Bacteria that live in rocks deep beneath the ocean live very slowly, reproducing only once every 10,000 years, according to research presented at a conference this week. In other words, the "parents" of these microbes existed prior to practically all recorded human history.

A search for life in rocks 1.5 miles (2.5 kilometers) below the ocean floor found 10,000 bacteria in a teaspoon-sized sample, the BBC reported. That may sound like a lot, but it's relatively little — a teaspoon of soil at the Earth's surface contains billions or trillions of bacteria. The study also found fungi, but the most abundant inhabitants were viruses: There were 10 viruses for every microbe, scientists from the Integrated Ocean Drilling Program announced at the Goldschmidt conference in Florence, Italy.

The microbes reproduce so slowly, some have questioned if they even constitute "life" in the ordinary sense. The finding also raises the question of what the limit to life will be. "The deeper we look, the deeper we are still finding cells, and the discussion now is where is the limit?" Researcher Beth Orcutt, of Bigelow Laboratory for Ocean Sciences in Maine, told the BBC.

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