More than 400 years ago, Giordano Bruno, an Italian monk, wrote that "in space there are numberless earths circling around other suns, which may bear upon them creatures similar or even superior to those upon our human Earth." Bruno deserves to be remembered -- he was burnt at the stake, in Rome, in 1600.

In the late 19th century, the science fiction of Jules Verne and HG Wells popularised the idea of alien life. Percival Lowell, a wealthy American, built his own observatory in Flagstaff, Arizona, primarily to study Mars. He believed that its surface was crossed by ‘canals’, dug by an advanced civilisation to channel water from the frozen polar caps to the ‘deserts’ near the Red Planet's equator. In 1900, a French foundation offered the Guzman Prize of 100,000 francs for the first contact with an extra-terrestrial species; but prudence led them to exclude Mars -- detecting Martians was then thought to be too easy!

How life began, and whether it exists elsewhere remains one of the most fascinating questions in the whole of science. Indeed, you don't need to be a scientist to wonder about this. But we still don't know the answer. Indeed, nobody now expects ‘advanced life’ on any of the planets or moons in our solar system. But our Sun is just one star among billions. And in the vastness of space far beyond our solar system we can rule out nothing. Astronomers have discovered, just within the past five years, that many stars have their own retinue of planets. There are millions of other solar systems. There would surely, among this vast number, be many planets resembling our Earth.

When we ask “where might we find extraterrestrial life?” the first place many scientists turn to, because of its similarity to Earth, is Mars. Mars may have been like the Earth in its past. Although no signs of life on Mars have been found, scientists will continue to search because they are aware of the potential for life in extreme environments.

What do we mean by life and living things? This simple question could become difficult for even biologists (people who study life) and they would have a tough time describing what life is! But after many years of studying living things, from the green mould on your old sandwich to monkeys in the rainforest, biologists have determined that all living things (at least living things on Earth) do share some things in common: They need to take in energy, get rid of waste, grow and develop, respond to their environment, reproduce and pass their traits to their offspring, and over time, living things evolve (change slowly) in response to their environment.

Sophisticated life forms are relative newcomers on Earth compared with bacteria. Because the environment of other planets is more primitive, life on other planets (if it exists) may be primitive and unsophisticated. If not, life would have developed the ability to withstand otherworldly environments as well as finding material for nutriment. We might have to imagine what such creatures would be like.

In 1996, scientists mistakenly thought that they had discovered life on Mars. But, there has not been any concrete evidence as of yet of life anywhere in the solar system besides Earth. In July 1996, it was announced that David McKay, along with a team of scientists at Johnson Space Centre (a division of Nasa), had discovered possible fossils of bacteria in a meteorite named “ALH84001” that came from Mars. It was found in the Allen Hills in Antarctica in 1984 after having landed there some 12,000 years ago. While many scientists were excited at first, much of the proof offered fell apart. Nasa said that after two years of study “a number of lines of evidence have gone away.”

Several different chemicals and molecular structures were exciting because they looked similar to byproducts of life on Earth. However, these chemicals and structures can also be created without life. Some are even present in deep space on comets, and scientists do not think that they came from Martian life anymore. The environment of Mars in the past was very different from what it is today. Conditions then may have been favourable for the existence of life. Even though the meteorite does not prove life once existed on Mars, it does not disprove the possibility.

In the past, liquid water used to flow on the surface of Mars. Both Earth and Mars should have been frozen in their early history because the Sun was weak at first, but both planets show that water was flowing, which suggests that they both must have had thick atmospheres in place to keep the surface warm. In this environment life may have once existed. The atmospheres on both planets came out of volcanoes. There were not many volcanoes on Mars, and those volcanoes were never very active. Compare this to Earth where volcanic activity continues today. Volcanic eruptions produce a lot of water. The water eventually falls to the ground or into the oceans. Mars is small, and so cooled off very rapidly. Mars was sufficiently cold for water to be absorbed into the ground and freeze like tundra in the Canadian northwest. Today scientists estimate that a large amount of water is frozen into the surface of Mars. They estimate this happened 2.8 billion years ago.

So it is not likely that Mars will become a haven for life in the future, unless it is life unlike that which we know. Whether or not humans spread beyond Earth during the next millennium, we'll still want to know whether we are alone. It would in some ways be disappointing if searches for alien intelligence were doomed to fail. On the other hand, it would boost our “cosmic” self-esteem that we are only one of its kinds. If our tiny Earth was a unique abode of intelligence, we could view it in a less humble cosmic perspective than it would merit if the galaxy already teemed with complex life. We’d have even stronger motives to cherish this “pale blue dot” in the cosmos, and not foreclose life's future -- a future that could be even longer than the time span over which simple life has evolved into humans. That is why we should expand our cosmic vision in the new millennium.

The writer is a doctoral candidate, Carnegie Mellon University, Pittsburgh, and knowledge editor of Financial Chronicle