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?

What 2 numbers equal 75 when multiplied? How many times does 9 go into 657? What are two or more angles that have equal measures? What is the place value of underlined digit 416275130? 10 qt equals how many gal and qt? What is 837 rounded to the nearest tens? What is 30 percent in lowest terms? How do you solve it what is the next in this see and tell sequence 111211121111221? What transformation produces similar figures? Is rhetorical appeal that includes facts figures scientific data and statistics? What is the median of 19 5 10 9 2? Is the decimal 0.12345... a repeating decimal? What is the Are of a largest circle that can be draw in a square that sides is 32.5 centimeters? List all the prime numbers from1 to 49? What is W divided by? What problem types can be solved using the law of cosines or sines? How do you solve xy equals 3? What is the answer for 3x plus 17? What type of number has an odd number of fractions? What does 500 and 500 equal?