Although the fossil record demonstrating their initial radiation across the Northern Hemisphere is very detailed , the fossil record from the tropics -- where primates most likely evolved -- is very poor , particularly around the time that primates and other major clades ( groups consisting of an ancestor and all its descendants ) of eutherian mammals were first appearing .
Although the fossil record of their initial radiation across the Northern Hemisphere is very detailed , the fossil record from the tropics -- where primates most likely evolved -- is very poor , particularly around the time that primates and other eutherian mammals were first appearing .