list and describe two ways to decide how long ago the organisms were alive?

 Carbon dating: It is a method of determining the age of ancient organic material, when bones are formed small amount of C-14 are incorporated and its amount remains constant towards life,Upon death the radioactive C-14 slowly decays to non-radioactive carbon i.e C-12.By measuring the ercent of C-14 and C-12, the age of the fossil can be determined

and for another point , refer NCERT book page number 154 "How do fossil form layer by layer"