Kenya’s Great Rift Valley, a 450- mile long volcanic trench ripped open by shifting tectonic plates, is known as the cradle of mankind for the million-year- old remains of human forebears discovered there. Oil drillers say the area also holds a string of fields that could make East Africa’s largest economy a major energy producer. The U.K.’s Tullow Oil Plc TLW and Canada’s Africa Oil Corp AOI. found crude at two wells last year and now plan as many as 11 more test wells in 2013. The valley could yield 10 billion barrels, Tullow estimates, enough to supply Kenya for three centuries or the U.S. for about 18 months.