The sentence that best helps develop the idea expressed in paragraph 2 is:
"Thanks to this analysis, the researchers were able to narrow down their original list of 3,000 genes to 43 specific sequences that might have contributed to Lonesome George’s long, disease-free life."
This sentence directly follows up on the idea of researchers studying the genetic material of long-lived animals to understand the factors that contribute to longevity, highlighting their findings and how they narrowed their focus to specific genes related to Lonesome George’s long life.