The number of people and even the percentage of the population both living in extreme poverty has really gone down over the past two centuries.

The idea wasn't properly articulated. both could be replaced with too above, but the non-finite clause heading by living would still be in the wrong place.

The number of people living in extreme poverty has gone down over the past two centuries, both in absolute terms and as a percentage of the total population.

The number of people and even the percentage of the population both living in extreme poverty has really gone down over the past two centuries.

The idea wasn't properly articulated. both could be replaced with too above, but the non-finite clause headed by living would still be in the wrong place.

The number of people living in extreme poverty has gone down over the past two centuries, both in absolute terms and as a percentage of the total population.