Darwin had very few fossils to support his theory of evolution by means of natural selection when he published On the Origin of Species. Why are fossils considered essential for the study of biological evolution? What can be learned by studying fossils

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Darwin had very few fossils to support his theory of evolution by means of natural selection when he published On the Origin of Species. Why are fossils considered essential for the study of biological evolution? What can be learned by studying fossils (think in terms of ecology, what role does the environment play in the evolution of species)?