However, over time it became clear that we were looking at something else. Life on Mars did share an ancestor with life on Earth, but they had diverged billions of years ago, *before* the last common ancestor of life on Earth.
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Life must have hitched a lift from Earth to Mars on debris from one of the many meteorites that impacted earth in the first epoch of Earth's biosphere. Or perhaps life originated on Mars, only to find a far more clement home on Earth. We'll never know for sure.
Either way, some of the adaptations that made life so successful on Earth had only arisen after that, with the result that Earth-life and Mars-life differed in fundamental ways.
They shared much of the same machinery for DNA replication and translation into proteins, as well as some of the core metabolic reactions, but the Martian genetic code used four base pairs to code for an amino acid rather than three.
This meant that Martian life could use many more than the 21 amino acids used by Earth-life, but it came at the cost that the code was more error prone and constantly subject to change, making it difficult for genes to be transferred between lineages.
In many ways Mars-life was better at evolution than Earth-life; by some measures Mars had a higher biodiversity than Earth. But Earth-life had an advantage despite that. It had found a set of 20 amino acids that worked and fixed them once and for all, allowing it to innovate in other areas.
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