related to monkeys and apes (see phylogeny below).

 

The evolutionary lineage that leads to humans split from its last common ancestor with chimpanzees about 7 Million Years Ago. Since that time, there have been a diversity of progressively more Homo sapiens like organisms, some our direct ancestors, some offshoots of the lineage that led to us.  We see this in the fossil evidence.  We have nearly complete skeletons of Ardipithecus ramidus and Australopithecus afarensis (see phylogeny below) and a diversity of skulls (see skulls below from oldest A to most recent N) and other bone from numerous species on the human family tree.  We also see our evolutionary relationships to other apes and extinct humans in our DNA. We can compare our DNA to that of modern apes, including bonobo chimpanzees with which we share 98.6% of our DNA, and even to Neanderthals for which researchers have pieced together the compete genome.  We are similar to enough to Neanderthals to have interbred with them and for people who are not of recent African origin to have 2% of their DNA come from Neanderthals.

 

 

Watch This Videos on Human Evolution

 

Click Here for the Second Video on Human Evolution (Links to an external site.)

Modern humans left Africa about 95,000 Years Ago and spread around the globe living primarily as hunters and gatherers until after the last ice age.  See map of human migration below.

 

Following the last ice age, we see progressive human developments that we recognize more and more as parts of our modern life styles: the development of agriculture 10,000 years ago, civilization 5,000 years ago, and the industrial revolution about 250 years ago.  The industrial revolution was the transition to new manufacturing techniques dependent on the use of fossil fuels, at first coal, but eventually, oil and natural gas.  These developments are the roots of how we live today.  From this point on, we see a rapid development in the size of the human population, the average resource use per person, and new technologies.

Human Population Growth

Watch This Video on Human Population Growth

 

It took all of our 195,000 years on the planet to reach one billion people around 1800, from there we doubled to 2 billion people by 1930, and then tripled to 6 billion people by 1999.  Today, there are approximately 7.6 billion people on the planet.  This rapid growth occurred because death rates declined while birth rates remained high during the early stages of industrialization.  Remember, that a population’s r-value is birthrates-deathrates.  If birthrates remain high while deathrates drop, the difference between birthrates and deathrates gets larger and the r-value is larger.  A larger r-value means more rapid population growth.  The deathrates declined following the industrial revolution as a result of greater food production resulting from advancements in agriculture and better health and longevity resulting from improved hygiene, water quality, sanitation practices, and medical care.

If we plot the growth of the human population over time, it looks like this:

 

This pattern of growth approximates the exponential growth model that we learned about two weeks ago.  Human population growth is behaving as if resources are unlimited.  For humans, this growth was the result of a growing r-value and a growing population size (N).

The worldwide growth rate of the human population peaked in the mid-1960 at about 2.2% (r=.022) and has declined since then to about 1.1% (r=.011) in 2017.  The r-value declined, not because deathrates went back up, but because birthrates declined.  The Total Fertility Rate (average number of children born to a woman in her lifetime) declined from about 5 in the mid-1960’s to about 2.5 today.  Birthrates declined as a result of better access to education and employment for women and better access to contraception and family planning services.  However, even though Total Fertility Rate is declining, the size of the human population is so large that the United Nations (U.N. 2017) predicts we will have 9.8 (95% CI: 9.4-10.2) Billion people on the planet by 2050 and 11.2 (95% CI: 9.6-13.2) Billion people on the planet by 2100.

 

The size of the human population is expected to peak around 11.2 billion people soon after 2100.  For the population to stop growing and stabilize, Total Fertility Rate would have to decline from 2.5 to 2.1.  Two children per woman is necessary to replace her and her mate.  The 0.1 accounts for deaths that occur before sexual maturity.  At a Total Fertility Rate of 2.1, birthrates will equal death rates so the r-value will be 0 and the population will no longer be growing.  The UN predicts a Total Fertility Rate of 2.0 by 2100 which means that after the population size peaks it will slowly decline. 

