On the geologic time scale are marked by major, striking, and worldwide changes in the type of fossils present.

An era on the geologic time scale are marked by major, striking, and worldwide changes in the type of fossils present.  

Explanation:

An era is defined as the one of the largest unit of time which is used to define a period in the history. Humans are known to be one of the oldest primates living on the earth till now. Our Earth is approximately more than 4 billion years old.

The oldest era of the Earth is named as Eoarchéen whereas the most modern era of the Earth is known as Cenozoic. The back dating is done by the method of carbon dating. In carbon dating process there is use of carbon 14 atom.  

gametes or germ cells are haploid cells (example: sperm and ova) containing only one set (or n) number of chromosomes and autosomal or somatic cells are diploid cells containing 2n number of chromosomes.