An investigator who first sounded the alarm about the growth of the human population and the ability of the earth to handle a large population of humans was?

Thomas Malthus

Explanation:

Malthusian theory of popualtion in his own words stated, "By nature human food increases in a slow arithmetical ratio; man himself increases in a quick geometrical ratio unless want and vice stop him. The increase in numbers is necessarily limited by the means of subsistence Population invariably increases when the means of subsistence increase, unless prevented by powerful and obvious checks."