It really depends on the Era. Are we talking pre-islamic or post? and what are we talking about in terms of Developed? like as in cultural, architecture, writing, science? I would argue their is different answers for all of them.

The few written accounts about ancient Mali were recorded by Arab travelers. One of the most famous primary sources is Rihlah by the African-born Ibn Battuta 1304-1368, Rihlah describes life in Mali between 1352 and 1353. Although other written accounts of the ancient West African empires exist, Ibn Battuta is one of the few who actually traveled to this area and wrote from personal experience. There is also an ancient Poem that I am forgetting the name of, about a famous king who ruled the Mali Empire, it is located as part of oral tradition, but I am unsure as to where you would find it or how accurate it is.

Basically the short answer is yes. Particularly once they embraced islamic tradition. But that being said. Arab traders were noticing the impacts of the empire long before it embraced islam. Most of what remains of their architecture are mosques dated to the 14th century, so it hard to know what the pre-islamic cities looked like, but primary sources indicate they used sandstone and mudbrick as well as wood structures. They were an advanced, and large aristocratic society. With an emphasis on oral tradition. They had advanced trade, and at its largest, held territory with around 40-50,000,000 people. Much larger than most Medieval European countries. They were technologically advanced, and had a working knowledge of blacksmithing and iron. That said, Slaving was (as with all african nations of the era) was a major part of their economy, along with gold, salt, and metal works.

http://www.learner.org/courses/worldhistory/support/reading_11_1.pdf

This is a great reading to get you started. (the full book is listed at the start, of the article.)

http://www.goodreads.com/book/show/130155.The_Royal_Kingdoms_of_Ghana_Mali_and_Songhay

This, i think is designed more for grade school students than actual research. But Frankly I found it incredibly useful to give me a broad overview of the subject in university.

edit. There is some debate over the population. I have read in a few places that it was 40-50 million, including all areas controlled under their direct borders, client kingdoms, and trade influence. The Mali Empire proper (Just within its own borders) was more likely 20-25 million.

http://africa.si.edu/exhibits/resources/mali/ Is from the Smithsonian Africa institute. They dont source anything on the article but they where the 40-50 million stems from, along with other things I have read.

50-40,000,000. That's means that it would have roughly the population of Yuan dynasty China Korea and Mongolia combined when it was conteporaneous with them. This in that part of the world the borders the Sahara on one side and barely arable dense jungle on the other seems wildly unlikely. Just agricultural capacity alone would have stopped them. For comparison, just modern day China holds 1.3 billion people, not counting the rest of the Yuan holdings. ALL of Africa is barely 1.1 billion. Edit: added bit about jungle and desert border, agricultural capacity.

Thank you , for answering . i will defintly put the book down on a list. So is the primary reason the alatnic slave trade , happened was west african nations had been doing it for centuries , and were not aware that alantic slave trade would probaly have a far worst long term impact then the pre-arab , and arab slave trades ?