The theory of evolution through natural selection is arguably the most important concept in all the life sciences. This fundamental idea has been derived and confirmed through more than 100 years of observations and countless experiments. Since the theory of evolution declares that all existing life forms on Earth are modified descendants of earlier life forms, it follows that there was once a number of species that no longer exist on Earth today. In fact, 99% of all species that ever lived on Earth are currently extinct . That causes a bit of a problem for scientists hoping to examine these species for evidence of evolution, since 99% of their subjects don’t exist anymore.
The best way we have to study organisms that no longer exist is through fossils. Imagine you are a paleontologist that has dug up a collection of fossils from horse-like creatures like the ones below (Fig. 1). You can look at the skeletons, particularly the bones in the feet and in the teeth, and arrange the fossils in what appears to be gradual changes from one form to another. The theory of evolution would predict that Hyracotherium *is an ancestor of *Mesohippus, which is an ancestor Merychippus, and so on, or vice versa. Either direction would be consistent with the theory of evolution and you wouldn’t know which way was correct. Alternatively, if the organisms from those fossils existed in any order other than top to bottom or bottom to top as shown in Figure 1, this finding would not be consistent with the theory of evolution. This is why being able to estimate the age of a fossil is so incredibly important to testing the theory of evolution.
Fig. 1 A diagram of tooth and hoof morphologies of ancestors to the modern horse. By “Mcy jerry”, Wikipedia commons
How do they estimate the age of fossils?
There are several methods for estimating the age of various objects. Which method you need depends on the type of material of interest and the approximate age range of the material. I’ll highlight just a few ways scientists have determined the age of fossils.
Relative dating Using Stratigraphy
The simplest and possibly oldest technique for estimating the age of objects is based on layers of rock in the Earth’s crust. Layers of rock closer to the surface are younger while deeper layers are older . Following this principle, one could estimate the relative ages of multiple fossils if they were collected from different layers of rock. Unfortunately, you still wouldn’t know exactly how old either of those fossils were using this method.
Fig. 2 Stratigraphy – Layers of rock are deposited on top of each other over time. A fossil collected from the green layer would be older than a fossil collected from the purple layer. (Source: Nature Education)
The term “carbon dating” is often brought up when referring to age estimation of fossil records. Carbon dating is just one example of radioactive dating. Radioactive dating is one method for estimating the true age of a rock or fossil. Radioactive dating makes its measurement based on the amount of radioactive isotopes present in the sample. For example, the element carbon has 6 protons, but can have 6-8 neutrons. Whether the carbon atom has 6,7, or 8 neutrons determines whether that particular atom is a C12, C13, or C14 isotope. The C12 and C13 isotopes are stable, meaning they will remain that way indefinitely. The C14 isotope, on the other hand, is unstable and will gradually turn into Nitrogen 14 (6 protons + 8 neutrons becomes 7 protons + 7 neutrons) over time. The rate at which C14 turns into N14 is constant and reliable. Using this knowledge, scientists can measure the amount of C14 and N14 in a sample. If the sample has a large amount of N14 and a small amount of C14, then the sample is old because much of the C14 has had enough time to decay in N14. However, after 50,000-70,000 years, nearly all of the C14 will have decayed into N14, and thus carbon dating is no longer valuable for estimating the age. So, carbon dating could not be used to estimate the age of a T. rex fossil because they lived more than 70,000 years ago. Although I used relative terms in this example, the rate at which C14 becomes N14 is very precise and thus very accurate estimates of age can be calculated using this method (Figure 3).
Fig. 3 A graph depicting the rate of decay of C14 over time. This precise rate of decay allows scientists to estimate the age of a sample based on the proportion of C14 remaining (Source: BBC.co.uk)
The same concept applies to other elements such as potassium (K) which decays into argon (Ar) at a slower rate, allowing for age estimations going back up to a billion years ! The major limitation to K-Ar dating is that potassium is not found in high enough levels in fossils to perform the test. Instead, this method can be used to date rocks and sediment surrounding where a fossil was found to give an age range for that fossil.
Determining the age of fossils has been an essential step in verifying the theory of evolution. Modern technological advancements have provided an arsenal of dating methods that can be used in conjunction to discover valuable insights into the past. What’s more, many of the things we learn for our planet’s long history may be able to teach us about how to plan for the future.
McKinney, Michael L. 1997. The Biology of Rarity: Causes and consequences of rare-common differences. Springer Science and Business Media. Page 110.
Peppe, D. J. & Deino, A. L. (2013) Dating Rocks and Fossils Using Geologic Methods. Nature Education Knowledge 4(10):1
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