Scientists Use Primitive Site to Model Life's Beginnings

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.


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