For around 3 billion years, life on this planet consisted entirely of simple, single-celled organisms, and then quite suddenly something astonishing happened. Around 570 million years ago, mother Earth hosted an unparalleled explosion in life.
Multi-celled plants and animals began to appear and proliferate in a period that may have lasted only a few million years, laying the foundation for life as we know it today.
It is called the Cambrian Explosion by some, or the Cambrian Transition by others, but by whatever name it was surely the most important period in the biological history of the Earth. For years, scientists have been asking why the planet was biologically boring for so long, and why it changed so rapidly during that era.
“One of the big puzzles about the Cambrian Transition is why, for so long on Earth, there were just these microorganisms with no major grazers consuming them,” says biologist James Elser of Arizona State University in Tempe. When grazers finally appeared they came on with gusto. “What took them so long?” Elser asks.
Primitive But Contemporary Model
Theories abound, but it isn’t possible to go back to the Cambrian period and run experiments to see which ideas are most likely correct. Still, it turns out that nature may have provided another way.
In an isolated area of Mexico, about an eight-hour drive south of the U.S. border, scientists have found what they believe to be a modern analogue of the Cambrian era. A series of streams and ponds near the Mexican village of Cuatro Cienegas harbor a simple biological system that appears to have diversified dramatically in only 40,000 years. What really excites the scientists is the presence of living stromatolites, bacterial structures that look a lot like coral reefs that are formed by the life processes of cyanobacteria, also known as blue-green algae.
Prior to the Cambrian Explosion, stromatolites literally covered the Earth, and they are now the most abundant fossils on the planet, according to ASU paleontologist Jack Farmer, who has made them his personal passion. But with the emergence of grazing animals, the stromatolites began to disappear.
“Once animals were in place, this whole complex ecosystem that we have today was possible,” Farmer says. “That changed the nature of the Earth as a habitat for life. It truly was one of the culminating events in biosphere evolution.”
Elser and Farmer are heading up a major research program, sponsored by a $750,000 grant from NASA’s Astrobiology Institute, to study the Cuatro Cienegas area for clues about the Cambrian Explosion.
“Something triggered that,” Farmer says, and the scientists want to know what.
Searching for the Trigger
The researchers will spend three years collecting samples from the area and modifying their environment in the laboratory to see how changes in such things as nutrients impact various organisms. And they will strap on snorkels to spend hours in the ponds, watching snails and other creatures dine on the stromatolites and the algae, just as others must have done so many millions of years ago.
It’s exciting, Elser says, because the stromatolites “are very active. They are growing, they aren’t just sitting there and becoming fossils.”
There are other places on the planet where stromatolites are still alive, but in most cases they are inaccessible.
At Cuatro Cienegas, “you can walk into a stream up to your shins and there they are, right there,” Elser says. The area, he says, is like a time machine.
The fundamental question the scientists would like to answer is this: What could have changed to give rise to the diverse explosion of life?
Was Oxygen Key?
The most widely accepted hypothesis is that atmospheric oxygen finally reached the level necessary to support multicellular organisms.
Even single-celled organisms produce oxygen through photosynthesis, but in the early years of the Earth there was so much iron in the oceans and on the land that the oxygen is believed to have combined with iron and then become trapped.
At some point, presumably in the period preceding the Cambrian, the “oxygen sinks” on the land and in the seas were filled, Farmer says. That caused the levels of oxygen in the atmosphere to increase to the point where animals could evolve.
That explanation “is not entirely unsatisfactory,” Elser says, but he has another hypothesis. Maybe instead of just more oxygen in the atmosphere, something else changed.
Expanding a ‘Marshmallow’ Diet
Complex organisms require carbon, nitrogen and phosphorus to build their tissues, Elser says, and maybe the stromatolites just didn’t have enough phosphorus, which controls growth, to meet the demands of grazers.
“It’s like feeding your kids marshmallows all the time, or sugar water,” Elser says. “The baby’s not going to grow. You have to give him good stuff.”
There is some evidence from the fossil record that ancient stromatolites were indeed low in phosphorus, he adds. It may be that just prior to the Cambrian explosion, enough phosphorus leached out of the Earth’s rocks and flowed into the seas to provide a new resource for the algae that produced the stromatolites.
If the algae became richer in phosphorus, it probably would have been nutritious enough to support multicellular organisms, which in turn could become food for more complex organisms, like snails and eventually fish.
Testing in a Time Machine
All of that could have happened relatively quickly once the “threshold” — be it oxygen in the atmosphere or phosphorus in the algae or a combination of these and other factors — was reached, Elser says.
Using their “time machine,” the scientists now have a chance to test those ideas out, although the findings from this single project are not likely to be conclusive, Elser says. Other sites, similar to Cuatro Cienegas, will have to be located to see if other experiments produce the same results.
Hopefully, it will lead to a better understanding of a remarkable period in the Earth’s history. It may also shed light on how one species could have evolved to the point of asking such bold questions.
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.