This shuttling of genes has had a huge impact not just on Earth today, but throughout the history of life. It was in the oceans that life got its start, after all. The oldest traces of life are fossils of marine microbes dating back almost 3.5 billion years. It was in the oceans that multicellular organisms evolved; their oldest fossils date back to about 2 billion years ago. In fact, our own ancestors did not crawl onto land until about 400 million years ago. Viruses don’t leave behind fossils in rocks, but they do leave marks on the genomes of their hosts. Those marks suggest that viruses
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