Some people have taken this trend toward stabilization to mean that we do not need to focus on reducing the size of the human population, but what’s important is not simply that the size of the human population stabilizes, but that it stabilizes below carrying capacity: the number of humans that can be sustained long term on planet earth.  Planet earth is a closed system, like an island.  If we get too many people, they cannot go somewhere else to find the resources that are no longer available on planet earth.  Planet earth is now a simplified system for us.  We have used our technology to remove our potential predators and competitors for resource’s, and we have overcome diseases and parasites with technology and modern medicine (we will overcome Covid-19 with treatments or a vaccine and likely both). 

Remember our reindeer on St. Matthew Island (see graph below)?  They lived in a closed and simplified system.  They overshot carrying capacity, damaged their resource base, and crashed.  If the carrying capacity for humans on planet earth is 12 Billion, then we are on track to level out below carrying capacity, but if it somewhere less than 11.2 billion, we should be concerned that the human population will be headed for a crash if we don’t reduce fertility rates more quickly.

 

What is carrying capacity for humans on planet earth.  How many people can planet earth sustain long term?

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The graph above is a model of the pattern of human population growth that has occurred globally since the industrial revolution.  We discussed this in Monday’s lesson.  Historically, human birthrates (green line) and deathrates (red line) were both high.  Birthrates were a little higher than death rates, so the population was growing slowly. Following the industrial revolution, we saw declines in death rates from improvements in agriculture, hygiene, water quality, and health care, while birth rates remained high.  This led to a period of rapid human population growth (shown by the rapid rise in the orange line in the middle of the graph).  Since the middle of the twentieth century, birthrates have declined, closing the gap between birthrates and deathrates and slowing the rate of population growth.  It is predicted that the trend in declining birthrates will continue and by the end of the century the size of the human population will be leveling off.

Carrying Capacity of Humans on Planet Earth

Nobody knows what carrying capacity is for humans on planet earth.  It is not one static number. It depends on technology and the standard of living that we want for people. However, we can get a sense of what carrying capacity is and whether we are on track for a sustainable future in a variety of ways.  The preferred way to get at this is using a concept called the Ecological Footprint.  This is a calculation of the area of biologically-productive land and water needed to provide the resources humans use and to absorb the wastes they produce.  It is an indication of the magnitude of the combined pressures humans are placing on the natural world.

The average ecological footprint for humans is 2.1 hectares per person.  A hectare is about 2 football fields put side by side.  So, the average human on planet earth uses about 4 football fields worth of land to produce the resource that they use in a year and to absorb the wastes they produce in a year.  However, this number varies based on the lifestyle that people lead.  In the developing nations of the world, it is 1 hectare.  In the developed nations of the world, it is almost 10 hectares.  An average person in a developed nation (like the U.S. and Europe) uses 10 times more resources per year than a person in a developing nation.

When we multiply this 2.1 hectares per person times the number of people on the planet, we see something worrisome.  The total global ecological footprint for people on planet earth is 50% greater than the size of the planet.  This means (by 2008), humans have exceeded the earth’s carrying capacity by 50%.  We are living as if we have 1.5 planet earths, but we only have one. 

As the number of humans on the planet increases, the UN predicts that by 2030 humans will be using twice the amount of resources that the planet can produce each year.  Remember, these numbers are based on the average global ecological footprint.  If everyone on the planet used the same amount of resources as an average U.S. Citizen, we would be using more than 5 planets worth of resources every year.  The global ecological footprint indicates that the human population on planet earth already exceeds carrying capacity.  We are using more than the earth can produce/absorb each year, which means we are using up the world’s capital.  Each year we live beyond carrying capacity, the carrying capacity of the planet declines (see graph below).

 

We can see the increasing demand humans are placing on the planet by looking at the annual consumption of natural resources (UN 2011).  In 1900, humans used 6 billion tons of natural resources.  By 2011, this had grown to 59 billion tons of natural resources used by humans each year, and by 2050, it is predicted to be 140 billion tons.  The largest consumers of these resources are the developed nation, including the U.S.  The 18% of the world’s population that lives in developed nations uses over 80% of the resources produced world wide each year and generates 75% of the world’s pollution and waste produced each year.

