Based on the context of the sentence, the bones are described as being a "vestige of one of evolution's many $\qquad$ experiments." This implies that these ancestors did not successfully lead to modern humans. The key phrases are "bones ... are those of a direct ancestor of modern humans," but "some scientists are skeptical" and believe they are a "vestige," or a remnant, of a less-than-successful evolutionary branch.

The words that would best fill in the blank must convey the idea of failure or a lack of success in an evolutionary sense.
- A. noble: This means having or showing fine personal qualities or high moral principles. It does not fit the context of evolutionary success or failure.
- B. unviable: This means not capable of working successfully; not feasible. This perfectly describes an evolutionary lineage that did not survive to the present day.
- C. intuitive: This means using or based on what one feels to be true without conscious reasoning; instinctive. This is irrelevant to the context of evolutionary history.
- D. unsuccessful: This means not successful. This is a direct and fitting synonym for "unviable" and conveys the idea that this particular evolutionary path did not produce a lasting lineage.
- E. archaic: This means very old or old-fashioned. While the remains are old, the word "archaic" doesn't capture the idea of a failed evolutionary branch.
- F. ingenious: This means clever, original, and inventive. This is the opposite of the meaning required, which is to describe a failed experiment.

The correct answers are B. unviable and D. unsuccessful, as they both describe an evolutionary experiment that was not capable of working successfully.