(A) In 1871, Charles Darwin published The Descent of Man, and before that, Gregor Mendel already had discovered the principles of genetics, using his famous pea plant experiments, and ultimately they would explain and justify Darwin's conclusions
(B) By the time Charles Darwin published The Descent of Man in 1871, Gregor Mendel already had discovered, during his famous pea plant experiments, the genetic principles that ultimately would explain and justify Darwin's conclusions
(C) Gregor Mendel already discovered the principles of genetics during his famous pea plant experiments, and although later Charles Darwin published The Descent of Man in 1871, these principles ultimately would explain and justify the conclusions of Darwin.
(D) With Charles Darwin publishing The Descent of Man in 1871, Gregor Mendel discovered already before this the principles of genetics during his famous pea plant experiments, principles that ultimately would explain and justify the conclusions of Darwin
(E) Before Charles Darwin publishing The Descent of Man in 1871, already Gregor Mendel had conducted the famous pea plants experiments and had discovered the principles of genetics, but these principles ultimately would explain and justify Darwin's conclusions
The Past Perfect Tense is one way to indicate that one past action preceded another. When is it necessarily? acceptable? redundant? For a discussion of these issues, as well as the OA & OE of this particular question, see:
Past Perfect on GMAT Sentence Correction
Mike
