Tiny fossil tracks embedded in a California rock formation that was once part of an ancient river may be evidence that freshwater ecosystems arose around 100 million years earlier than what is generally believed. The existing fossil record dates the evolution of freshwater ecosystems to around 435 million years ago, according to ScienceNow. Before then, the oceans were virtually the only hubs for life. But this new set of fossils, found in rocks dating between 520 and 542 million years, suggests entire communities of organisms were already thriving in lakes and rivers at this time.

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