extinct 440.ext.002 Louis J. Sheehan, Esquire

On Earth, such extinct life can be found in the form of microfossils and stromatolites. Such forms, as found in western Australia, are the oldest evidence of life on Earth, dating from 3.5 billion years ago. Microfossils are individual fossilized organisms (typically algae), as much as a few millimeters in diameter. Stromatolites are formed when layers of microbial organisms in shallow lakes or pools are covered with sediments. The organisms migrate toward the light after being covered, and the remaining organic material forms a characteristic layered or domed structure.

Stromatolites are important because they may be large enough to be seen by lander (or perhaps even high-resolution orbiter) cameras, and so some researchers have suggested searching for them near features that appear to be ancient lakes or bays. While definitive proof of biological origin would require microscopic imaging or sample return, the discovery of such features would lend credibility to the idea of extinct life.