How Microscopic Algae Kick-Started Life As We Know It

Some 650 million years ago, algae took over the seas, which may have been a needed spark in the formation of complex life

This slimy green algae is far more complex than the type that helped fuel the formation of modern creatures, but it's a distant relative. Wikimedia Commons

In our planet's infancy, life was pretty tiny. Simple, single-celled critters (mostly bacteria) dominated the seas. But from these microbes eventually evolved the many creatures that stomp the earth today—from sharks and snails to labradoodles. But what kickstarted that change? A new study suggests suggests one possible answer: algae.

As Roland Pease reports for the BBC, the study, published in the journal Nature, argues that a sudden dominance of the green stuff some 650 million years ago was the push our planet needed in the development of complex life.

Algae didn't always hang out in the seas, but one fateful day, about a billion years ago, a simple bacterial cell swallowed a cyanobacteria cell, writes Ed Yong at The Atlantic. This chance occurrence allowed the bacteria to produce energy from the sun, a process known as photosynthesis.

After its formation, algae hung around for a few hundred million years, co-existing alongside the many other microbes of the seas. But suddenly, something happened to kick-start that humble algae, allowing it to become the dominant form of life.

To find out exactly when and why the global algal bloom took place, Jochen Brocks of the Australian National University and his team searched for the remains of algae in samples of ancient ocean floors brought up in drilling cores produced by the oil industry. Because the soft and squishy algae and bacteria does not easily fossilize, the researchers looked for their chemical fingerprints—a group of compounds common to algae, steranes, and compounds found in bacteria, hopanes.

According to Yong, the team found that bacterial hopanes dominated the layers of the cores until about 645 to 659 million years ago. After that, the algal steranes became dominate. "The signals that we find show that the algal population went up by a factor of a hundred to a thousand and the diversity went right up in one big bang, and never went back again," Brocks tells Pease.

The algae boom corresponds to a time in geologic history dubbed Snowball Earth, explains Sarah Kaplan at The Washington Post. Around 700 million years ago, ice sheets covered our planet. But then some 650 million years ago the cold was disrupted. Volcanic eruptions built up greenhouse gasses in the atmosphere that helped heat the planet, melting the ice and warming the oceans. Around 14 million years later, the Earth cooled and the Snowball returned.

It was during this time that algae rose to global dominance.

Before the big freeze, the oceans were low in phosphate. But the millions of years of glaciers grinding and pulverizing rock created vast amounts of the nutrient that eventually flowed into the oceans, feeding the algae and causing it to move from the fringes of life to the center stage, according to a press release.

“They all come so close to each other—phosphate came first, algae came second, animals came third,” Brocks tells Yong. “The algae provided the food and energy source that allowed organisms to become big. I just don’t think an ecosystem with sharks in it would be possible with just bacteria.”

Nick Butterfield of Cambridge University tells Pease he thinks the dating of the bloom is accurate, but Brocks may have gotten the cause and effect backward. “There’s no evidence for animal evolution being constrained by a shortage of food,” he says. Instead, he thinks the rise of other animals like sponges made the proliferation of algae possible.

The team expects that there will be many challenges to their interpretation. “The causes and consequences of that rise are controversial, and I’m looking forward to people fighting about it,” Brocks tells Yong, though he does not think the algae bloom itself is in dispute.

So perhaps, we all still have algae to thank for our existence on the Earth.

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