Old as Dirt: Four Dating Methods Used For Ancient DNA

By: Jim Brewster

Learn how scientists are able to provide an estimated date to the ancient samples we add to the Time Tree.

We’ve all heard about groundbreaking discoveries of archaeological sites that push back the dates of the earliest human settlements further and further. We even hear about human remains that are dated to be extremely old. How do we date the remains and find out their age? Despite popular belief, Tinder, Match, Farmers Only, and other dating sites are not involved. What is involved is a number of pretty neat sciency things, so let’s dive in and find out all about ancient DNA.

Radioactive Decay is the Cornerstone of Dating Methods

There are several different dating methods used in the natural sciences, and they have varying degrees of accuracy and age ranges. Many of them utilize the principle of radioactive decay. So before we jump into them, let’s explain radioactive decay

Isotopes Are Variations of Elements Found in Nature

Just like people, elements come in many shapes and sizes. You probably all remember the periodic table. This table has a lot of numbers, including size, weight, number of protons, neutrons, etc. All of these numbers interact in a…wait for it….number of ways. *ba-dum tss*

The same element can have a number of different varieties that differ in their number of neutrons. We call these varieties “isotopes.”

Some of these isotopes are more stable than others because they eat their Wheaties and have a balanced diet of electrons. When the “weight” of the neutrons is out of sync with the electrons and protons, some bits get flung out as the isotope slowly balances out and stabilizes. These tiny bits of nuclear shrapnel whizzing about are what cause things like radiation poisoning, if the thing they are whizzing through is you.

As more and more material is flung out of an isotope, it “decays” into a more stable form of that element, or even into a simpler element.

One example is uranium-lead decay. Uranium, as you probably know, is a super-dangerous element that is simply radiant, but in a scary way. Give a sample of uranium a few billion years, and it will eventually fling off all the excess neutrons and settle down into a nice, stable adult lead atom. It does this via a method called “half-life.” The fact that it takes so long to decay and fling off so many particles is one reason it is ideal for creating nuclear energy.

In any given amount of uranium, half of it will have decayed into lead in 4.5 billion years. Then half of the remaining sample will take another 4.5 billion years, and so on. The half-life is set and known by scientists.

The important takeaway here is that you can take a sample of uranium and measure the amount of uranium vs. lead to know how long that uranium has been decaying. In other words, you can tell how old that chunk of uranium is.

Different elements “radiate” particles in different ways and at different rates. They each have their own half-life. Each isotope of each element also has a known half-life, and we can use this attribute of elements to date them.

Four Different Methods of Dating

Now that we know all about radioactive decay, let’s take a look at a few different types of dating methods:

• Carbon Dating: Measuring the ratio of carbon-12 to carbon-14.
• Argon Dating: Measuring the ratio of Argon to Potassium
• Stone Patina Dating: Measuring the buildup of patina on a surface
• Dendrochronology: Measuring tree growth by counting rings and comparing to native trees at the excavation site.

Carbon Dating Measures How Long a Human Has Not Been Breathing

One well-known example of radioactive decay is carbon dating. Carbon-12 is a stable carbon isotope with 6 protons and 6 neutrons in the nucleus (hence 12) that are neatly balanced by 6 orbiting electrons. It’s pretty abundant, so it is super handy to use as the main component of carbon-based life.

Since we are focusing on archaeology, let’s just lump all this carbon life under the term “humans” to make this a nice, well-balanced, human-centric article. Coincidentally, since we are all made of carbon, every date you have ever been on has technically been a carbon date. *ba-dum tss*

I know what you’re thinking: “I have met a lot of humans that I would definitely not describe as stable.” Well, dear reader, you’re right. The problem is that people have to take this nice, stable carbon-12 and screw it all up by breathing.

Carbon is in the atmosphere as CO₂, and humans absorb this when they breathe. This adds extra particles and turns their carbon-12 into the unstable carbon-14. There is a theory that all this carbon-14 eventually leads to death. That’s right. Breathing is not your friend. The alternative, of course, leads to death much more quickly, so I guess it’s a cost-benefit thing.

The entire time humans are alive, they build up more and more carbon-14, but that stops once they stop breathing. Carbon-14 eventually decays into carbon-12 and then onto nitrogen-14, so you can use the good ol’ half-life method to tell how old a human body has been not-alive.

While not breathing may not be your friend, the lack of breathing is definitely a friend of archaeologists, as it is helpful for gauging the age of archaeological remains. The downside of this method, however, is that the maximum half-life of carbon-14 is around 60,000.

