Читать книгу Exploring the Solar System - Peter Bond - Страница 94

No one knows how and when life began on Earth. Some believe it originated in a random manner, possibly in a chemical “soup” enriched by early atmospheric gases such as methane and carbon dioxide, with energy supplied by lightning or solar ultraviolet light. Others suggest the raw materials, or life itself, were delivered by comets and meteorites – a theory known as “panspermia.”

Yet another possibility is that the first life forms lived deep beneath Earth's surface, where the temperature was far above 100°C, with sulfur in the rocks as its source of nutrition. Warm, chemical‐rich, environments around deep sea volcanic (hydrothermal) vents, known as black and white smokers, are another alternative.

Figure 3.35 White hydrothermal smokers on the ocean floor near the Mariana Islands in the Pacific Ocean. Small white chimneys emit a cloudy white fluid, as well as columns of bubbles rising from the surrounding sediment. The vents release hot water, carbon dioxide, and minerals – one reason why the oceans are salty. Although they are often too deep to receive any sunlight, such smokers are often home to many organisms, such as bacteria, worms, and shrimp. They may have been sites where life first appeared on Earth.

(NOAA)

Once the surface began to cool and life was able to establish a foothold on Earth, it was probably extremely precarious and short‐lived. The massive bombardment by incoming asteroids and comets may well have wiped it out on a number of occasions until the rain of planetary debris tailed off about 3.8 billion years ago.

Nevertheless, the fossil record indicates that life was flourishing 3.5 billion years ago. Microfossils thought to be the remains of blue‐green algae have been found in rocks of this age in parts of Australia. If this is the case, it shows that photosynthesis began very early in Earth's history.

Although micro‐organisms seem to have spread quite rapidly across the oceans and the land surface, the oxygen produced did not build up immediately in the atmosphere, since it was involved in chemical reactions with rocks, particularly oxidation of iron. Even today, most of the oxygen produced over Earth's lifetime is locked up in ancient rock formations.

Levels of oxygen in the atmosphere seem to have remained quite low until the Great Oxidation Event, about 2.4 billion years ago. Only when the minerals in the crust became saturated and the rate of oxidation dropped dramatically did the oxygen content of the atmosphere increase – sporadically at first, and then irreversibly.

At the beginning of the Devonian period, about 400 million years ago, there was a wide variety of plants and animals in the oceans, including the first fish and sharks. Despite the steady evolution of life in the oceans, there seems to have been little change on the continents until the invasion of the land by algae, lichens, and mosses. Exactly when this occurred is uncertain, although fossils suggest land plants may have existed by 500 million years ago.

At that time, Earth's surface underwent a major change as North America collided with Europe, creating the Appalachian Mountains of the USA and the highlands now found in Norway, Scotland, and Greenland. Meanwhile, the huge land mass of Gondwanaland rested over the South Pole. This coincided with an explosion in the number and variety of land creatures.

Once large plants such as ferns moved onto the land, animals soon followed. The first were the arthropods, creatures such as crabs, spiders, and scorpions. Then came amphibians, fish‐like animals that were able to breathe air and spend short times out of water. Insects began to appear about 325 million years ago, evolving from land‐dwelling arthropods.

During the next 50 million years, there were major evolutionary changes in both plants and animals. By 280 million years ago, large areas were colonized by swampy forests with trees up to 30 m high and 1.8 m in diameter.

The final break of life's tie with water came with the arrival of the first reptiles, which laid hard‐shelled eggs on the ground. These eventually developed into the most fearsome (and successful) creatures that ever roamed the Earth – the dinosaurs. Their reign came to an end 65 million years ago, after a massive asteroid impact coincided with huge volcanic eruptions, opening the way for the dominance of warm‐blooded mammals.

Vegetation cover protects and stabilizes the soil, reducing erosion by heavy rain and wind. When this ecological balance is upset, perhaps by logging or clearance for agriculture, the exposed soil may suffer large‐scale erosion, causing rivers to fill with sediment. This results in landslides, increased flooding, and transport of soil to the sea.