In just 50 years, the reach of humankind in our solar system has expanded enormously. We have sent probes to orbit every planet, and landed on (or dived into, in the case of gas giants) most. Probes have seen the sunrise on Mercury and felt a cool breeze on Mars. Some missions have even succeeded in acquiring rock and dust samples, and returning them to earth. Apollo 11 in 1969 was the first to do this; later the process was automated in the Soviet Luna probes of the 1970s. In the 1990s samples of the solar wind were captured, and in 2006 particles from the tail of a comet were brought to earth for further study. Sample return mission to Mars have been proposed for the future, and given enough time, mission to and from the Jovian moons may be possible.
When planning an exploratory mission to another celestial body that could (in theory) support life, scientists make every effort to sterilise the probe of all earthly lifeforms. Microbes from earth might colonise the foreign world, and might even displace the native wildlife (in other words alien microbes). Proof of life on another planet those that would be of immeasurable interest to scientists and the public alike, but earthly organisms in the equipment would most likely pollute the result of the off-earth experiments.
All sugars found on earth are dextrorotatory (where the molecule has a right-handed configuration). Levorotatory (left handed molecules) sugars cannot be used by living organisms as source of energy (because they cannot be phosphorylated by hexokinase, the first step in respiration). In contrast, all amino acids (the building blocks of proteins), are in nature all found to be dextrorotatory. Proteins constructed of just one of the wrong type of amino acid have a altered shape, and so a reduced or eliminated functionality.
If alien microbes possessing the wrong type of either molecules were to invade earth and even partly displace our microbes, their effect may be to reduce nutrition up the food pyramid. Organisms at the top of the pyramid (such as humans) by definition receive little of the available nutrition of the ecosystem as a whole. Following an alien invasion, higher organisms would find themselves with less material to fuel and construct their bodies. In other words, they starve.
The current situation where only one of the two possible forms has come to dominate all earthly sugars and proteins is thought to have evolved by chance. When life emerged on Earth, it is likely that many alternative biochemistries ran together in parallel. The only lifeform that succeeded in leaving descendants to the present day happened to use the now ubiquitous system. It follows then that a similar roll of the die might determine the prevalent biochemistry of Martian or Europan microbe. Can we risk our ecosystem to the hands of fate?