After reading this response, I went to the African Forum over on AccurateReloading where many African hunters and African Professional hunters post and asked the question of minimum caliber required in Africa for elephants.  The following is a detailed response for all African countries.  It basically says that the .375 is the minimum centerfire cartridge for elephants in all African countries, and that .40 caliber is the minimum black powder caliber for elephants in Zimbabwe.

"Not true other than in the 60's or 70's there was a East African min of .40 maybe for Keyna and Tanganyika ( -- cant remember that was before my time).

Here is the current min's:

Benin
• There is no minimum equipment requirement for rifle hunting in Benin.
• Benin does not require a minimum energy (Eo - muzzle velocity) for calibers used.

Botswana
• The minimum equipment requirement for rifle hunting in Botswana is .222 caliber for any game other than dangerous game.
• The minimum equipment requirement for rifle hunting in Botswana is .375 caliber for dangerous game or big game hunting.
• The maximum equipment allowed for rifle hunting in Botswana is .577 Nitro Express caliber.
• Botswana does not require a minimum energy (Eo - muzzle velocity) for calibers used.

Burkina Faso
• There is no minimum equipment requirement for rifle hunting in Burkina Faso.
• Burkina Faso does not require a minimum energy (Eo - muzzle velocity) for calibers used.

Cameroon
• For Group 1 - Small Game, the equipment requirement for hunting is .240 caliber or less, shotgun may be used as well.
• For Group 2 - Medium Game, the equipment requirement for hunting is .240 to .354 caliber.
• For Group 3 - Big Game, the equipment requirement for hunting is .354 caliber or larger.
• Cameroon does not require a minimum energy (Eo - muzzle velocity) for calibers used.

Central African Republic
• The minimum equipment requirement for rifle hunting in Central African Republic is .375 caliber for dangerous game or big game hunting.
• Central African Republic does not require a minimum energy (Eo - muzzle velocity) for calibers used.

Ethiopia
• There is no minimum equipment requirement for rifle hunting in Ethiopia.
• Ethiopia does not require a minimum energy (Eo - muzzle velocity) for calibers used.

Mozambique
• There is no minimum equipment requirement for rifle hunting in Mozambique.
• Mozambique does not require a minimum energy (Eo - muzzle velocity) for calibers used.

Namibia
• Smallest caliber allowed 7 mm (.284).
• Minimum energy (Eo - muzzle velocity)
• Big Game
5400 Joule
(Elephant, Cape Buffalo, Rhino, Lion, etc.)
• Large Game
2700 Joule
(Greater Kudu, Cape Eland, Oryx / Gemsbok, Red Hartebeest, Blue Wildebeest, Black Wildebeest, Hartmann's Zebra, Burchell's Zebra, Giraffe, Sable Antelope, Roan Antelope, Waterbuck, Tsessebe, Leopard, etc.)
• Medium to Small Game
1350 Joule
(Springbok, Impala, Blesbok, Gray Duiker, Steenbok, Ostrich, Caracal, Black-Faced Impala, Red Lechwe, Damara Dik-Dik, Klipspringer, Black-Backed Jackal, Warthog, Cheetah, Nyala, Chacma Baboon, Game Birds, etc.)

South Africa
• Most provinces do not have a minimum equipment requirement for rifle hunting and rely on common sense.
• Some provinces require a minimum of .375 caliber for dangerous or big game hunting.
• No provinces require a minimum energy (Eo - muzzle velocity) for calibers used.

Tanzania
• The minimum equipment requirement for rifle hunting in Tanzania is .240 caliber for any game other than dangerous game.
• The minimum equipment requirement for rifle hunting in Tanzania is .375 caliber for dangerous game or big game hunting.
• Tanzania does not require a minimum energy (Eo - muzzle velocity) for calibers used.

Zambia
• Zambia does not have a minimum equipment requirement for rifle hunting plains game and rely on common sense. Caliber in the .270 range will be well suited for some of the smaller plains game in Zambia.
• The minimum equipment requirement for rifle hunting in Zambia is .300 caliber for dangerous game such as Leopard and Lion.
• The minimum equipment requirement for rifle hunting in Zambia is .375 caliber for dangerous game or big game hunting such as Elephant, Buffalo and Hippo.
• Zambia does not require a minimum energy (Eo - muzzle velocity) for calibers used.

Zimbabwe
• Class A Game
5300 Joule
Minimum caliber 9.2mm in diameter
(Elephant, Hippo, Buffalo)
• Class B Game
4300 Joule
Minimum caliber 7.0mm in diameter
(Lion, Giraffe, Eland)
• Class C Game
3000 Joule
Minimum caliber 7.0mm in diameter
(Leopard, Crocodile, Kudu, Oryx / Gemsbok, Hartebeest, Wildebeest, Zebra, Nyala, Sable Antelope, Waterbuck, Tsessebe, etc.)
• Class D Game
850 Joule
Minimum caliber 5.56mm in diameter
(Warthog, Impala, Reedbuck, Sitatunga, Duiker, Steenbok, Jackal, Game Birds, etc.)
• Black Powder Rifles
Minimum caliber .40"

My choice of one rifle to hunt all of the big game animals of the world would be my .375 RUM.