Stories about creatures from space are nothing new. Nearly 2,000 years ago, a Syrian writer named Lucian made up a fantastic tale in which a sailing ship was carried to the Moon by a whirlwind. Almost as soon as they arrived, members of the ship's crew were attacked by moon-men riding three-headed vultures!

 

Today, thanks to the exploration of its surface by the Apollo astronauts, we know that the Moon is a totally lifeless place (see Figure 1). There are no people flying around on the backs of fierce-looking birds. In fact, there are not even any tiny bugs crawling around among its dry rocks and dust.

 

Yet our quest for life beyond the Earth has only just begun. The Universe is incredibly vast. It may be filled with living things – even with alien beings that can fly in spacecraft from star to star as easily as we can fly from one country to another. On the other hand, our planet may be the only one that supports life, or one of just a handful of planets in all the Universe that is inhabited. Either way, it would be a remarkable discovery. But how can we judge whether life in space is common or very rare?

 

To begin with, it would help to know something about how life came to exist on our own planet. If the process by which living things developed on Earth could be easily repeated on other worlds, then alien life forms might be quite common. But if life on Earth came about through a chain of extremely unlikely events, then finding creatures elsewhere in space would be a much more difficult task.

 

In the Beginning

About 5 billion years ago, scientists believe, the Sun slowly formed from a large cloud of gas and dust that came together through the pull of gravity (see Figure 2). Parts of the cloud that were not drawn into its center circled around the newborn Sun as a broad, gassy, dusty, pancake. Over millions of years, the material in this spinning pancake gradually stuck together to make the planets and moons of the Solar System.

 

Slightly more than 4½ billion years ago, a small planet, the third out from the Sun, neared the end of its fiery birth. This was the Earth, but it looked nothing like the Earth of today. Dust and rocks and, at times, boulders as big as a house slammed into the surface of the infant world. The heat from the steady bombardment helped keep the Earth molten, red-glowing, and soft.

 

Slowly, the blizzard of dust and rocks eased. Most of the loose material that had been swirling around the Sun had now been scooped up by the planets and their moons. The rocks at the Earth's surface cooled. Eventually, they hardened into a thin skin, or crust.

 

Below the crust, however, the rocks remained very hot. Everywhere, volcanoes and cracks appeared through which huge amounts of carbon dioxide, nitrogen, and water vapor poured into the atmosphere. These gases joined others, such as hydrogen, methane, and ammonia, that had been captured when the Earth first formed.

 

In time, the escaping water gathered into thick, heavy clouds, Then it began to rain. On the ground, small pools collected. Gradually, the pools grew into lakes and, later, into oceans. The heavy rain also washed down much of the methane and ammonia from the atmosphere, so that these chemicals began to build up in the top layers of the ocean.

 

At this time, there was nothing to stop the most powerful ultraviolet rays from the Sun from reaching the Earth's surface. Today, smaller amounts of weaker ultraviolet rays are what cause sunburn. But 4 billion years ago, all of the Sun's ultraviolet rays penetrated straight through to the surface of the ocean. Along with energy from lightning bolts, these rays caused chemicals such as methane and ammonia in the water to split apart. Then pieces of the broken up chemicals rejoined in different ways to make more complicated substances. The most important of these new substances were sugars and amino acids, because they were capable of linking up further into long chains.

 

Over millions of years, the sugars and amino acids combined to form still more complex chemicals known as proteins and nucleic acids. Finally, tiny "bags" of proteins and nucleic acids came together that could do a remarkable thing. They could use other chemicals around them to grow and make exact copies of themselves. These were the first living things on Earth.

 

From Proteins to People

To begin with, the earliest life forms simply ate the rich chemical soup around them. But they could do this only as long as they ready-made food supply lasted. Soon, other, more advanced life forms started to appear that could feed on more primitive creatures. Eventually, living things evolved that could use the limitless energy in sunlight to help them change simpler substances in the ocean into food. As part of this process, they produced oxygen.

 

Over time, some of the newly made oxygen rose high into the atmosphere. Here, it reacted with the Sun's ultraviolet rays to form a new gas, ozone. Ozone acts as a shield, absorbing the Sun's most powerful ultraviolet rays and thus preventing them from reaching the Earth's surface. With this protective shield in place, it was safe now for life forms to grow at the surface of the ocean.

