It had been building for some time before everything fell apart. To this day there was a lot of finger pointing but no answers. Earth’s atmosphere was turning toxic to humans. Fleeing to Mars, humanity set up and rebuilt on the satellite planet.
Up until the “Collapse” Mars was just a launch off planet, used to dock between Earth and real space travel. With a fully developed atmosphere and breathable air, overnight Earthlings became Marsians.
After a decade on Mars, people got used to their new home. It was now like Earth but… Mars. Most expected to live out their lives forgetting about Earth. That is until a probe returned one day saying life was sustainable on Earth. With the poison only affecting humans, Earth had recovered over the decade. Earth was not only a viable place to live again, but there were huge opportunities.
While most immediately turned down the idea of returning to Earth, others jumped at the opportunity.
Pros:
Free Land, if you claim it, it’s yours
Unpopulated, versus the overpopulation on Mars. Moving humanity from Earth to the smaller planet made everything feel tight.
Fresh start for society
No government, or at least yet
Over a million people left and started a new life again. And for the most part, people were happy. Earth started sounding like a paradise again. But as people on Mars communicated with Earth, they began noticing family and friends on Earth looking different. It was being hushed at first, but soon word got out. Earth was safe to live on, but the atmosphere was changing its humans into anthropomorphic creatures.
They were still humanoid, most still looking very much like themselves with just animal features. Tests showed mentally everyone was still their old selves, but physically they were somewhere in between.
Women turned dog woman went into heat twice a year and birthed multiples
Men turned deer men grew antlers every year and rutted in spring
The examples went on. Soon, all Earthlings were animal human hybrids. Overnight the idea of returning to Earth was undesirable; many thinking Earthlings were now less than human.
Despite the challenges, Earth and Mars continued working together and space travel reopened; although most only travelled to Earth on short visits to avoid turning into an animal hybrid. This fear remained even though science proved this effect was lessened and eliminated over time.