"Unfortunately, all life on earth - the only life we know - represents, for all its current variety, the results of a single experiment, for every earthly species evolved from the common ancestry of a single origin. We desperately need a repetition of the experiment (several would be better, but let's not be greedy!) in order to make a judgement. Mars represents our first real hope for a second experiment - the sine qua non - for any proper answer for the question of questions."