Why are the last 570 million years of earth's history divided into three different eras while preceding the four billion years comprise only one era?

How do you reset plantronics explorer 210? What is the lowest common multiple of 34 and 45? Where can you apply inflation in real life? How much rock will you need to do a driveway 100 feet by 15 feet? How do you solve the following identity sec x cos x equals sin x tan x? What is the average seventh map test scores? What is the answer of expanded form 33 56? What is the square root of 593 in radical form? What are the five rules for stewing? If 24 counters are the whole set how many are five-eighths of the set? What is 12 over 18 eqivalent to? What is the value of 1978 US proof set? What is xxi represent in roman numerals? What is non-parallel forces? How will you convert 0.1to 0.5N? How many times is the word Diamond mentioned in the Bible? Demical numbers in words 0.23? What number goes into 53 and 12? When x-3 equals 5 find x? What is 36 over 65 as a percentage?

Why are the last 570 million years of earth's history divided into three different eras while preceding the four billion years comprise only one era?