One of the key characteristics that distinguish mammals from other vertebrates is their reproductive mode. Mammals are viviparous, meaning they give birth to live young, as opposed to laying eggs like birds or reptiles.

This characteristic is a significant evolutionary advancement because it:

- Allows for more complex and intimate parental care

- Enables mammals to nurture and protect their young in a more effective way

- Supports the development of more complex social behaviors and bonding between parents and offspring

The other options are not unique to mammals or do not represent a significant evolutionary advancement:

B. Paired limbs are found in many vertebrates, including birds, reptiles, and amphibians.

C. Terrestrial mode of life is shared with many other vertebrates, including birds, reptiles, and amphibians.

D. Possession of a heart is a characteristic shared with all vertebrates.