Biogeography, Fossil Record, and Radiocarbon Dating
by Jason Amores Sumpter
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now because of continental drift. That means that the distribution of organisms and ecosystems across the continents has shifted over. Time is continents break apart. And as continents break apart and ecosystems and species become divided well, they are influenced by different evolutionary forces and therefore go down different evolutionary pathways. Now we call this study of the distribution of species and ecosystems in geographic space and across geographic or geologic time bio geography. And you can see a little image here trying to represent how certain species were distributed, distributed across continents. And when those continents would break up those species, those pockets on populations of that species would be separated. Evolutionary forces would act differently upon them, and we would get, um, different evolutionary courses resulting from these different species. Now, our picture of the ancient world and the life on Earth that existed before we did comes from the fossil record. Problem is, the fossil record is not only limited, it's biased. What does that mean? Biased? Well, let's say that organism a lives in conditions that are very good for forming fossils so that when uh organism dies, there's a really high chance that it's going Thio have its body preserved as a fossil, whereas organism be on the other hand, organism be lives somewhere where it's very likely that its body will be degraded when it dies, meaning there's a very low chance that it would actually turn into a fossil. Because of this, you know, millions and millions and millions. And millions of years later, when humans were digging up, fossils were much more likely to dig up a fossil of organism A than organism. Be not because there were mawr of organism a back in the day when it was still alive, but because organism A had a higher chance of dying and turning into a fossil. So there is a bias in our fossil record. Now that little example. I just gave eyes an oversimplification, and, moreover, Onley illustrates one way that a bias would arise in the fossil record. Their many other ways, However, it's just important to note that the fossil record does not give us a complete picture of life on Earth. In fact, it gives us a very, very limited, limited snapshot of some of the things that used to exist. Now we date fossils by radiocarbon dating and we talked about this, uh, in a previous lesson wave at the beginning of this course. And basically radiocarbon dating gets down to comparing the ratio of carbon 12 to carbon 14. Now, what's interesting to note is that the oldest fossil of a living organism call this dramatic light is actually still in existence today. So here we can see the fossil of a stream, a delight. And this is a structure that's created by photosynthetic cyanobacteria, sometimes called blue green algae. Though there's nothing algae about them, they're a type of bacteria, not algae. So these stromatolites are formed by the metabolic processes of cyanobacteria isa, byproduct of their metabolism. And they're still around today. As you can see right here, these Airstream satellites with living cyanobacteria. This photo was taken in the recent past. These things they're still around today. So pretty amazing that our oldest fossils of living organisms are actually of things that still exists to this day, that you and I could go see if we want pretty incredible now getting back to radiocarbon dating. When we talked about radiocarbon dating in this comparing of the ratio of carbon 12 to carbon 14 we brought up something called Half Life, and that represents the time it takes for half of a sample to decay. So let's get back to our whole carbon 12 carbon 14 thing. Give a little refresher of how this all works. So basically, um, there is a small percentage of carbon out there that is an isotope called carbon 14. Now most carbon is carbon 12. Very stable. Adam. That's why so many living things are made of it. Carbon 14, on the other hand, will decay, and it will stop being carbon 14 and turn into something different. So let's assume, and these numbers are not accurate. I'm making them up just to make it easier to understand. Let's assume that 10% of all the carbon out there in the world is carbon 14. Well, that means when an organism is alive like me, for example, I'm incorporating carbon 14 into my body on a regular basis, meaning that if you were to look at the carbon composition of my body, about 10% of it would be carbon 14. Of course, when I die, I would stop incorporating carbon 14 into my structures so it would start to degrade. And let's say, you know, a million years from now there would be less than 10% of the carbon in my body would be carbon 14. So getting back to a half life, uh, scientists know the half life of carbon 14 and they know the percent composition of carbon 14. That's out there in the world. So using that information, they can roughly determine the age of fossils by looking at how much carbon 14 is present. Now, one important thing to note about half life is that every half life that occurs on Lee, half the sample degrades. So here we have a nice example of half life. All of our sample starts out as thes red dots in one half life. So let's call this. This is a half life now, only half of our sample is red dots, right? And then another half life occurs, and now half of that sample is red dots and one last half life. And again, the amount of red dots halfs again. That's how half life works. And this is an example of exponential decay so important to note that with each half life, your Onley losing half your sample. Um, as opposed to losing the same amount each half life. Right. So we go, we lose four in the first half. Life two in the second. As opposed to losing four in the first half life and then four again in the second half. Life here in this example with red dots. So half life works Well, tricky way. Just remember that and you'll be all set. All right, let's turn the page.