"Lactose intolerance" is the default state for most humans (65%) on the planet and was historically normal for all humans until comparative recently.

Lactose tolerance is a recent mutation that seems to have developed sometime between 2,000 and 20,000 years ago (opinions vary) and it seems the first instances of it were in Turkey and from there spread north and west into Europe. Later it spread across to the USA.

It is still the case that it is mainly Europeans and people of European descent that are tolerant of lactose; the rest of the world remains lactose intolerant and that applies to most of Africa.

So what happened in Niger.?

• A strong indicator is that Niger was a French colony for 58 years from 1900 to 1958. The influx of lactose tolerant French Europeans, interbreeding between them and the indigenous population is the likely reason why the LI mutation was successful there.
• Also significant is that the indigenous population at the start of the French rule was tiny. It rose to 2.5 million by 1950 and 24,5 million today, giving it one of the highest growth rates in the world. This growth has been largely achieved by reproduction from a very small genetic base rather than immigration which means that small genetic changes achieved during the French period will have amplified today.
• A further contributor is that the Nigerien economy is largely based on livestock with 72% of the population being involved with the farming of milk producing animals (goats, sheep and cattle) as well as camels. There is some evidence that suggests that in communities where milk is stored and drunk, being lactose tolerant has an evolutionary advantage and that may also have encouraged the trend.

How Did Humans Develop Lactose Tolerance?

Background Notes for Niger: January 2009 Bureau of African Affairs

Decalo, Samuel (1997). Historical Dictionary of the Niger

Nigerien demographics: World Population Review

[Incidentally - it may seem strange to talk about "evolution" when we are only looking at a period of a hundred or so years. The evolution of the lactose tolerance gene may well have taken 100's of thousands of years to happen, but once present, its transmission and multiplication through a society - particularly a small one that is rapidly expanding - can happen very quickly.]