Paleo Exo-Earths

As we discover Earth clones around nearby stars, they will be of a variety of ages.   SIM will be able to locate several Earth-sized terrestrial worlds in our stellar vicinity.  Its data will provide the initial characterizaton of these planets since SIM will be able to determine the mass, as well as the orbit, of each world.  Those findings from SIM will provide the targets for follow-on missions that will analyze the light reflected from each Earth twin.  These data will enable us to determine whether that planet is habitable, and in what geologic age that planet happens to be in. 
    The image below shows how the Earth appeared during various time steps, back to 750 million years before present.  During those eras, the Earth's environment was sometimes different from that of today.  In fact, sometimes quite different.   For example, the Earth has experienced numerous Ice Ages during which ice caps covered significant fractions of the continents.  At the extreme, Earth was essentially blanketed by ice during several episodes that are referred to as "Snowball Earth."
   So, as we venture out to study nearby Earth-like worlds, both virtually and, one day, in person, we should expect those terrestrial worlds to be in any of a number of geologic phases.  Statistically, most may be in the "Pre-Cambrian" stage.  On the Earth, this refers to the time before multi-cellular life evolved.  This stretches from 500 million years ago back to the Earth's origin at 4,500 million years ago.  It is thought that one-celled life developed at anytime from 4 billion to 2 billion years ago.  Therefore, many of our New Earths may be "populated" by algae! 
 

   There is another possibility.  Some of the Earth clones may be in "future" geologic ages; perhaps, post-civilization.  We do not know the lifespan of a typical intelligent species, such as human beings.  It is quite possible that there are the ruins of extinct civilizations scattered across the Milky Way.  However, we won't know until we look, and SIM is the first step in that long voyage of exploration.