Layers of fossils in pillars, the rock covering them is dated and we get an exact time of each layer.
Lead.
"Due to insufficient water treatment, lead leached from water pipes into the drinking water, exposing over 100,000 residents to elevated lead levels."
If the original cell had 52 chromosomes then the two daughter cells produced would be identical, both with 52 chromosomes
No; some mice may have developed lung cancer in less than 500 days of cigarette smoke exposure.
