The aliens come to earth to destroy and conquer, but they got here to early there's nothing to destroy or to conquer. So there thinking "What do we do now?", and decide to leave you behind on the planet with a seed and require you to produce life for them to return and conquer. Its your job to guide the path of evolution through its stages and produce a race to be conquered or build one to defend the oncoming attack and get revenge for leaving you behind. The choice is up to you, your in charge of what we do next.