How Are Fossils Dated?
The premier listener email is from Roger, who writes: “Any time I talk with my Christian family about evolution, I am teamed up against for even questioning anything, which I find humorous, because I can hold my own when it comes to reason, but I’d like to ask a question about the evolution of humans. i listened to one of your podcasts dealing exactly with the transitions in to Homo sapiens and I learned a great deal, but I was still wondering that when I argue that humans and chimpanzees split at some point in time, how I can get it across to those arguing against me, when they make comments like "why aren't chimps still evolving in to humans!" i usually try to explain it that at some point they split off, but i feel like I’m missing that one big nail to close the coffin on that subject for good. i have also been questioned about why humans aren’t evolving further, and I suggested that in fact we are because of our increasing knowledge that has grown significantly in recent times and other small aspects, but anyway, that is a constant argument with family and friends back home, and if you could help with the "why aren't chimps still evolving in to humans" question better than I can, I’d be very appreciative. Thank you, and keep up the good work!”
This is, as many of you are no doubt aware, a common objection. Given the fact that Roger’s friends and family are Christians, I would suggest making an analogy which they can understand from their religious perspective. According to Christianity, all humans alive today are descended from Adam and Eve. It's also an easily observable fact that different groups of humans have very distinctive physical features (Africans, Asians, Caucasians, etc.) In the same way that we wouldn't expect an African person to give birth to an Asian baby, we also wouldn't expect a chimpanzee to give birth to a human baby. Instead, just like Africans, Asians, and Caucasians all have a common ancestor, chimpanzees and humans have a common ancestor. So asking why "chimps aren't still evolving into humans" makes as much sense as asking why "Africans aren't still evolving into Asians."
Now, this will be a tricky analogy for two reasons. The first is that I’m analogizing different human races with different species. This DOESN'T mean that the different races are in actuality different species. Many creationists have jumped to this conclusion for the purpose of denouncing evolution as racist. Be sure you make it clear that you're not arguing that the different races are actually different species- just that you're using them as an analogy. The second reason is that I’m using a Biblical concept (Adam and Eve) in my analogy, which may make them think that you accept their existence as part of evolution. Again, this is only to illustrate the analogy- if you're familiar with the concept of the Mitochondrial Eve, you might want to bring it up at that point.
Regarding the second objection, humans are indeed continuing to evolve. All species do, it's just that the rate of change depends on the selective pressures of our environment. For the past several millennia, humans have been able to control their environment significantly, and so few physical changes have been necessary. However, a new study has shown that there are several genes which are continuing to evolve, all of which are related to brain function. This makes sense, because the most crucial human organ that's tied into our reproductive success is our brain.
Okay, well I hope that’s helpful, and again, I’ll be looking for your questions. On to this week’s topic.
I’ve also received several emails asking about how scientists are able to accurately date fossils to the millions and millions of years old they often are claimed to be. I’ve been avoiding answering this question because it’s not really a biological issue, but since it is of close interest to evolutionary theory, I figured that it might be a good idea to do an episode on this topic anyway.
We’re going to have to start with some very basic nuclear physics. All matter is made up of atoms. An atom is essentially the smallest unit of matter which can be described as still having unique physical and chemical properties. Imagine that each kind of atom is a different kind of car. So, a hydrogen atom would be like a Mini Cooper, a carbon atom would be like a Toyota Camry, and an iron atom would be like a Chevy Suburban. Now, even though each type of atom is different in terms of size and capabilities, each has the same basic components. In fact, the particles that are smaller than atoms are basically interchangeable. That is, you could take a particle from a carbon atom and trade it with the same particle from an iron atom, and you wouldn’t be able to tell the difference. Just like you could take a steering wheel from a Chevy Suburban and get it to work in a Toyota Camry. I’m not too much of an mechanical expert, but I know that it’s not exactly the same, but it’s close enough for this analogy.
Well, these subcomponents in atoms are three basic types. They’re called electrons, protons, and neutrons. Protons and Neutrons have the same mass, but protons are positively charged, whereas neutrons don’t have any charge at all. Both protons and neutrons clump together, and form what is called the nucleus of the atom. Electrons are much smaller than either protons or neutrons, are negatively charged, and exist in a kind of an orbit around the nucleus. The number of protons determines the basic physical properties of that substance, and defines that atom as one element of matter or another. For example, all atoms with one proton are considered hydrogen atoms, all atoms with six protons are considered carbon atoms, and all atoms with 26 protons are considered iron atoms. Electrons give atoms specific chemical properties, and the number of electrons can be fairly fluid, but it’s not really relevant to the point I’m making, so I’m just going to move on.
