Sixth grade would be eleven or twelve years old. The opossums in North America are indeed native, and not transplanted. Marsupials were probably a earlier mammalian design, and later died off from over competition from placental mammals which would be the later more efficient design. The exceptions being Australia which was geographically isolated for so long, and a few lone survivors. Opossums are often maligned for looking outright disturbing, but they are true survivors.
Try this article if your curious about the group.
http://en.wikipedia.org/wiki/Opossum










