Cyanobacteria built ancient coral reefs known as

stromatolite

Explanation:

if you want/need to know the details of the layering, it's on the internet (e.g. wikepedia) :)

the diploid number of chromosomes is sometimes represented by the symbol 2n. for the fruit fly, the diploid number is 8, which can be written as 2n = 8, where n represents twice the number of chromosomes in a sperm or egg cell.