“But Jim! If the upper limit is 60,000 years, how do they come up with an age like 2 million years?” Thank you for pointing that out. Good question. The good news is that everything around us is crumbling away, and we can use that inspirational world view to utilize other elements with longer half-lives.

Argon Dating Can Provide Information Further in the Past

If there is no carbon-14 left in the remains, there is always all the stuff around the remains that you can measure. For example, the soil the remains are found in probably contains potassium. Potassium eventually throws off all the irresponsible particles of its youth and matures into a responsible, upstanding Argon isotope.

The downside is that potassium’s half-life is 1.25 billion years. The minimum age we can reliably verify from this half life is 20,000 years. Younger than that, and the amount of argon is so small we can’t reliably measure it. This long time scale means that there is a proportionately larger margin of error. It can also be affected by a number of other factors, like the introduction or subtraction of potassium or argon into the sample.

Another method is Argon-40 to Argon-39 dating, but it is somewhat dependent on determining quantities established through potassium-argon dating and subject to other variables. It does narrow the field to something like 268,000 thousand years. It can also have a wider age range than carbon dating.

Stone Patina Dating Can Help Provide Context From the Environment

Patina refers to the buildup of material on a surface. This can be any material on the surface of something, like:

• sediments
• rinds
• soot
• contaminants
• the discoloration inside of your coffee cup

Whenever something is exposed to environmental factors, it starts to build a patina. By measuring the thickness and age of this patina, you can determine how long it has been exposed to air relative to the surrounding material.

For example, a rock carving will have less patina in the carved lines than in the rock they were carved into. You can date the rock using other methods, like those described above, to tell how long ago that carving was made. This, of course, is subject to those same limiting factors.

Say you have a stone tool made of chert. You can measure the patina on that tool to tell how long ago that chunk of chert was chipped and fashioned into a tool. Of course, if some thrifty human finds, say, a worn old hand ax and reshapes it into a spear head, then the patina will be reset to the age of the new tool.

Dendrochronology: Answers May Come From Counting Tree Rings

This one is not based on radioactive decay but on the growth patterns of trees. More commonly known as tree ring dating, dendrochronology compares the thickness of tree rings to build up a chronology like one long barcode stretching back in time.

Each year, a tree grows a new ring. The thickness of the ring is dependent on the climate. This means all the trees of the same species in the same area will grow the same size ring each year. You can line up these rings in overlapping segments to build a chronology that stretches back hundreds or thousands of years. This chronology is specific to a climate/region, so some areas have better records than others.

This means that if you find a cut beam for a roof, post, etc., you can compare the ring pattern to the known chronology to pinpoint the age. The downside of this is that it can tell you the age when the tree was cut down, but this may differ from the age of a site. What if the log was found in older, unrelated ruins than our dig site and was simply repurposed into a structure at the newer site? What if the structure had existed for generations before the individual died?

This technique was actually developed in my home town of Flagstaff, Arizona, to study sun spot activity in the world’s largest ponderosa pine forest. Only for the fact that you can see part of the forest in the background, and no other vainglorious reason, here is a shameless picture of me standing atop the highest mountain in the state, just outside town.

There Are Restrictions to Radiometric Dating

It’s always a bit of an age range with any dating method for a variety of reasons.

You Must Locate The Pure Isotope In Your Sample

One factor in determining the radiometric decay rate of any element depends on getting a pure, undecayed sample of that element in the first place. Elements like to mingle and make compounds and complex molecules, so you have to find that element, and that element alone, to test.

Then you have to observe that element either by measuring its ratio over time or by measuring the radiation coming off the element to get an estimate of how long it would take to decay. It’s not feasible to sit around watching an element for thousands of years, so we have to do some math. The larger the control sample and the more precise the measuring device, the more precise the date you can get.

Measuring devices have improved over the years, and more control samples have been studied. This has improved the accuracy of the readings, but there is still a margin of error.

Your Sample Must Be Well Preserved

Once you have the standard decay rate nailed down (as best as possible), applying it in the field presents its own set of obstacles.

First, you have to make sure that whatever you are testing has not been contaminated by surrounding materials. For example, make sure the bones you test are clean so you don’t inadvertently add some soil with decayed plant matter to them. That soil is going to be younger, so it will throw off the readings.

Next, you have to factor in how it has been preserved. Has it been exposed to other types of radiation? Sunlight? Nearby sources of radiation in the soil? Near a waste dump? Handled by the Incredible Hulk? All of these can alter the rate of decay and need to be taken into consideration.

You Must Calculate a Margin of Error

All of these factors combine to form a margin of error. Often, when you read the age, it is listed as something like “20,000 years plus or minus 300 years.” This range can vary from one sample to the next, and improvements in technology are always reducing the range, but it is still a factor.

