They haven't.
It's a myth inspired by The Bible. Apart from rare genetic differences, or surgery, Everyone has 24 ribs. Check any anatomy book.
Around 1 in 500 (0.2%) people have a cervical rib (extra rib or neck rib). It is more rare to have two cerical ribs.
The latest research shows that 1 in about 200 people have an extra rib. The preponderance is in females.
One in twenty people have an extra rib.
If a mutation caused it, it's possible.
The second rib is classified as an atypical rib because it does not follow the typical rib anatomy pattern. It has a longer and more curved body compared to other ribs, with a prominent angle where it articulates with the sternum. These differences make the second rib distinct from the other ribs in the rib cage.
Your neck does not contain any ribs.
Condoms?
Rib vaults are groin vaults with extra stone ribbing.
Neck...as in cervical rib, meaning an extra rib arising from the neck. It may also refer to the narrow part of the uterus (its "neck"), as in cervical mucus, cervical cancer, etc.
As long as they have extra circular activities which allows them to interact with children for the other gender it should be fine.
I believe that in the first year of production, 1912, you could order as an extra what Winchester refered to as a matted rib. What Winchester meant by this I'm not sure as 1913 is generally given as the first year for a raised matted rib. In 1919 Winchester received a patent for a ventilated rib, they probably did not show up on gun until 1920 though.
The term "Gorilla rib" is used to describe a Lumbar rib. Most humans only have ribs bilaterally attached to their 12 thoracic vertebra but a number of variations exist with cervical ribs, lumbar ribs and even so called sacral ribs. Approximately 8% of humans have a extra rib attached to their lumbar vertebra which also occurs in some types of gorillas