A team of researchers in Australia has shed light on how the first animals appeared on the planet and paved the way for human existence as we know today.

For almost three billion years, life on Earth largely remained on the single-cell phase, where bacteria dominated in the watery world. Then the melting of “snowball Earth” happened some 650 million years ago, and frozen equatorial oceans turned into water, which led to the explosion of algae bloom.

Post-Snowball Earth

"The Earth was frozen over for 50 million years,” said lead author and Australian National University Professor Jochen Brocks in a statement. “Huge glaciers ground entire mountain ranges to powder that released nutrients, and when the snow melted during an extreme global heating event rivers washed torrents of nutrients into the ocean."

Much larger algae, feeding on nutrients from glaciers and mountainsides sliding toward the sea, replaced microscopic bacteria that had dominated Earth. They ushered in “a revolution of ecosystems” and changed the base of the food web to make our existence possible, Brocks told AFP when he attended a geochemistry conference in Paris and presented the findings.

The team found proof of the transition in samples of ancient sedimentary rocks extracted from the central Australian desert. They crushed the rocks to powder, extracted molecules of ancient organisms from them, and found a drastic shift or up to a 1,000-fold rise in the amount of molecules indicating complex organisms.

Algae Bloom And The Rise Of Complex Life

This rise of algae occurred so closely with the appearance of the first animals, making it unlikely that they were a mere coincidence, Brocks noted.

During that time, staggeringly high levels of nutrients in the ocean as well as the cooling of global temperatures created the right conditions for algae to rapidly spread. Huge amounts of nitrogen-rich nutrients went to see as the planet’s surface melted, letting photosynthesizing algae increase versus far tinier organisms.

These large, nutrient-laden creatures at the base of the food web offered the energy required for complex life forms to emerge and thrive, Brocks said.

Until the present, why and when animals first showed up on Earth has been a subject of debate. One group argues that there are internal and non-environmental factors in the evolution of animals, while another believes that something got in the way for animals to evolve more quickly.

Lack of oxygen, for instance, is thought to hinder the rise of multi-cellular life forms, as large and energy-intensive organisms need oxygen to burn their fuel.

The findings have been detailed in the journal Nature.

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