Life
  Definition of Life
  Fossils
  Martian Meteorite

Data Analysis
  Thin Section Analysis
  Isotope Analysis
  Gas Chromatograph Mass Spectrometer
  Infrared Spectroscopy
  Alpha-Proton X-Ray Spectrometer
  Detection of Carbon Complexes
  Labeled Release
  Gas Exchange
  Pyrolytic Release
  Stimulus Response Experiments

The oldest fossil on Earth is only 3.5 billion years old. For an interesting, thorough discussion on the origins of life, the oldest terran foosils, and evidence for a common origin for life on Earth, visit NASA's origin of life page.

This oldest Earth fossil is accepted by the scientific community to be valid, fossilized remains of life. In order to conclusively identify a martian fossil, it needs to be compared against this ancient fossil and the minimum characteristics a sample needs to have in order to be a fossil. A National Academies workshop for the Size Limits of Very Small Microorganisms established a scientific basis for accepting terrestrial fossils. This basis can also be applied to Martain fossils.