Human Prehistory to the Industrial Age: Evolution, Agriculture, Climate, and the Structure of the Atmosphere

Human Prehistory and the Hunter-Gatherer Era

Between roughly 1 million and 10,000 years ago, the hunter-gatherer stage dominated human existence. Several australopithecine species were gradually replaced by Homo habilis and then Homo erectus. Modern humans, Homo sapiens, originated in Africa.

Early humans lived in savannahs and forests, moving in small groups and competing with other animals. They were omnivorous, with a broad diet, and their ability to make tools was a key factor in their success. Over time, they learned to find shelter, locate water, use medicinal plants, predict weather, make clothing, and hunt with increasingly sophisticated tools. Their rudimentary means made them comparable to intelligent predators.

The discovery of fire by Homo erectus was the first genuine revolution. Fire allowed humans to cook food, regulate body temperature, and keep dangerous animals away. Early hunting also became more organized, with groups pursuing large prey.

Key Developments of the Paleolithic Period

• Tool making from stone, wood, and bone
• Controlled use and production of fire
• Cooperative hunting of medium and large animals
• Dispersal across Africa, Eurasia, and beyond

The Agricultural and Livestock Revolution

Around 10,000 years ago, at the end of the last Ice Age, a warmer climate and increased humidity favored the expansion of forests. This created a new relationship between human populations and their environment, made possible by the appearance of agriculture and the domestication of animals.

The Neolithic transition meant that food became more reliable, and populations grew rapidly, in some cases multiplying many times over. People learned to cultivate plants and, over time, to domesticate animals. Women began gathering wild cereals and imitating natural crops in their fields. Wheat, rice, barley, and maize became staple crops that sustained human societies.

In livestock raising, people selected wild species for traits such as size, docility, and meat production. Goats, sheep, pigs, cattle, and horses were domesticated over time, transforming the relationship between humans and animals.

Consequences of Settled Life

• Abandonment of nomadic life and the rise of permanent settlements
• Villages built near rivers, lakes, and lagoons
• Construction of houses from mud, wood, and later stone and brick
• Development of cities, ports, and roads
• Discovery of metals and invention of pottery, the wheel, the plow, and the hoe
• Use of water and wind power, and the development of stone mills
• Specialization of labor, social stratification, and trade

These changes laid the foundations of economic and social life, and human action on nature began to leave a lasting mark, particularly in populated areas.

The Industrial and Technological Era

From the late eighteenth century to the present, the defining feature of human history over the last three centuries has been its growing energy demands. Rising productivity led to an immediate increase in the consumption of coal and mineral resources, and later to the uncontrolled exploitation of new energy sources, most notably oil.

The burning of fossil fuels and the release of harmful pollutants are the main causes of the current environmental crisis. Oil, coal, and natural gas are being depleted at a rate far faster than they can form naturally. The economy has grown at an unprecedented pace, and the human population has surged from a few hundred million to billions of people.

Our planet has finite limits, and the carrying capacity of Earth is approaching its maximum. It is necessary to reduce the rate of population and consumption growth in order to avoid irreversible environmental degradation. Inequality is a major problem: a privileged minority of the world's population consumes resources in an excessive and unchecked manner, while large numbers of people remain deprived.

Environmental Impacts

• Deforestation and the spread of deserts
• Contamination of soil and water sources
• Unprecedented concentrations of CO₂ in the atmosphere
• Accumulation of non-biodegradable waste exceeding recycling capacity
• Intensification of the greenhouse effect and rising global temperatures
• Damage to the ozone layer caused by CFCs, increasing harmful UV radiation

Humanity now faces the consequences of climate change, a threat that is still poorly understood by many. Despite advances in science and technology, and a significant increase in life expectancy, which now averages nearly 80 years in many countries, the path forward requires urgent and coordinated action.

Structure and Composition of the Atmosphere Today

The atmosphere is a layer of gases surrounding the Earth, held in place by gravitational attraction. Its density is greatest at sea level and decreases rapidly with altitude. The atmosphere extends up to about 10,000 km, although most of its mass lies within the first 80 km.

The chemical composition of the lower atmosphere is fairly uniform and consists mainly of a mixture of gases known as air. This region is called the homosphere. Dry air is composed mainly of nitrogen, oxygen, and carbon dioxide, with argon also present in notable amounts. Carbon dioxide and water vapor play an enormous role in the natural greenhouse effect.

The homosphere is divided into three layers based on temperature variation:

1. Troposphere

The troposphere extends from the surface up to an average altitude of about 12 km. It contains most of the atmosphere's water vapor and carbon dioxide, as well as most of the air. Heat from the Earth's surface warms this layer, so temperature decreases rapidly and steadily with altitude. Fine particles such as dust and sea salt are found here and act as condensation nuclei, which is why clouds form in this layer. The boundary between the troposphere and the next layer, the tropopause, reaches temperatures of around -70 °C.

2. Stratosphere

The stratosphere extends from the tropopause to an altitude of about 50 km. In its lower part, temperature stays nearly constant, but from about 20 km it begins to rise. This is due to the ozone layer, or ozonosphere, which absorbs solar ultraviolet radiation. Temperature rises to around 0 °C at the top of the stratosphere, between 50 km and 60 km, which marks the boundary known as the stratopause.

3. Mesosphere

The mesosphere is the last layer of the homosphere, extending from about 50 km to 80 km, where the mesopause is located. Temperature decreases again, reaching values of around -100 °C. This layer is where most meteors burn up on entering the atmosphere, producing the fleeting streaks of light known as shooting stars.