What Do You Know?

One in a series of letters to my son that I wrote to strengthen him as he began his high-school years

Dear Son,

As you read the evolution chapters in your biology textbook, keep in mind the difference between interpretation and fact. Also watch for generalizations and omitted details. 

Your book begins its discussion of evolution on page 252 with a section on "The Age of Earth". Right away you "learn" that Earth is 4.5 billion years old. Is that a fact? No, it's really an interpretation. The age is presented as fact, but the very next section shows that 4.5 billion years is really an interpretation of some facts. 

The next section is "Measuring Earth's Age". Paragraph 1 says that "scientists have estimated" Earth's age by measuring the amount of radioactive isotopes in, well, the paragraph doesn't say. The phrasing "scientists have estimated" is a generalization. Other phrasing in the paragraph points to omitted detail. Here are some questions you could ask yourself after reading that paragraph:

  • Who are these scientists?
  • Which isotopes are we talking about?
  • What objects did they test? Presumably they looked at rocks, but the paragraph omits mention of that.

Paragraph 2 goes on to give an example of potassium-argon dating, involving the decay of potassium-40 to argon-40 and calcium-40. There is nothing to link paragraphs 1 and 2. Was potassium-argon the method used to compute the 4.5 billion years? Your textbook implies that from the juxtaposition of the two paragraphs, but you shouldn't assume something not stated explicitly. 

In fact, one of the earliest people to compute a 4.5 billion-year age was Clair Patterson, and he used Uranium-Lead dating as a basis for that age. Why then did your textbook authors choose potassium-argon dating for their example? We don't know. The authors do not explain why they chose the example they did. 

(By the way, Clair Patterson tested a meteorite -- a rock that crashed to Earth from space).

My goal in this letter is only to get you thinking about what you really know, personally, versus what you read in your textbook. For example, here is what I personally feel that I know after reading page 252:

The authors believe that Earth is 4.5 billion years old.

The fact I hold in my head is that "The authors believe...". We know they believe in a 4.5 billion-year age, because they said so in print, in their book, that they wrote. 

Make some time in life to dig into details when a topic is interesting or important. Research the trail from facts to interpretation. Nail down specifics. You'll learn the most when you do those things. And it's fun too! Isn't it cool to know that Clair tested a rock from space to determine the age of Earth?