Which unit of geologic time is the oldest? cenozoic mesozoic ordovician precambrian

The oldest unit of geologic time is Precambrian.

Explanation:  

The period of Precambrian prolonged from 540 to 490 millions of years ago. Likewise for Ordovician, 490 to 443 million year ago, whereas for Mesozoic period extends from 248 to 65 million year ago. At last, Cenozoic time lies between 65 million to 10,000 years ago.  

The organisms in each period are as follows,  

they each offer a fresh glut of resources to the organisms that can arrive and exploit them. as individuals and new species continue to appear, they actively and passively alter the environment. competition for shared resources increases, local abiotic and biotic conditions shift, and the relative abundances of species change accordingly. the process is called ecological succession, and it is one of the earliest and most well-known concepts in ecology.

it also supports my hypothesis. if you rest first you can squeeze the clothespin more times. from the data i collected the average of the eight people concluded exercise makes your muscles more fatigued and less able to squeeze the clothespin compared to resting before squeezing.