As spacecraft probe the solar system for life, such as Cassini, in and around Saturn and its moon, Titan; and the rovers Spirit and Opportunity on Mars; it’s good to have a reference of just what exactly life is.

A nice, succinct, readable website that contemplates the properties of life is Top 10 Properties of Life. The author appears to be a fellow named John Kyrk. He also has an evolution page that has a slider which can be moved from 13.7 billion years ago to the present, which is really quite interesting. The site is well organized with many embedded hyperlinks that lead to more detailed information. Below is a table from his site with time scales of key moments in the development of life on Earth.

Earth and the formation of life:

 Formation of Earth  4.5 billion yr ago
        Earth coalesced from space dust 
 bombarded by interplanetary dust & comets  4.5 to 4.0 bya
        water source for oceans 
 Heavy comet bombardment stops > life begins   3.9 bya
 Oldest fossil rocks  3.8 bya 
 Life (anaerobes)   3.5 bya 
 advent of oxygen* (evolution of aerobes)  2.0 bya
 eucaryotes  1.0 bya 
 multi-cellularity   0.7 bya 
 early animals   0.6 bya 

There is a lot of information to study on this site. One item I read I didn’t know had a name attached to it is Panspermia; the idea that living microbes colonized Earth from space driven by radiation emitted by stars. This idea, although dating from as far back as the 5th century B.C., was developed in 1908 by Svante August Arrhenius, a Swedish scientist who won the Nobel Prize in Chemistry in 1903 for his electrolytic theory of dissociation.