Why? Brocks was puzzling over that mystery when he read a study, published last year, showing that the oceans used to have very low levels of phosphate—a vital nutrient. Phosphate levels only started to rise between 800 and 635 million years ago—exactly before the algae took off. “I thought: Wow, this can’t be a coincidence,” says Brocks.
When phosphate levels are low, bacteria do better than algae because their cells are much smaller. With more surface area for their size, they’re better able to absorb nutrients from their surroundings. “If nutrient concentrations are low, small size wins every time,” says Brocks. “In a low-phosphate world, the larger algae had no chance.”
This competition started tilting in the algae’s favor during the first snowball Earth, when mighty glaciers ground mountainsides into powder, releasing phosphate into the ocean. When the planet warmed, increased rainfall hit the newly exposed ground and washed even more phosphate seaward. It was an overkill of nutrients, the likes of which the planet hadn’t seen before. And, according to Brocks, it broke the bacterial stranglehold in the oceans.
Here’s what he thinks happened. At first, the glut of nutrients would have gone to the dominant cyanobacteria, which would have been eaten by microscopic grazing cells. These grazers capped the bacterial numbers, freeing up nutrients for the larger algae, which could finally flourish. The presence of so much algae would, in turn, have fueled the evolution of predators like rhizarians—single-celled hunters that devour around 50 percent of the ocean’s algae every day.
Entirely new food webs came to be, as did arms races between predators and prey that led to the evolution of larger and larger creatures. By the 635-million-year mark, at the dawn of the Ediacaran period, centimeter-sized organisms showed up. And it was during that period that the first animals appeared. “They all come so close to each other—phosphate came first, algae came second, animals came third,” says Brocks. “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.”
“It presents a feasible scenario and brings together the best new data,” says Robin Kodner from Western Washington University, who studies algae both modern and ancient. “But like all historical geobiological studies, there are some oversimplifications.” For example, there are many types of algae. Of these, the green algae—and specifically, a group called the prasinophytes—are the only ones of any great ecological importance. It’s unclear if the rise in algae that Brock found is a rise in prasinophytes specifically.
Also, many prasinophytes are tiny, and only slightly bigger than cyanobacteria. There’s no particular reason why their rise should have dramatically reworked food webs in the way that Brocks suggests. The reality is that such webs are complicated. Cyanobacteria and algae coexist, filling different roles. The predators that devour one group will often eat the other too. So working out how exactly the rise of algae led to the rise of animals—or even if they did—will take more work. “And really, I don't think we can do any better than this with the data from the geological record,” Kodner says.
Events that happened hundreds of millions of years ago are necessarily hard to reconstruct, and will always be open to speculation and debate. But Brocks’s study is valuable because it unites a bunch of disparate observations into a cohesive framework, against which future discoveries can be compared, writes Andrew Knoll from Harvard University in an accompanying commentary.
And as Brocks says, “If someone comes to me and says they have a better explanation, I’d be happy to accept that.”