Question

the main, or central, idea of the passage is chimpanzees and humans have surprisingly similar dna. how does the detail in bold develop the main idea?
humans and chimpanzees descended from a common ancestor, a gorilla - like creature that lived millions of years ago. even in light of this fact, it is astonishing to learn just how similar humans and chimpanzees are when it comes to their genes. in fact, the two species dna differs by only 1.2 percent. a laboratory comparison of chimp and human chromosomes—strands of dna each containing approximately 1,100 genes—reveals a strikingly similar pattern. perhaps surprisingly, chimpanzees are genetically closer to humans than to gorillas. admittedly, the apparently small 1.2 percent variation between chimps and humans still corresponds to forty million differences in dna base pairs. still, the tremendous genetic similarity indicates that the chimpanzee (along with the bonobo, another ape) is our closest living relative.
it offers an alternative interpretation of dna research.
it suggests the initial reason why scientists pursued research on dna similarity.
it specifies the extent of the genetic similarity.

Explanation:

The bolded detail "the apparently small 1.2 percent variation between chimps and humans still corresponds to forty million differences in DNA base pairs" provides a more in - depth look at the genetic similarity. It shows that while the percentage difference (1.2%) seems small, the actual number of base - pair differences (forty million) is significant. This helps to clarify and specify the nature of the genetic similarity mentioned in the main idea. It is not about an alternative interpretation of DNA research (no indication of that in the text) or the initial reason for DNA research (not discussed).

Answer:

It specifies the extent of the genetic similarity.