In the year 2240,
Most of the Earth's natural resources Have been harvested whether it be plant or mineral. To the point that the sustainability of the world is beginning to dwindle. It's become so much that the planet has begun to become near inhabitable.
People and creatures of all kinds are starting to die in droves. Most of the Earth's natural and lush wildlife has been turned into more concrete jungles of buildings and houses. Most of native wildlife are able to be supported with the new environment. Most natural inhabitants of the once undisturbed woodlands have mostly started to die off or the lucky few are taken to the zoo to keep the species from completely dying off. Most of the animals you could once find in the wild are now currently being kept and raised in zoos or other isolated reserves built for the animals.
Very little plant life still exists nowadays except for a farm here or there but even they too are becoming rare as most food is now inorganic and lab-made and not really grown naturally nowadays.
Most of the nonhumans living amungst the human have too started dying off too as a result of the change in environments that and lack of resources. What was once a flourishing hidden society of nonhumans living amongst the humans has started to dwindle as the resources too began to dwindle themselves.
When the populace started to realize their dire situation they started to take action. Soon the humans started to take great interest in space travel, namely to find a new inhabitable world to call home. So the job of space voyager and explorer came to be where groups of a few hundred people were trained to board spaceships to travel the galaxy looking for hopeful new planets to explore and see if they meet the criteria to be deemed habitable for what is left of Earth's current populace. Thus the six massive spaceships were erected with crews of at most one hundred were made to set off into space to find their new objective.