Answer:
Africa is a tropical place filled with a variety of landscapes such as rainforests, tropical deserts, and savanna grasslands. Since scientists and archeologists have, over the years, found many more human-like fossils in Africa than anywhere else in the world (many of them being older than other fossils found in other regions) they have concluded that early humans have originated from Africa.
My personal thought for the second question, is that early humans (before modern humans) inhabited Africa long before they spread out to the other continents. They lived there, where some evolved into more "anatomically correct" humans, and thus Africa was not only the only known home of our early ancestors, but it was and can also be considered the birth place of modern day homosapiens. I assume this is already tying into what I said earlier, but I just felt like bringing it up. Anyways I hope this answer isn't too lengthy for ya :)
(Because, you really only need the top part.)
