Cataloguing the History of Life on Earth – How the Age of Fossils Is Determined

United States Geological Survey

From the very beginning to the present day, the Earth boasts a rich and vibrant history beyond our wildest imaginations. But how do we actually know when everything happened?

Historians can piece together much of the last few thousand years of history thanks to the availability of things like written accounts. For example, the earliest writing was invented in Mesopotamia over five millennia ago, giving us invaluable insights into the dawn of civilization. Where there aren’t written records, however, historians instead must rely on various artifacts to reveal insights into that culture, using no small amount of educated guesswork in the process.

Unsurprisingly, things get exponentially more complicated as we delve deeper into the past. The first time someone put pen to paper may have been 5,200 years ago, but no triceratops has walked on Earth for 66 million years. We’ve now entered the realm of an unimaginably long time spans. However, triceratops, which was one of the last of the non-avian dinosaurs, may as well have lived yesterday when we consider the oldest fossils ever found, which date from almost 4 billion years ago.

We now know that life on Earth first appeared in the form of microbes sometime between 3.8 and 4.1 billion years ago. From this point forth, we’ve managed to create an accurate chronology of the rich history of life on our planet. We’ve managed to plot the course of evolution and identify thousands of different species by the time they lived and the time they disappeared, usually long before even our own primate ancestors were even thought of.

Ordering Events by Comparing Rock Strata

Kevin Walsh

Every layer tells a story. Pictured here are rock strata in Cornwall, UK. These ones date from the Carboniferous Period, between 358.9 and 298.9 million years ago.

Until 1905, when radiometric dating was invented by British physicist Ernest Rutherford, there was no way to determine the age of fossils or, for that matter, anything older than a dated written account. Instead, palaeontologists had to rely on relative dating to establish what happened and in what order, but there was no remotely reliable way to tell when the story started. Nonetheless, relative dating does, for example, tell us that stegosaurus lived long before triceratops.

Imagine a layered cake. Obviously, the bottom layer of the cake is put in place first, the following added in succession. Rocks work in the same way, in which individual layers of rock (strata) define different geological timespans, such as aeons, eras, periods, epochs and ages. Distinct layers, characterised by their unique compositions, are left behind by volcanic activity (in the case of igneous rock) or by water activity (in the case of sedimentary rock).

Well-preserved rock strata have been found all over the world. For example, the Colorado Plateau of south-eastern Utah exhibits rock strata spanning some 150 million years from the Permian to the Jurassic Periods. In this case, the lowest layer of rock represents the Permian, the middle layer presents the Triassic, and the final layer presents the Jurassic. Any fossils in the formation can be confidently attributed to the period defined by the layer.

Palaeontologists have been relying on relative dating for more than 200 years, when it was first noted that individual layers of rock were home to distinct fossil organisms. In other words, you’re not likely to find a triceratops fossil in Permian strata, simply because the Permian period ended long before they existed. However, there is one major drawback of relying on relative dating alone: it doesn’t give any hint as to when events occurred, instead only revealing the sequential order of things.

Revolutionising the World of Palaeontology with Radiometric Dating

Ruth_W

Radiometric dating using carbon-14, a radioactive isotope of carbon, has allowed historians to determine that the ancient village of Skara Brae in the Orkney islands was occupied between 3180 and 2500 BC.

Thanks to radiometric dating, we now have a remarkably accurate calendar detailing the history of life on Earth, including even the age of the Earth itself. This method relies on a natural phenomenon known as radioactive decay. Unstable isotopes of an element, such as carbon, are inherently radioactive. Radioactive atoms decay over a predictable period of time known as half-life, which is the time it takes for half of the atoms in the group to decay into stable isotopes.

When determining the age of organisms up to around 60,000 years old, we use carbon-14 dating. Carbon is an unstable, radioactive version of the normal carbon isotope carbon-12, which is abundant in all living things. However, organisms stop taking in more carbon when they die, but any carbon-14 atoms in them continue to decay over time. By comparing the ratio of carbon-14 to carbon-12 in the sample, they can determine, quite accurately, when the organism died.

The problem with carbon-14 is that it has a half-life of only about 5,700 years. This means that the radioactive output of the isotope drops by half over that amount of time. After 10 half-lives, or about 57,000 years, carbon-14 has less than 0.1 percent the radioactive output of what it did when it formed. As such, carbon-14 dating cannot be used reliably on specimens that are more than around 50,000 years old. This makes it utterly useless for determining the age of dinosaur fossils.

Following the Clues

Glen Larson

This image shows Cretaceous and Paleogene rock stata in Alberta. The boundary between the two contains unusually high amounts of iridium, which is now believed to have been left behind by the impactor that wiped out the dinosaurs and ended the Cretaceous Period.

Because the last non-avian dinosaurs disappeared 66 million years ago, there won’t be a single carbon-14 atom left in their remains. Even if there were, it would not originate from that organism. Additionally, rather than contain the actual remains of the organism, fossils are more often an imprint in the mineral deposits in which the it died. As such, there’s often not any organic matter left to date. Instead, it’s often necessary to determine the age of the rocks to determine the age of the organism.

While carbon-14 is great for dating many human remains and human-made artifacts, palaeontologists need to rely on isotopes with much longer half-lives to determine the age of fossils. After all, some fossils are billions of years old, and the Earth itself has been around for some 4.7 billion years. Unfortunately, although some isotopes have half-lives running into the billions of years, they’re not present in the fossils themselves or, in most cases, in the surrounding sediments either.

The best isotopes for measuring the age of extremely ancient objects are those with immense half-lives. For example, the Uranium-238 isotope takes a staggering 4.468 billion years to halve its potency. However, isotopes like these are usually only found in igneous rock, which is solidified magma. Unsurprisingly, any organism unlucky enough to be caught up in a magmatic flow would be completely destroyed, leaving no traces behind. As such, it becomes necessary to focus on other rock layers.

Putting the Story Together

MathKnight

Thanks to a combination of stratigraphic layering and radiometric dating, we can now tell, with a remarkable degree of accuracy, when this triceratops was roaming across what is now North America.

Volcanic ash also contains igneous rock, which may contain the radioactive isotopes necessary for dating. However, fossils are normally only preserved in sedimentary rocks, which are formed when soft materials, such as clay and sandstone, are deposited by bodies of water. Unfortunately, sedimentary rocks don’t tend to contain enough radioactive isotopes for accurate dating. Instead, scientists need to look for igneous rock layers, such as those formed by volcanic ash.

Let’s say our triceratops died while drinking at the floodplains of the Hell Creek Formation in Wyoming. Shortly after it died, the river floodplains deposited sand and clay over it, protecting its remains from the elements. Over millions of years, this soft material turned into sedimentary rock. Eventually, the rock erodes to expose a fossil to a lucky palaeontologist. However, to approximate the age of the sedimentary layer, scientists will need to determine the age of any neighbouring igneous layers.

For the purposes of simplicity, let’s assume that there are layers of igneous rock, created by volcanic ash, both directly below and directly above the sedimentary layer. By determining the age of these rock strata using radiometric dating, we can find a minimum and maximum age of the sedimentary rock between them and, consequently, determine the approximate time when our triceratops was last seen wandering around.

Conclusion

The above is a highly unlikely scenario, but even fossilization itself is an extremely rare occurrence that requires a very specific set of environmental conditions. Nonetheless, the abundance of fossils discovered over the last two centuries and the development of modern technology have allowed us to garner incredible insights into the distant past. Although estimates for the age of different dinosaur species and specimens is presently only accurate within the tens of thousands of years, that’s not bad when we’re talking about tens of millions!