The answer you are looking for is Natural Selection.
As the earth has active tectonics we know that rocks are recycled and the oldest rocks may therefore just be the oldest rocks that have not been recycled yet. We know the true age of the earth by looking at radioactive isotopes of things like spacedust and meteors that formed at the same time as the earth as they have not been affected by the tectonic process of recycling here on Earth.
The best example of hypothesis which leads to new experiment methods was done by Morgan where he used fruit flies.
Thomas Hunt Morgan continues on genetic research which is of Gregor Mendel.
Fish have <span>Eukaryotic cells
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