Dr. Knoll opened the session by summarizing the challenges of recognizing biological signatures in extremely old rock samples from Earth or Mars.
Everyday experience suggests that the gulf between biology and the physical world is conspicuous.
This impression arises, however, because the biology most familiar to us is principally that of organisms found on distal branches of the tree of life. The difficulty in distinguishing between biogenic and abiogenic features lies at the other end of the tree. Life arose as a self-perpetuating product of physical processes, and it is likely that the characteristics of Earth's earliest organisms their size, shape, molecular composition, and catalytic properties bore a close resemblance to the products of physical processes that gave rise to biology. For this reason, detecting the remnants of early life in terrestrial rocks is difficult. In martian or other extraterrestrial samples, it is doubly challenging.