Scientists estimate that there are about 8.7 million species of animals on Earth – give or take 1.3 million. But in the beginning there could be only one.

That animal was very likely the simple sea sponge, according to a study by researchers at the Massachusetts Institute of Technology in Cambridge.

Based on new genetic tests, the team of scientists can say with confidence that molecules produced by sea sponges have been found in 640-million-year-old rocks. These rocks significantly predate the Cambrian explosion, the period 540 million years ago in which most animal groups took over the planet, suggesting that sea sponges may have been the first animals.

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