The Sahara was like a giant wall. Not much (if any) commerce took place there. That meant trade in North Africa was with a much different group than the trade with sub-saharan Africa. That’s probably worth thinking about and how those foreign influences might have manifested.

“Since the end of the last ice age, the north and sub-Saharan regions of Africa have been separated by the extremely harsh climate of the sparsely populated Sahara, forming an effective barrier interrupted by only the Nile River.”

But I copied that from an Encyclopedia, so if you use it in a homework submission, you may get an F for plagiarism.

We could, but we’re not here to do your homework for you (see Help>The Guidelines>Question Guidelines). Also, your question asks why they’re different in the title but asks how they’re different in the description. These are two different questions.

I’m sure there are more than one way they are different. Why not try to read about them both and then find the topic you like best.

I like gorillapaws’s answer which is quite good, I’m just going to add to it. The sahara isn’t quite impassable, but it’s such an endeavor, it may as well be. North Africa traded throughout the med in ancient times when the Phoencians set up trading outposts all over (Carthage was Phoenician), and then again later when the Muslims conquered it. It’s only in the post-Islamic Golden Age era that North Africa has fallen on hard times.

But sub-saharan Africa has a harder time connecting to the rest of the world. We know some trade occurred over the Indian Ocean, but even that is harder than the easy connections in the Med and over the near eastern land routes. It’s easy to forget how FREAKIN’ HUGE Africa is because we’re all used to the mercator projection maps that make it look smaller than it really is. And much of sub-saharan Africa’s coast is rockier, making for less quality ports.

Then you get another factor – rainfall. The best places to build large stable civilizations is in an area where moist air hits a mountain range. The result is a ‘rain shadow’ where rain falls predictably and the rivers that feed it also become predictable. Africa has no such feature until you get to the far eastern edge. As a result, while Africa gets a decent amount of rain below the sahara, it’s erratic and undependable from year to year. Rivers surge some years and go dry others. Imagine how hard it is to build a stable empire under those conditions.