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 un