Ectopic 1st molar teeth are relatively uncommon and occurs relatively early in life as these teeth come into the mouth around the age of 5- 6 years. Typically the first molar destroys the back root of the primary tooth in front and becomes wedged under the crown of the primary tooth as shown on the arrowed upper right first molar in this case.  Extraction of the primary tooth is sometimes considered but this will certainly complicate orthodontic alignment later.  Early treatment can involve various appliances to push the first molar backward, as in this case.