How Life Began
NO ONE KNOWS exactly how life began. Conditions on the young Earth were very different from those found today. Its surface was hot, its oceans were boiling, and its atmosphere was unbreathable. In an attempt to discover the origins of life, scientists have
recreated this early inhospitable environment in the labouratory. They have also investigated places such as hot springs and salt flats, where conditions are similar to those found more than 4 billion years ago.
The first chemical reactions
Scientists believe that Earth's early atmosphere was a reaction chamber for the production of certain molecules that, over millions of years, evolved into living organisms. The energy for these reactions was provided by ultraviolet rays from the sun and by lightning flashes. This energy combined gases such as methane, hydrogen, ammonia, carbon dioxide, and water vapour to form simple substances such as amino acids and sugars. When rains fell, these newly formed substances were deposited into the warm oceans, where further reactions took place. These produced, by chance, larger and more complex molecules. Some of these had the ability to copy themselves, like the DNA found in all living organisms. Other molecules could make chemical reactions work faster, just like the enzymes that are found in all cells. The raw materials for life had now appeared.
The first cells
Cells are the basic units of life. Essentially, they consist of many molecules working together under the control of DNA - the key molecule - and protected within a fatty membrane. Scientists believe that the first living cells appeared when certain molecules grouped together to form spheres, like soap bubbles, within which other molecules collected and reacted together. Over time, the reactions grew more complex, becoming more like those found in modern living cells.
Where life began
The first cells appeared about 4 billion years ago. But exactly where they first appeared is a mystery. It may have been in the open oceans. Some scientists believe that life first appeared around the deep-sea volcanic vents (openings) from which boiling hot, mineral-rich water pours into the ocean. Archaebacteria, which live around these vents, are the most primitive life forms on Earth. They are probably very similar to the first cells to appear. Other scientists, however, have suggested that life arrived with meteorites that fell to Earth from outer space.
Energy sources
The first cells found energy to power their life processes by using compounds containing sulphur, nitrogen, or other elements extracted from the oceans. But soon other cells evolved that used the sun's energy to give themselves life. Some of these cells, the cyanobacteria (blue-green algae), are still found in today's oceans and lakes. They made sugars using sunlight energy, water, and carbon dioxide, and released oxygen as a waste product. By 1.8 billion years ago, this oxygen had started to fill the atmosphere. Cells appeared that could use this new gas to release energy from their food, just as our cells do. In time, these cells evolved into all the life forms - trees, fish, dogs, birds - that we see around us today.