What is the difference between 100% whole wheat pasta and 100% whole wheat couscous?

Couscous is a type of pasta. As you say, there are many different types and shapes of pasta. Couscous is simply a form that pasta can take.

According to Wikipedia, Couscous was first introduced to Europe around the thirteenth century. While creating pasta that is many small ball shapes is not something you may consider to be revolutionary or new, the method of preparation was, I believe, novel.

Edit: It is worth mentioning that there are other distinguishing characteristics, such as a lack of dough during preparation of Couscous, and the difference in boiling vs. steaming.

Personally, this seems like an insufficient difference to warrant an entirely new categorization of food, but that gets into opinion-based territory, so I'll simply leave the differences to speak for themselves.

Other than the shape, obviously, not that much in the grand scheme of life.

the couscous grain is coarse wheat semolina; when wanting to eat it, it is usually boiled (boiling water added to couscous) or steamed (using a couscoussière)

Pasta is a dough made from finer semolina, it is made with water or eggs.

From the pasta dough, you can make many different type of pasta.

Some small pasta shape can look like couscous, for example Kushku or Fregola (larger than normal couscous) ...

As to know which came first, I would venture to say Couscous as it is easier to prepare.