Thomas took the seat vacated by Marshall, but in no way can anyone say he filled Marshall's shoes.
When he was appointed to the Court for life by George Bush the Elder (confirmed, 52-48), Thomas was 43. He graduated in the middle of his class at Yale and laments publicly that his degree is tainted because of Yale's affirmative action program (it set aside seats for minority students). Earlier in hs education he had also been the beneficiary of minority recruitment when he was accepted to the College of the Holy Cross, a religious school in Worcester, MA.
Thomas had one year of judicial experience; Bush appointed him to the United States Court of Appeals for the District of Columbia Circuit in 1990 before elevating him to the Court in 1991 -- something that would not have happened had Marshall not paved the way for him.
From 1974-1977, Thomas served as Assistant Attorney General of Missouri. Like many Republican nominees to the Court, he also served time as a corporate lawyer (three years with Monsanto). Most of his career (1979-1990) was in government, including (ironically) a stint as the head of the Equal Employment Opportunity Commission.
Marshall, on the other hand, was 59 when Lyndon Johnson appointed him to the Court (confirmed, 69-11). He was denied admission to University of Maryland Law School in 1930 because he was black. Instead, he attended Howard University Law School, a historically black university located in Washington, DC, graduating at the top of his class (magna cum laude).
Marshall brought a history of judicial experience (he won 29 Supreme Court cases) and a passion for equal rights, especially in education. In 1933 he forced (via lawsuit) the University of Maryland to admit a black student, but he is probably best known for his 1954 case, Brown v. Board of Education.
He was chief counsel for the National Association for the Advancement of Colored People (NAACP) and, at the request of the United Nations, helped draft the constitutions for Ghana and what is now Tanzania.
Although I recall the Thomas hearings (particularly Anita Hill's testimony), I did not realize the extent to which Marshall's legacy was being handed to a judicial lightweight. [Read the transcripts.]