How did African towns often begin

The urbanization of most of Africa is moving fast forward, especially south of the Sahara. It is estimated that in 1900, about 89% of inhabitants lived from the primary occupations of farming, hunting & gathering, cattle nomadism, and fishing (Aase, 2003:1) meaning that 11% or less were urban. At the start of the independence period in 1957, 14.7% of Africa's inhabitants were urban, in 2000 had it risen to 37.2% and it is expected to rise to 49.3% in 2015, in effect 3.76% to 3.35% per year (UN, 2002). In sub-Saharan Africa in 1960 "only one city, Johannesburg, had a population of one million; ... in 2009, there were fifty-two cities with such large populations. The Nigerian city of Lagos that in 1963 had 665,000 inhabitants (Rakodi, 1997) and 8.7 million in 2000 is expected to become the world's 11th biggest city by 2015 with 16 million inhabitants (UN, 2002).