Once the concept of planets as worlds like our own was established it was generally accepted that every planet was inhabited by people like ourselves. This made sense in the context of a belief in gods or a God, who would not waste his time making dead worlds. But, as more was found out about the planets, it became obvious that only Earth had the conditions to support life as we understand it.

We have no real idea about how life began on Earth, except that the planet was very different from the way we know it. For example there would have been no free oxygen in any appreciable amounts, and the air would essentially be a reducing atmosphere, that is oxygen-losing, hydrogen reactive. Recent discoveries of extremophile bacteria living in boiling water; at very low temperatures in Antarctica; highly alkali or acidic environments; in radioactive waste; deep in the sea by hydrothermal vents; within rock and at depth in the Earth's crust; tend to suggest that these are the true ancestors of all life on Earth. Evidence from the samples of the American probe Surveyor 3, which were brought back by the Apollo 12 astronauts, showed that earthly bacteria could survive on the Moon.

This obviously has profound implications for the search for life on Mars. Mars is within the "zone of habitability" of our Sun - this stretches from just within the Earth's orbit, to just outside the orbit of Mars. The earliest fossils we know of are about 3,600 million years old. The Earth is about a 1,000 million years older . The fossils are of bacteria, but already complex, so there must have been a simpler beginning. It is therefore likely that life began on Earth very soon after its formation - basically as soon as it could. If conditions on Mars were more clement in its past, then life could well have got started there as well. Martian conditions are no longer apparently suitable for terrestrial life, but could Martian life have adapted to the deteriorating conditions of the environment?

Perhaps life was carried from Mars to Earth on a meteorite? In which case we are the Martians. Perhaps life originated in space, as spores. This means we would never know exactly where life began, and discovering it on Mars would not really prove anything.

There are also implications for space probes. Is it possible to sterilise a spacecraft completely to avoid contamination of Mars by Earthly microbes? Will our instruments give false readings because they have brought passengers with them? Will we risk bringing Martian microbes back from the surface, and will these pose a risk? The unit suggests that this should be tackled by asking the students to think about the hygiene methods we use around the home. Are we running a risk by trying to be too clean? Do we actually need a certain amount of "dirt" in a home to give our immune systems something to work on? If we continue to look for better and better hygiene, will we end up like H.G. Wells' Martians, having forgotten all about microbes, and so fall victim to them?

This unit tries to get the student to think critically about the origin and possibilities of life.

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