Watch This Video on Carrying Capacity

 

Read The Summation of the State of the Natural World on the Next Page

 

 

Environmental Impact of the Human Population

When we look at what is happening to natural systems around the world, we see that impacts of human activities are accumulating in very destructive ways that provide evidence we are living beyond carrying capacity.  Some of those statistics are highlighted in the graphic above.  A few of the most common impacts we see are:

1. Habitat Destruction: This occurs when humans damage natural areas where species get food, water, shelter, and find mates, usually converting these natural systems to some form of human use, such as cities or farmland.  In marine environments, this commonly occurs as a result of destructive fishing practices. 

2. Overexploitation: This is where humans remove individuals from a population faster than they can be replaced through reproduction.  On land this is often a result of poaching activities.  In the marine environment, this typically occur through overfishing.

3. Non-native Species: Non-native species are species that evolved in one part of the world and were introduce by humans to a different ecosystem type in some other part of the world.  When a non-native species is introduced to a new ecosystem, it may have a negative effect on the species that live there. 

4. Global Climate Change: Humans conduct a variety of activities that release greenhouse gasses into the atmosphere, including carbon dioxide and methane.  Each molecule of these gasses can hold a certain amount of heat based on its chemical structure.  As humans release more and more of these gases into the atmosphere, the amount of heat each molecule can hold is multiplied across a larger number of molecules, resulting in more heat being held in planetary systems.  This changes temperature and rainfall patterns around the globe.  Producing greenhouse gasses faster than the earth can process them is a substantial contributor to why humanity's ecological footprint exceeds the earth’s carrying capacity.

5. Ocean Acidification: In addition to accumulating in the atmosphere, carbon dioxide released into the atmosphere gets absorbed into the ocean where it goes through a series of chemical reactions that change the PH of the ocean and reduce the availability of carbonate ions that many shelled organisms in the marine environment use to build their shells. This inhibits the ability of these species to grow.

6. Toxic and Nutrient Pollution: Greenhouse gas emission are one form of pollution, but humans also release nutrients, like nitrogen and phosphorous, into the environment through fertilizing farm fields, resulting in nutrient pollution in the environment.  This throws off the balance between organisms in rivers and oceans, and results in marine dead zones in many places in the world where there is not enough oxygen in the water for most species to survive.  Toxic pollution includes heavy metals, industrial chemicals, pesticides, plastics, etc., that accumulate in the environment and have negative effects on living organisms.

We will explore some of these environmental issues in upcoming lectures.  We will look at the role these factors play in species endangerment and extinction next week and take an intensive look at global climate change the following week.

All of the impacts discussed above have accumulated and accelerated over the last 250 years and particularly since the middle of the 20th century.  They are a product of the growing size of the human population, the increase in the average resource use per person, and the development of technologies that allow/cause us to exploit natural resources at a faster and faster rate. In regards to the carrying capacity and population size, it is important to note that the environmental damage discussed above has not resulted from us living with 7.6 billion people on the planet long term, but has occurred just ramping up to 7.6 billion people.  When I was born 52 years ago, there were half as many people on the planet as there are today. 

We can represent the drivers of environmental degradation using a semantic model.  A semantic model is a word model.  Like all models, it is a simplification of reality that helps us understand complex phenomena.  Semantic models are not meant to be solved mathematically.  The model of human environmental impacts is:

I = P x A x T

I -- Environmental impact

P -- Number of people in population

A -- Average resource use per person (Affluence)

T -- Environmental effects of technologies

As the number of people on the planet grows, the average resource use per person grows, and our use of technology grows, humans have a greater impact on the environment.  We see that in all of the impacts described above that have gotten progressively worse over the last century.  As a side note on technology, some technological developments could help us to reduce the impact we have on the natural world.  If we produce energy from wind power, rather than coal, oil, and natural gas, we will reduce greenhouse gas emissions and lessen the impact of climate change.  However, some people argue that technological developments will get us out of whatever trouble we get ourselves into.  We have to be careful about relying on technologies that do not yet exist to get us out of problems that currently exist because the technological solutions we hope for might never be developed.

