What is the oldest kingdom of life on Earth? 

The oldest kingdom of life on Earth is considered to be Kingdom Monera. This kingdom is composed of prokaryotic organisms, which lack a defined nucleus and other membrane-bound organelles. It is believed that these organisms first appeared on Earth around 3.5 billion years ago, making them some of the earliest forms of life on the planet. Today, the classification of Monera as a kingdom has been challenged, and it is now more commonly divided into the domains Archaea and Bacteria based on genetic and biochemical differences. Nonetheless, the prokaryotic organisms that make up this group remain important in a wide range of ecological processes and continue to be the subject of ongoing scientific investigation.