Answer:
I would say climate change. Though the sun is getting larger, therefore Earth is getting warmer, we can reduce greenhouse gases in the atmosphere in order to reduce Earth's temperature. I selected this issue because at one point, if we do not stop increasing the heat on Earth, it will eventually be so hot we cannot grow crops or plants, leading to Earth's oxygen levels to significantly drop. A way to reduce global warming is to stop burning fossil fuels. This would be a good solution because burning fossil fuels, like other gases, will heat up Earth's atmosphere.
They were on separate landmasses, but I'm not really sure this answers the question, you should try just asking google.
I would say plate movement caused it partially however flooding may also have helped
