In the sentence. "Digging in sediments in ..." is modifier
This modifier cannot modify "evidence"
Eliminate A and B
In option D,
"than that which was". which refers to "that".
"which cannot refer to "that"
Also, there is a comparison present in the sentence.
When ever there is comparison, "that" is used a pronoun.
When "that" is used as pronoun, "that" must be followed by "some preposition". For example, "that of something"
here, the usage of "that" is wrong.
Eliminate D
Similarly
In E, " than that previously thought"
Usage of "that" is wrong
Also, there is a comparison present in the sentence.
When ever there is comparison, "that" is used a pronoun.
When "that" is used as pronoun, "that" must be followed by "some preposition". For example, "that of something"
here, the usage of "that" is wrong.
Eliminate E
Hence C is the correct answer