 

By 3 billion years ago, conditions on Earth had become much less severe. The most dangerous ultraviolet rays were being screened out, and the amount of oxygen in the atmosphere was increasing. From this point on, more and more complex creatures began to appear (see Figure 3). The most complex of all – those in the animal kingdom – breathed oxygen. At first, they still lived in the ocean and filtered oxygen from the water with gills. But over millions of years, some of them ventured onto land and developed lungs.

 

The Earth's creatures gradually adapted, or changed, to suit their surroundings. A great variety of animals and plants spread throughout the oceans and continents. Each of them depended in some way on the others. At last, there was a creature with nimble hands and an unusually large brain. It began to use tools, make fire, work metals, raise animals and crops, and build cities. This new and highly developed creature was a human beings.

 

Briefly, this is how scientists explain the appearance and rise of life on Earth. It also leads to an important question. Could such a process have taken place on other planets, either within the Sun's family of worlds or beyond?

 

Testing the Soup of Life   In the 1920s, scientists first claimed that life may have developed from a chemical "soup" in the Earth's earliest oceans. To begin with, they were not taken very seriously. Then, in 1953 at the University of Chicago, an American researcher, Stanley Miller, carried out a remarkable experiment (see Figure 4). Into a glass vessel, Miller put a mixture of methane, ammonia, hydrogen, and water. This is similar to the mixture that probably existed in our planet's ocean around 4 billion years ago. Then he passed electrical sparks through the vessel's contents to represent the effects of lightning strikes and ultraviolet rays from the Sun. After a week, a brown, tarlike ooze lay in the bottom of the container. Miller removed this strange liquid and tested it. To his amazement, he found that it contained a blend of amino acids and other substances that form the basis of all life on Earth.   More recently, two other scientists have suggested that the firs living things arrived on our planet from outer space. According to Fred Hoyle and Chandra Wickramasinghe, life on Earth started from viruses – disease-causing particles – that came from the tails of comets. These researchers also claim that diseases such as colds and influenza may be caused by viruses that continue to arrive from space   In 1990, Hoyle and Wickramasinghe announced that their studies seemed to link major outbreaks of colds and 'flu with large storms on the Sun's surface. These storms reach a peak every 11 years and cause unusually powerful particles to move outward from the Sun. As the particles strike the Earth's atmosphere, the theory goes, they push down viruses from space that have collected in the top layer of air. When the viruses reach the surface, say Hoyle and Wickramasinghe, they infect millions of people around the world almost at once. Although this theory would explain the surprising speed with which some cold and 'flu epidemics spread, it is not an idea widely accepted by other scientists.

 

Biology in Space

All life forms on our planet are made in much the same way. Giraffes and jellyfish, cactus and cockroaches, may look totally unlike each other. But all contain very similar, tiny parts called cells. Cells are the smallest living units of animals and plants. Billions of cells of many different kinds make up your body. Yet the simplest creatures, such as germs, consists of just a single cells.

 

With cells the most important substances are proteins and nucleic acids. One nucleic acid, called DNA, carries the code for passing on characteristics of one generation of living things to the next. The information stored in DNA controls every chemical process in your body.

 

Proteins and nucleic acids are made in part from sugars and amino acids. Sugars and amino acids, in turn, are made from tiny particles called atoms. The main atom contained in living things on Earth is carbon. No other kind of atom can link together to form such long chains and complex shapes. Carbon is essential to all life on Earth.

 

Elsewhere in the Universe, living things may have developed in a different way than they have on their own world. But it is hard to see how the complicated chemicals needed to support life could be based on any other substance than carbon. Life on other worlds may also depend on amino acids, and even on proteins and nucleic acids. Amino acids, for example, have been found inside meteorites – rocks that sometimes crash to the Earth from space. Such discoveries prove that these building blocks are found outside our planet.

 

Many scientists now believe that if alien life exists, then it is probably based on chemicals similar to those inside your own body. This is no more than a reasonable guess, and it may be wrong. Yet, it is a god place from which to start. If life elsewhere developed as it did on Earth, then only certain kinds of stars and planets could create the right conditions for living things.

 

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