Complex sentence: Clarence P. Cazalot joined Marathon Oil as President in March 2000 and became the President and Chief Executive Officer of the company on January 1,2002, when USX's steel and energy businesses separated.
Split into simpler sentences: Clarence P. Cazalot joined Marathon Oil as President in March 2000. On January 1,2002, he became the President and Chief Executive Officer of the company when USX's steel and energy businesses separated.