Neutrons give atoms stability. Usually, there are about as many neutrons as protons in an atomic nucleus, although the larger the nucleus, you tend to find slightly more neutrons. The number of neutrons added to the number of protons gives the atomic weight, which is essentially the measure of mass for the atom, since electrons don’t really have much mass to them at all. As I said, usually there are the same number of neutrons as there are protons, so in the average atom of carbon, there are six protons, as I mentioned before, and there are also six neutrons. This gives the carbon atom the atomic weight of twelve. But not all atoms of carbon will have six neutrons. A few will have eight instead. This gives some carbon atoms the atomic weight of fourteen. Now, since the both have six protons, they’re both defined as carbon, but since they have different atomic weights, we classify them differently. Different atoms of one particular element that differ in terms of atomic weight are called “isotopes.” We can differentiate between them by referring to them as “Carbon-12” and “Carbon-14” based on their respective atomic weights.
Now, you remember that I told you that nuclei prefer to be stable, which means that they keep about the same number of protons and neutrons. So, since Carbon-14 has more neutrons than protons, it’s unstable- which means that something interesting happens. One of the extra neutrons ejects an electron, which means that it loses a negatively charged particle. Thus, the neutron becomes a proton. This changes the atomic number of the atom, raising it from six to seven, which means that the atom itself changes from carbon to nitrogen. The electron that’s ejected is thrown out of the atom, and is a form of radiation called beta-radiation. What’s particularly interesting about this process is that this change occurs at a measurable rate. We can determine empirically the amount of time it takes for one-half of an unstable isotope to decay into a stable isotope. This amount of time is called the “half-life,” and is unique to every different isotope.
As you may have guessed, we can use the known half-life of a particular isotope to calculate backwards in time, assuming we know the ratio of unstable to stable isotope to expect. And as it happens, there are several isotopes for which we do have this information- and carbon-14, which I already mentioned, is one of them. Carbon-14 makes up a small fraction of all the carbon in the environment, but it is basically a steady fraction. And since all living organisms take up carbon in any number of organic molecules, each living organism- including you- has the same ratio of carbon-14 in its body to carbon-12 as can be found in the environment. Now, of course this carbon-14 is being decayed to carbon-12 according to its half-life, but as long as an organism is taking in carbon from the environment, that carbon-14 is being replaced. The only time that the ratio stops being maintained is at death. Once an organism dies, the amount of carbon-14 in its body slowly but steadily becomes converted to nitrogen, leaving only the regular carbon-12. The half-life of carbon-14 is 5730 years, which means that 5730 years after an organism has died, there is only half as much carbon-14 left in its body as when it was living. After another 5730 years, there will only be a quarter as much, and then only an eighth as much, and then a sixteenth as much, and so on. Because the amount is halved every time, it never drops to nothing, but after about 60,000 years, it’s dropped too low to measure. This means that anything organic which was alive prior to then can be dated with reasonable accuracy according to the amount of carbon-14, what is called “radiocarbon dating.”
Now, you may be thinking at this point, “60,000 years is a long time, but most fossils are much older than this. How do you measure farther back in time without carbon?” Well, carbon is only one of several useful isotopes. You may have heard of uranium, the element that is usually used in nuclear reactors- well, no surprise, but it’s radioactive, and decays into lead at a very slow rate. Two rates, actually- two different isotopes of uranium decay into two different isotopes of lead, one with a half-life of 700 million years, and the other with a half-life of 4.5 billion years. That’s right- billion. In addition, potassium decays to argon with a half-life of 1.3 billion years, and rubidium decays to strontium with a half-life of 50 billion years. Now, obviously, this is far older than any existing fossil- but these dating techniques are used on the rocks which surround the fossils. Fossils exist in very specific and discrete layers of rock strata, and so all a geologist has to do is date the strata layer using one of these radiometric methods, and then any fossils found within that layer are placed roughly within that time frame.
But, are there any problems with these methods? Well, they’re not perfect, of course- no measurement is. But when scientists make measurements, they use the power of statistics- that is, if a measurement accurately reflects a particular phenomenon, then multiple, independent measurements of that same phenomenon should distribute around a clear average, with proportionally little variation. And that’s what happens- many measurements are made when dating a particular strata, or organic sample, and only if those measurements show a clear consensus is the date accepted. In addition, depending on the phenomenon, radiometric dates can be cross-checked with other observable dating methods- dendrochronology, for example- the counting of tree rings.
So, to review- certain radioactive and naturally occurring isotopes of various elements are known to decay into other elements at measurable rates, and by analyzing the ratios of the starting isotope and its product, scientists are able to reliably date organic objects only a few hundred years old, as well as inorganic objects more than a billion years old. These methods are independently verifiable, and can also be compared with other empirical dating methods for calibration.