Field of Science

Evidence of early life on Earth?

How old is life on Earth? The Earth itself is 4.5 billion years old, give or take a few million. The oldest rocks on Earth are 4.28 billion years old, and the oldest fossil evidence of life is about 3.55 billion years old. The chemistry of the universe seems to be very life friendly leading some to speculate that life of simple sorts may be very common especially because 20 years ago planets around other stars were unknown and now some 5000 have been detected. So planets are pretty common too. Back to Earth, prior to the oldest fossils there are some biochemical signatures of life, skewed carbon and sulfur isotope ratios now only associated with metabolic activities.  Now some zircons have been found that are 4.1 billion years old, and some graphite (carbon) inclusions showing skewed carbon isotope ratios of the type associated with photosynthesis. This has nothing to do with radioactivity (carbon 14), but the primary, and very ancient carbon fixing enzyme nick-named "rubisco" has a preference for carbon 12 over the much less common carbon 13, so in carbon from an organic source the ratio is skewed toward carbon 12, i.e., there is more present than expected from its 90:1 natural ratio. This may seem like rather skimpy evidence, but to say this isn't evidence of life requires that you explain the carbon isotope skew, how it happened in the absence of life. This is how science works. You don't get to deny what is known.

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