 As the major defining characteristic of the eukaryotic cell , the nucleus ' evolutionary origin has been the subject of much speculation . Four major hypotheses have been proposed to explain the existence of the nucleus , although none have yet earned widespread support . 
 As the major defining characteristic of the eukaryotic cell , the nucleus ' evolutionary origin <a href='uninflect' title='1'><mark>has</mark></a> been the subject of much speculation . Four major hypotheses <a href='been_done' title='2'><mark>have</mark></a> been proposed to explain the existence of the nucleus , although none <a href='been_done' title='3'><mark>have</mark></a> yet earned widespread support . 
As the major defining characteristic of the eukaryotic cell , the nucleus ' evolutionary origin have been the subject of much speculation . 4 major hypotheses done been proposed to explain da existence of tdanucleus , although none done yet earned widespread support .