Why did they use a metagenomic approach when their objective was to sequence the genome of one species, the cave bear

As the chart shows, prokaryotes, such as bacteria, were the first forms of life on Earth. They eventually led to the evolution of more complex life forms, including plants and animals. Green bacteria were important because they allowed some organisms to make energy from the sun. The green bacteria also released oxygen in the atmosphere, allowing for more diverse forms of life on Earth.

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