This statement is true. Early earth was much cooler than it is today.
This is because the pollution as well as the population number was very small compared to nowadays population and pollution.
The pollution mainly is causing a decay in the ozone layer which protects the earth from uv rays. Thus, more of these rays enter the atmosphere and increase the temperature.
The increase in population along with the decrease in green areas leads to the increase in the percentage of carbon dioxide in the atmosphere, which again leads to an increase in the temperature.