You can find the first post in this series here.
Before we can really dig in (if you’ll forgive the pun) to what the fossils tell us, we need to establish a timeline. Many (most?) detractors of evolution maintain that the earth is less than 10,000 years old, because if you take the Bible literally and go backwards through its timeline, you’ll find that creation should have happened somewhere between 6000 and 10,000 years ago. So this is primarily a faith position, and not something based on scientific reasoning. But I don’t want to simply dismiss it out of hand — let’s look at the evidence that would show the earth and universe are much older than a literal reading of Genesis would suggest.
In order to measure time, you need a clock. This is no less true when measuring times on an evolutionary scale. And as it so happens, we have a number of clocks that help us do this. It’s well known that you can tell the age of a tree by its number of rings. But those rings also provide additional details, like whether or not a specific year was good or bad for trees. Good years with lots of rainfall differ substantially from years of drought. Some summers are especially hot — some winters are especially cold. There are even more extreme weather events like El Niño, or large volcanic eruptions. All of those events affect the way a region’s trees grow, and this can be seen in the trees’ rings.
Our oldest living trees measure their age in the thousands of years, but these growth patters in tree rings allows us to back even further through overlaps. By comparing the ring pattern of living trees (whose age is known) with trees that have died some time back, we can match the patterns in a way that allows us to figure out when that tree lived. Once its timeline is established, we can compare it to even older trees and establish their timelines, working our way back through the millennia. This is called dendrochronology. Currently, the longest chain that’s been worked out goes back a little over 11,000 years from now. One of the nicest things about tree rings is that it is year-specific. Whereas many of our other dating techniques are accurate to within a certain standard deviation, tree rings can take us to the exact year. That makes it very useful in comparing the results of techniques like carbon dating.
Like tree rings, glaciers have layers of ice and sediment that can be broken up by seasons or years. The same can be done with coral reefs, making both of these very handy in measuring time as well.
Carbon dating, which almost everyone has heard of, is a type of radiometric dating. Radiometric dating is highly useful, and it works with many more elements than just carbon. I don’t want to get too technical with this, but I think it’s important to have an idea of how radiometric dating works.
Elements come in different versions known as isotopes. Some of these isotopes are unstable, meaning that over time, they will degrade into different elements. What’s cool here is that we know the rates at which these various isotopes degrade, and it’s different for each element. The decay rate is known as the “half-life.”
For example, carbon-14’s half-life is 5,730 years. This means that a specimen with 10 grams of carbon-14 at death will have 5 grams 5,730 years later. In another 5,730 years, it will have 2.5 grams. In another 5,730 years, it will have 1.25 grams, and so on. So in this way, it takes a very long time for all of the carbon-14 to change into something else (nitrogen-14, to be exact). But there’s still a limit to carbon-14’s usefulness, and it maxes out at around the 60,000 year mark. So while carbon-14 is very useful for archaeology in examining human artifacts, it’s not as useful in examining evolution.
However, a number of other elements are useful on an evolutionary timescale. Potassium-40, for instance, has a half-life of one billion two hundred sixty million years. Scientists can test the potassium-40 to argon-40 ratio in igneous rock (rock formed by lava) to determine approximately when the rock was formed. Potassium-40 is just one of several elements with long enough half-lives to test the approximate age of the earth, and they currently all agree that the earth is between 4 and 5 billion years old.
Another aspect tied to radiometric dating that indicates an old earth is the following:
Of the 158 unstable [isotopes], 121 are either extinct or exist only because they are constantly renewed, like carbon-14. Now, if we consider the 37 that have not gone extinct, we notice something significant. Every single one of them has a half-life greater than 700 million years. And if we look at the 121 that have gone extinct, every single one of them has a half-life less than 200 million years. Don’t be misled, by the way. Remember we are talking half-life here, not life! Think of the fate of an isotope with a half-life of 100 million years. Isotopes whose half-life is less than than a tenth or so of the age of the Earth are, for practical purposes, extinct, and don’t exist except under special circumstances. With exceptions that are there for a special reason and that we understand, the only isotopes that we find on Earth are those that have a half-life long enough to have survived on a very old planet.
— The Greatest Show on Earth p. 102-103, Richard Dawkins
Did you catch his point? If the earth is not billions of years old, how do we explain the lack of naturally occurring isotopes with (relatively) short half-lives?
Astronomy and the Hubble Constant
The last piece of evidence I want to give for an old earth is astronomy. Once we began to understand just how far away the stars really are, it caused a problem for a literal reading of Genesis. We can see stars that are millions of light-years away from us, meaning that we shouldn’t be able to see the light from those stars if the universe is merely thousands of years old. And it’s more than that. We’ve discovered through things like the Hubble constant that the objects in our universe are moving away from a central point at an ever-increasing rate. If we trace those trajectories back through time, we find an origination point around 13.77 billion years ago.
Hopefully, I’ve successfully shown that our reasons for thinking the earth (and universe) is very old come from many different lines of evidence. These various methods are frequently used against one another to weed out inaccuracies. As far as I know, there are no scientific reasons to think our universe in only thousands of years old.
As I said in the first post I did referencing evolution, I don’t think evolution and religion have to be at odds. Maybe accounts like that in Genesis were meant to be taken figuratively. Or maybe it’s totally accurate and God just made things look like they’ve been here much, much longer. Either way, we should be able to accept that the physical evidence overwhelmingly points toward our universe’s age being measured in the billions of years, not thousands. If we could all come to that conclusion, then maybe we could stop trying to sabotage the science taught in our classrooms.