The story of the origins of animal life on Earth has long been a hazy one, but some pieces of the puzzle may be coming into focus.
Life on Earth was simple, with single-celled organisms dominating the planet for billions of years. Then, sometime around 600 million years ago, the multicellular organisms that we call animals burst onto the scene. It all seems a bit sudden in the fossil record, so what happened?
Scientists suspect global environmental turmoil that occurred around that time had something to do with it. One of the main changes researchers suggest may have triggered the surge of animal evolution is a dramatic change in oxygen levels. But, if that's the case, why did oxygen suddenly rise after billions of years of being at low levels?
The sequence of events that occurred so long ago has mystified scientists, but a team of geochemists may have just found a crucial clue: A spike in phosphorus levels that occurred around 800 million years ago.
And, they say, this may have been a key step in the story of the origins of complex animal life, a story that set the stage for life as we know it on Earth today.
Christopher Reinhard, a geochemist at Georgia Institute of Technology, Noah Planavsky, a geochemist at Yale University, and colleagues compiled about 15,000 samples from the sedimentary rock record to build a picture of the evolution of phosphorus levels across the last 3.5 billion years of Earth's history. Their results are reported in a paper published Wednesday in the journal Nature.
"The first thing that jumped out at us is there's a change in the average amount of phosphorus that's moving through these kinds of environments," Dr. Reinhard says in a phone interview with The Christian Science Monitor. "And this change occurs conspicuously right around the time that some of these major innovations in biological complexity are inferred to have occurred."
The shift, from low-phosphorus conditions to high-phosphorus conditions, also occurs just before major shifts in climate and oceanic chemistry are thought to have occurred, he adds.
So did this dramatic change in phosphorus levels kick off the process that made life on Earth what it is today?
Reinhard, Dr. Planavsky and their colleagues are hesitant to say that just yet, but it seems like a "viable explanation," says Louis Derry, a biogeochemist at Cornell University who was not involved in the research.
Previous models have suggested that a limited availability of phosphorus could have acted as a sort of throttle on life, explains Dr. Derry, holding it down to limited amounts. That's because phosphorus is a nutrient that is crucial to all forms of life as we know it. (It is also one of the most common nutrients found in fertilizers used today.)
"This compilation of data suggests that these ideas that have been floating around for a couple of years actually are consistent with what we see in the sedimentary rock record," Derry says in a phone interview with the Monitor.
Where does oxygen fit in?
The idea that phosphorus played a crucial role in kickstarting animal evolution doesn't minimize the role of oxygen, it actually supports it, explains Graham Shields-Zhou, a geochemist at the University College London who was not involved in the research. "The phosphorus cycle is assumed to be very important for oxygen in that if you have more phosphorus available, you can drive more productivity," he says in a phone interview with the Monitor. That's productivity of the simple organisms that lived on Earth before this major transition.
With more of those organisms living, there would also be more of those organisms dying. And that means more organic carbon (that is locked away in their bodies) getting buried in the sediments. That process ultimately yields oxygen, Dr. Shields-Zhou explains. "So if you have a phosphorus-limited world in the Precambrian, then the potential for building up oxygen is much lower." But with increased phosphorus just before the Cambrian, oxygen levels have the potential to explode.
But where was all that phosphorus before?
There are many ways that phosphorus can be locked up and kept out of the environment, Reinhard says. Iron may have something to do with it, he says, as phosphorus can be incorporated into iron-based minerals.
And that makes sense, Shields-Zhou says, because in a low-oxygen system, phosphorus can be "more efficiently scavenged" by iron.
"We're kind of imagining this oxygen-lean, iron-rich world in which iron is kind of locking up nutrients, like phosphorus, and preventing the biosphere from flexing its muscles," Reinhard says. "This may have been the norm for maybe 3.5 billion years of Earth's history. So the question now is how did we flip over to the modern world?"
That is still unclear, but Reinhard suggests that disturbing that system may be the key.
Explaining Snowball Earth, too
An extreme shift in the chemistry on Earth and in the atmosphere wasn't the only dramatic change the occurred around the time of this transition. Major glaciations are thought to have completely covered the Earth in ice in what is called the Snowball Earth some 716 to 635 million years ago or so.
And that is likely linked to the chemical changes, too, says Shields-Zhou.
Remember how more organic matter is being buried with increased production from increased phosphorus? Well, that buried organic matter doesn't just generate oxygen, Shields-Zhou says, it also draws carbon dioxide out of the atmosphere. And, as carbon dioxide is a greenhouse gas, pulling lots of it out of the atmosphere could plunge the world into an ice age.
Settling the story
"You have this confluence of all these very radical changes in the chemistry of the ocean-atmosphere system, and the climate system, and the complexity of the biosphere all around the same time," Reinhard says, but it's long been unclear exactly what's driving what.
"We think that this change in the nutrient availability is actually a really critical piece in understanding why all of this crazy stuff happened so fast," he says, which would suggest that "the Earth today is really a result of a short and dramatic change in the phosphorus cycle, 800 million years ago."
But at this point, that's just a hypothesis.
"I don't think they're changing the story yet," Shields-Zhou says. "I think it's sort of a shot across the bow to remind us that phosphorus is important. I don't think they've clinched that case yet, but they've provided at least the evidence that we can start using to make this case."
Reinhard agrees that a lot still has to be done to figure out just how the shift in the phosphorus cycle might be related to the other chemical, biological, and climatic changes that occurred at the time.
And in addition to painting a picture of the history of life on Earth, he says, understanding how this system works could also provide key insights in the hunt for life on other planets.
"I think that this sort of provides a case study for better understanding the links between the phosphorus, iron and oxygen cycles that really allow for an oxygen-rich atmosphere to emerge and be stable on long periods of time," Reinhard says. And such a long-term oxygen-rich atmosphere is a key condition for life, as we know it.