Explain why scientists think that life on earth began after 3.9 bya

Scientists found evidence such as fossils that were almost 4 billion years old, concluding that life on earth began after that.

Explanation:

1.the evidene of the reaction

2.yes

3.yes

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).