Having more than one sample helps to corroborate the ages we find, and being able to use more than one method of aging helps even more. Because there is a margin of error for each type, having a large sample size and other data to compare can help us narrow down the range and verify the accuracy of it. So now that we know how to date a sample, we need to find some remains to date. I would recommend updating your dating profile.

“But Jim! I still have more questions; can’t you please spend another 1.25 billion years explaining this next topic?” Well, yes, dear reader, I certainly can!

Consistent Environments are Ideal For Preservation

Some climates are better suited than others for preservation. Remains can be found in pristine or near-pristine conditions in wildly different environments, including:

• tundras
• deserts
• underwater
• cemeteries

While vastly different, they all have one thing in common: consistency.

The biggest detriment to preserving organic matter and material culture (the stuff people make) is variation in hot/cold and wet/dry. Whenever organic matter is frozen, ice crystals form that damage cells. Repeated thaws and freezes damage the cells more and more. This is why meat that has been thawed, cooked, frozen, and thawed again is not as good as freshly cooked meat. The same goes for when something is waterlogged, then dried, then soaked again.

• Ötzi the Iceman was frozen and never thawed.
• The mummies of Egypt were dried and stayed that way.
• The Bog mummies of England were submerged in stagnant water and never surfaced.
• Wooly mammoths fell into tar pits and stayed permanently encased.

These same principles exist for things people make: wicker baskets, grass huts, etc.

Whatever the climate is, as long as it does not experience a great deal of variation, it is ideal for preservation. This is one reason you may often hear about remains found in caves.

Not every area has conditions well suited for preservation. The middle of a rainforest, for example, is not a great place. This means that some areas have a more robust sample set to draw from.

What does this mean for geneticists? Ancient DNA is more abundant in some areas than others. That means that some haplogroups like E and J (found most abundantly in the Middle East and North Africa) have more samples than other haplogroups like O and P (found most abundantly in Southeast Asia and Oceania.)

Advancements in Science Provide More Data For a Small Pool of Ancient Samples

I had an archaeology professor in college who told us that really only 10% of the stuff that people make and people themselves are preserved in the archaeological record, and funding only allows us to find about 10% of that 10%. So we are talking about less than 1% of all the people and places that ever existed are available to draw conclusions from.

As time goes on, we are making more and more discoveries that revise previous conclusions and expand our knowledge. The availability of genetic testing has exploded in the last 20 years and has allowed more and more ancient DNA to be found and analyzed.

Improvements in dating techniques mean that we have a more accurate assessment of absolute ages than we did even 10 years ago. Advanced sequencing techniques allow us to accurately test older and older samples and to get more accurate genome sequencing. When you factor in all the caveats and limitations of the techniques described in this article, it is really quite amazing that we can pinpoint the age of an ancestor to within a few hundred years.

Combining This Information With Genetic Genealogy Provides Insights on Your Ancient Ancestors

“But Jim, now I feel like we can’t trust any of this data!” Well, dear reader, let me assure you that you can trust the data. The fact that we know the limitations and potential age ranges means that we know what to compensate for and what the margin of error we might see is. We have been able to prove the data by comparing it to other types of data, other sites, and other labs.

We know what is reliable data and what is not because we have seen plenty of both. The SNPs on the tree with a narrow age range have them because the sample size, both ancient and modern, is so robust that any irregularities can be easily identified.

The reliability of these methods is exemplified by the lack of data and the broad time ranges of some branches. This is probably not very satisfying news for those on the less well-represented branches of the trees, but it means that we are only providing data we are confident in. Personally, I would be more wary of someone promising exact down-to-the year estimates on slim to no data. Even these branches are slowly growing, and the time ranges are slowly narrowing.

All of these amazing techniques come together to form our Time Tree, and when you factor in all the caveats and limitations of the techniques described in this article, it is really quite amazing that we can pinpoint the age of an ancestor to within a few hundred years. Now, the explosion of testing made possible with NextGen sequencing, the Big Y-700, and testers like you has allowed us to bring this ancient DNA to the present in a way like never before.

About the Author

Jim Brewster

Subject Matter Expert at FamilyTreeDNA

Jim Brewster was born at a very early age and gradually became older. He has been in the genetic genealogy field since 2014 and delivered numerous presentations at genealogy conferences. He has helped with collaborations between FamilyTreeDNA and non-profit organizations and for some reason FamilyTreeDNA decided to let him write stuff too.

With a proven track record of both doing things and accomplishing stuff, Jim enjoys presenting and writing about genetic genealogy methods and the science of DNA testing. In his free time, he enjoys puns and cat pictures.