Last time I left you with the cliffhanger that the oldest rocks known to man show the chemical signatures of life, rather than non-life. In other words, as far back as we can study geological formations, life has been here. Let me put this in perspective.
After the earth was formed roughly 4.5 billion years ago, it was unfit for life (or formless and empty). There were many impactor events that would have melted the earth's crust and vaporized all of its water in this early, "hadaean" period, which lasted from the time of earth's formation until roughly 3.9-3.8 billion years ago. The end of this era was particularly harsh, and was called the "late heavy bombardment", when the intensity and frequency of these impactor events were quite high.
These events are planet-sterilizing, so any life that could have existed was snuffed out. (Interestingly enough, there is recent research that argues that some life, if it existed, may have survived. However, this conclusion is controversial at this point. We'll have to see how that theory shakes out over then next year or so.) In addition to destroying all life, these impactors also melted all of the rocks, such that the oldest known rocks on the planet today (which are in Greenland I believe) date to the end of the late heavy bombardment, roughly 3.85 billion years ago.
In these rocks, the carbonaceous materials that we can find all bear the signature of life! In other words, the oldest known rocks date from a period that is also the earliest possible moment that the earth was habitable, and yet they show signs that life existed then! Not only was there no prebiotic soup (see my previous post), but life arose essentially instantaneously.
Wow. OK, next time we'll get to the third point I brought up in my last post: the initial complexity of life.
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