>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.