AP Biology · Unit 7 · Topic 7.12
Three dates from rocks bracket when life began. A fourth line of evidence says what the first genetic material probably was.
Learning objective 7.12.A Describe the scientific evidence that supports models of the origin of life on Earth.
Start with what is firmest. This topic contains one set of claims that almost nobody disputes and one set that is actively argued about, and it is worth being able to tell them apart. The dates are the firm part. They do not say how life began — only when it must have, and the evidence for them is not biological at all.
A single date from a single measurement would not be worth much. What makes these dates firm is that decay systems with completely different rates give the same answer for the same sample.
| Decay system | Half-life, billions of years | Half-lives elapsed | Parent isotope left | Age it gives |
|---|
People sometimes object that radiometric dating must be unreliable because carbon dating cannot reach these ages. That is true and it is not an objection: carbon-14 is the wrong instrument for the job, in the way that a kitchen scale is the wrong instrument for weighing a ship. Choosing a clock whose half-life is comparable to the age being measured is part of the method.
Every model of the origin of life has to get from simple chemistry to something that reproduces and inherits. The route is usually drawn as four steps. They have been investigated separately, and they are not equally well supported — so this tool marks how strong the evidence is for each one, and what is still open.
What has been shown:
What is still open:
| Step | Produces | Evidence |
|---|
Whatever the first genetic material was, it had to do three things. Score the three molecules we know against those three requirements and only one of them is left standing — which is the whole argument for the RNA world, and it is a matter of chemistry rather than of preference.
The commonest objection to any of this is a probability calculation, and it is worth doing properly rather than waving away. Both halves are computed below.
| The calculation | Result | Is it correct? |
|---|
This is the most important sentence in the topic, so it is worth reading twice. The reply to the odds argument is completely convincing about everything that happened after there was heredity, and says nothing whatever about how heredity itself started. That is step 4, and step 4 is the one marked weakest. Being able to say precisely which part of an argument a piece of evidence supports — and which part it does not — is most of what scientific reasoning is.
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