Associated Press

Scientists in Australia believe they've solved the mystery of when and how animals first appeared on Earth: it was linked to the rise of algae, preceded by 50 million years of huge glaciers pounding entire mountain ranges into powder.

"These large and nutritious organisms at the base of the food web provided the burst of energy required for the evolution of complex ecosystems, where increasingly large and complex animals, including humans, could thrive on Earth," said the lead researcher, Jochan Brocks.

Why it matters: Understanding the conditions that led to complex life here could help to determine whether and how live evolved elsewhere in the universe.

The back story: Scientists have known for awhile when complex forms of life began to show up on Earth, triggering evolutionary processes that culminated in the broad diversity of life we now see across the planet. It generally happened after a time known as "Snowball Earth" (a period when the planet was frozen beginning about 700 million years ago lasting 50 million years). What they didn't know was what triggered the evolution of complex life.

To answer that, a research team crushed ancient sedimentary rocks from central Australia into powder and then extracted molecules from organisms inside them in order to study them more closely.

What they found: Glaciers may have pounded mountain ranges into dust 650 million years ago, releasing nutrients into the oceans and creating perfect conditions for the rapid spread of algae. The researchers think the world's oceans then transitioned from a place dominated by bacteria to one inhabited by algae that served as food for more complex life. Eventually, some of that complex life slithered and crawled onto dry land. The rest is evolutionary history.

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