The biosphere – the Earth’s life envelope

The Earth is about 4.6 billion years old, and the first signs of life appeared roughly 3.5–3.8 billion years ago as tiny invisible organisms. The Earth’s life envelope – the biosphere – is the shell where organisms live and interact: it includes the lower part of the troposphere, the whole hydrosphere and the upper part of the lithosphere (the textbook gives a thickness of 30–40 km); organisms are most numerous near the surface, in soil, in the lowest air layer and in the upper water. Plants make organic matter from water, mineral salts and carbon dioxide in sunlight. Herbivores eat plants and carnivores eat other animals; microbes rot dead plants and animals and turn them back into minerals. The spread of life depends on climate: hot, humid equatorial forests are very rich, deserts have small-leaved, spiny, long-rooted plants, and the polar ice deserts have almost no life. The oldest ancestors of humans appeared about 2 million years ago; the use of fire, farming, herding and factories increased the human effect on the biosphere: some animal species have declined or disappeared and the climate is changing. Places in dry lands that people have watered by digging ditches and canals and turned into gardens and fields are called oases.