 

Slowing Human Population Growth 

We have seen that environmental damage is accumulating from the growing size of the human population, the increase in the average resource use per person, and the development of technologies that allow/cause us to exploit environmental resources at a faster and faster rate.  We have also seen that analysis of humanities ecological footprint indicates that we are already living beyond carrying capacity.  It seems unlikely that we can provide a high quality of life for people in the future unless we reduce human population growth and average resource use per person.  We can look to countries where Total Fertility Rate has declined significantly and rapidly for guidance in how to slow the rate of human population growth. 

In China, the Total Fertility Rate was 5.7 in 1972, but was reduced to 1.8 by 2000.  China provided free and widely available access to contraception, couples were urged to postpone marriage, and not to have more than one child.  If families had less than on child, they received extra food, better housing, larger pensions, preferential treatment in employment, free medical care, salary bonuses, and free school tuition.  Although apparently successful in reducing total fertility rate, there were unintended consequences, as well.  Widespread reports on female infanticide and a shadow population of children hidden from the government among them.  Upon analyzing the Chinese approach, most western democracies don’t believe this is the best approach because of its focus on heavy regulation of peoples lives and the unintended consequences that resulted.

Another example is Thailand.  Thailand’s Total Fertility Rate declined from 6.4 in 1971 to 1.9 in 2000.  Thailand improved literacy rates, access to education and employment among women, and health care for mothers and children.  There was support for family planning services and contraception from government and religious leaders.  This is seen as a more widely applicable model than China’s approach.

The preferred approach to reducing Total Fertility Rate is improving education and employment opportunities for women, providing free and widespread access to contraception and family planning services, and cultivating the support of government and religious institutions in these endeavors.

Some people argue we don’t need to pursue population growth reduction strategies, but should focus on economic development instead.  If you look at the currently developed nations, reductions in Total Fertility Rate and improving education and employment opportunities for women accompanied economic development.  This is an idea known as the Demographic Transition.  When we look at the developed nations of the world, Total Fertility Rate is at replacement rate or below.  As an example, the U.S. was ~ 1.9 children/woman in 2017.

 

Population growth through the rest of the century is expected to occur primarily in developing nations in Asia and Africa (see graph above).  If Total Fertility Rate in developing nations follows the same pattern as it did in developed nations, economic development would lead to a stabilizing world population.  This is happening, and it is what’s leading to the leveling off of the population around the end of the century.  Unfortunately, the population will level off in 80 years at a level that is far beyond carrying capacity. 

There are a variety of other problems with the Demographic Transition approach:

1. The problem of Humans exceeding carrying capacity is not just about population numbers, but also about increases in average resource use per person that are prevalent in developed nations. If we speed up economic development in developing nations and it results in the population levelling off at 9 billion by 2050 (hypothetically), but everyone of theses people consumes resources at the rate of an average person in the U.S., the pressure on the planet would be significantly greater than it is today.  Remember, if the 7.6 billion people on the planet today all lived like we do in the U.S., we would be using over 5 times what the planet can produce each year.  We need to reduce average resource use per person in the developed nations of the world while facilitating improvements in the quality of life for people in developing nations.

2. It may not be possible for developing nations to proceed through the same developmental pathways as developed nations. Developed nations went through these processes over a period of centuries in a world that was much richer in natural resources.  When developed nations ran low on resources, they began to exploit and import the additional resources they needed from other countries all over the world. The developed nations facilitate their resource consumptive lifestyles by drawing in resources from other countries which means the resources aren’t there for the development of those countries.

3. Government, cultural, and religious institutions of countries may not support improvements in women’s rights and improved access to contraception and family planning services. Even in the U.S., progress made in these areas over the last century has seen significant set backs.

This is not an exhaustive critique of the Demographic Transition model, but it gives you a sense that economic development alone can not be our answer to the growing environmental problems we face from the growing size of the human population, the increase in the average resource use per person, and the development of technologies that allow/cause us to exploit environmental resources at a faster and faster rate. 

The graph below illustrates the idea of demographic transition.  It shows the stages of development that developed nations proceeded through and how they affected population. It places countries today along the spectrum of where they would be in the transition if they follow the same model.

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