The two types of of pure substances are elements and compounds.
It is important because each species has a role in the environment, and they all need to do their part.
Explanation: We can turn of the lights when were not using them and use bike rids instead of car rides use solar panels.
To my knowledge it would be during the development stages or if it were exposed to harmful chemicals. There could be mistakes made when the DNA is replicating- like cancer cells- multiplying when its not needed