Humans and chimpanzees diverged about 6 million years ago.If 12 nucleotide substitutions have occurred between human and chimpanzee lineages in a sequence of mitochondrial DNA and 36 have occurred between humans and gibbons, according to the molecular clock hypothesis, humans and gibbons separated _______ million years ago.
A) 2
B) 6
C) 12
D) 18
E) 36

Question

Quizplus · Accepted Answer

According to the molecular clock hypothesis, the number of nucleotide substitutions that have occurred between two species can be used to estimate the time since they last shared a common ancestor. If 12 nucleotide substitutions have occurred between human and chimpanzee lineages, and humans and chimpanzees last shared a common ancestor approximately 6 million years ago, this suggests that nucleotide substitutions occur at a rate of 2 substitutions per million years (12 substitutions / 6 million years). Therefore, if 36 nucleotide substitutions have occurred between humans and gibbons, this suggests that humans and gibbons last shared a common ancestor approximately 18 million years ago (36 substitutions / 2 substitutions per million years). Therefore, the answer is D.

Humans and Chimpanzees Diverged About 6 Million Years Ago