Mica minerals include biotite (on the left) and muscovite (on the right.) Click on image for full size (304 Kb) Lisa Gardiner/Windows to the Universe

One day last spring, scientist Helen Hansma was splitting mica apart under her microscope. Mica is a type of mineral that splits into very thin sheets. Because it has this special pattern of breaking, called cleavage, mica is layered like little sandwiches. As Dr. Hansma peered into her microscope and split the mica layers apart, she found that the inorganic mica minerals were covered with organic material – the molecules of living things, called biomolecules.

"As I was looking at the organic crud on the mica, it occurred to me that this would be a good place for life to originate -- between these sheets," said Dr. Hansma. The tiny space between layers of mica could have been just the place for molecules to organize into the Earth’s first living cells.

So from the tiny mica layers Helen Hansma developed a hypothesis to explain how life first formed on Earth. She called it the mica hypothesis. The hypothesis claims that life may have developed between the layers of a chunk of mica sitting like a many layered rocky sandwich in ancient ocean waters.

When scientists develop a new hypothesis, they test it by looking for evidence. Sometimes the evidence supports the hypothesis. And other times the evidence points out that the hypothesis is not correct. In the case of the mica hypothesis, the evidence that Dr. Hansma has found so far supports it.

Dr. Hansma compared the spaces in the mica sandwich with the insides of living cells and found that they have things in common. Both are rich in potassium and both are negatively-charged. She also found evidence that the spaces in the mica sandwich would be a good environment for biomolecules and evolution. For example, the space between sheets of mica is an isolated place, and isolated places are known to be ideal spots for the process of evolution to happen rapidly. Plus, the layers of a mica sandwich move around as the surrounding water warms and cools or as ocean currents move to and fro. These little movements may have helped rearrange molecules and the bonds between them, forming biomolecules, the building blocks of life, out of other molecules.

Ancient rocks that record what happened several billion years ago also support the mica hypothesis. In Greenland, some of the Earth’s oldest rocks contain huge amounts of mica minerals and are found near places where ancient life existed.

The mica hypothesis and its sandwich of biomolecules is not the only scientific hypothesis about how life evolved on Earth. Other hypotheses, which for some reason also refer to food, include the idea that ancient oceans were like a “soup” that contained all of the ingredients needed for biomolecules to form and those ingredients eventually got together. Also, the idea known as the "pizza" hypothesis says that the earliest cells developed on land atop minerals, much like toppings on a rocky pizza. Scientists continue to look for evidence to test all of these hypotheses so that we will someday know more about how life formed on Earth.