It is thought that the New World monkeys and rodents migrated to South America on rafts to find new homes. Paleontologists know that their ancestors originated in Africa and that South America had been an island continent at the time of the first fossils on record. If it's true, it's amazing how mammals could migrate like this. Steve Brusatte calls it "Hail Mary migration", wherein thousands of species attempt to cross the ocean on natural rafts as a last resort from some cataclysmic event. In a hail mary pass, there is only a slight chance of success, but that chance can add up to 10-20 species that successfully relocate on a distant continent if enough of them attempt to cross. 99% of the other migrant species would have died on that journey.
That only monkeys and rodents made the trip make me makes me suspicious: why weren't there other groups? Perhaps these groups were so small and resourceful that they were the only ones capable. A more reasonable explanation is that there are older fossils dating from before the breakup of South America and Africa. When cross-migration was easier. We just haven't found them yet.
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