History of Life on Earth

Hi. In this video, we're gonna take a brief look at the history of life on Earth. And to begin that discussion, we need to take a look at the Earth itself. And first, we're going to examine the theory known as plate tectonics, which is basically the theory that the earth's crust this outer layer right here crust is basically made of large, solid plates that more or less float on the hot inner mantle. So here is the mantle. These are the inner and outer core. I'll just write cores. We don't need to worry too much about that. That's more the realm of geologists were just interested in, how the crust moves around on top of the mantle so that crust made of a solid, uh, plate, are made of solid plates, floats around on this incredibly hot mantle, the interior and those plates air actually broken up, as we see here in this image. Now you can see the continents and the oceans have been illustrated on this image, and the black lines through the image represent the boundaries of the various plates that, uh, the continent's rest on, and in some cases, their plates that don't have continents resting on them as you can see in the Pacific plate, for example, no continents there, whereas the North American plate you can see right here. This North American plate has both North America, Greenland, Greenland being this landmass right here and a little bit of eastern Russia, including the Kamchatka Peninsula. Right there. Now, what is the point of all this? How does this relate to biology? Well, because the earth's crust with the continents on it, aka Earth's landmasses, are on these plates, and these plates float on top of the mantle. Over time, these plates actually shift positions, they move around on the earth. As a result, we have something called continental drift, which is the movement of the tectonic plates, resulting in the shifting of the continents over time. So you may or may not be familiar with this image right here. The super continent, as it's sometimes called, that used to exist on Earth known as pan JIA. Now hang JIA was, as I said, a super continent, meaning it was a massive land mass made up of all of Earth's continents, kind of smashed together now, over time due to continental drift. These landmasses separated, and one of the first major separations that occurred is what we see in this image right here. And I'll just take myself out of the shot so you can see this better. And this you can see that Laura Asia, as it's called, includes what waas or what will be, I should say North America and Eurasia sort of this chunk I'm circling here, and this lower portion that I'm circling in red broke up into what is called Gondwana. Now, collectively, this formation of land masses is known as Gondwana land. And of course, it is made up of the two giant continents, Laure Asia and Gondwana, which will, of course, over time, break up into the continent's that we're familiar with today. All right, let's turn the page.

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.

now, things haven't always been great for life on Earth. In fact, there have been about five mass extinctions which are periods where the majority of lineages meaning the majority of different species out there, actually die out in a very short period of time, basically like less than a million years, which doesn't sound like a lot. But when you consider that, um, life has been around on Earth for billions of years Well, now it puts it in perspective, right? So the majority of lineages die out in a short period of time, and this results in a dramatic reshaping of life on the planet. And, of course, one of the most famous mass extinctions, as pictured right here by this totally freaked out t rex was the extinction of the dinosaurs, um, thought to occur, have occurred due to an asteroid impact. Now this table right here shows you some of the other mass extinctions. So here we have the end of the dinosaurs and let me actually change my colors. So it's easier to see. Here's another mass extinction. Here's another one known as the Great Dying, and they're actually to earlier ones that are also considered mass extinctions, but we don't really need to worry about specific mass extinctions. Just know that they have occurred throughout Earth's history, and essentially, it's a one door open one door closes another door open situation because so many of the species out there die off. It gives great opportunity to the remaining species to fill a lot of the niches left behind by those species, and it allows life to completely reshape the environment. So when that happens, we often have something called an adaptive radiation, which is the process by which organisms rapidly diversify into many new forms, often in response to changes in the environment that shift evolutionary pressures and open niches. So, on a grand scale, when we have mass extinctions, we frequently have adaptive radiations following that, but on a very grand scale. However, adaptive radiations can also happen on a much smaller scale, and a wonderful example of this is in the Galapagos finches. And yes, this is the same example that Darwin took note of which led him to developing his theory of descent with modification. So the Galapagos Islands present many different environments, and the finches on each of these islands has a have evolved to unique forms. So they all started from a common ancestor, Finch pictured here this little guy, and depending on the environment the finches found themselves in, they were experiencing different evolutionary pressures and evolved to fill different niches, as evidenced by all these finches on the outside of the circle. All right, let's flip the page.

Now let's take a quick look and I really mean brief Look at the course of life on Earth. Now the different time periods of Earth's history have been divided into various subsections. And here in this diagram, you can see that we have these various subsections Haiti In our key in, uh, pro tears OIC Haley is OIC, Mesozoic Santa's OIC. But I'm actually just gonna make to big divisions in the course of Earth's history what I'm going, what is called pre Cambrian time and the Finnair's OIC era, which is the modern era in which we live now. Pre Cambrian time is the period of time between the formation of Earth and the appearance of most animal groups around the Cambrian explosion, which was a period of rapid, rapid evolutionary diversification. Now, during the majority of Earth's history, life has been Eunice Cellular. In fact, you can see from this figure multi cellular life is quite recent. In terms off the billions of years three earth has existed. Also very important thing to note, especially about the early ages of Earth's history, said Oxygen was absent. In fact, it wasn't until the photosynthetic process processes evolved that oxygen became abundant in the atmosphere and in fact, way back in time of the dinosaurs. For example, oxygen was farm or abundant than it is now, which is part of the reason that living organisms were so much bigger back then. It's in part because of the lower oxygen concentrations that life has gotten smaller over the over the course of history since the dinosaurs, until now, now very important event occurred in pre Cambrian time. That is the formation of the first you Kerasiotes and they were formed through a process called Sim Biogenesis. What is sin Biogenesis Sim biogenesis which you probably know is Endo Symbian Theory is when two separate life forms fused together to create a new life forms. So what is Endo Symbian theory will remember We had our big, um, big giant member in a cell that engulfed itty bitty little pro carry out, which eventually became the mitochondria. That was a sim biogenesis event. So the first you Kerasiotes formed through sim biogenesis through one cell engulfing another and the two living symbiotically Um additionally, multi cellular organisms appeared during this time another really important development in the course of life on earth. Now, in the Fanara Zoe area, we've had a couple pretty major events in the Paleozoic era. We have the Cambrian explosion, which again kind of defines the boundary between pre Cambrian time and this modern era that we live in. Um, so the Cambrian explosion created a great diversity of life. Also, during the Paleozoic era, we have the terrestrial ization of plants and animals. Yes, before this time, all life existed in the oceans and it was in this era where life actually finally made it toe land first plants and animals. Then in the Mesozoic era, we have the dominance of Gim No sperms and dinosaurs give no sperms are plant organisms like pine trees, for example, Um, these were the dominant plant life form on terrestrial earth back then, and dinosaurs were the dominant animal life form on terrestrial earth. Back then, of course, we had a great or a mass extinction. See you later. Dinosaurs and we move into the Seine is OIC era where now the plant scene is dominated by angiosperms, which are flowering plants like the ones that produce delicious fruits and vegetables that we so love to eat and also the dominance of mammals in the animal scene and not to get too deep into it. But in many ways, the dying out of the dinosaurs opened the door for mammals to become the next dominant animal form. So again, those mass extinctions, it's kind of like one door closes, another door opens Now. The last idea I want to touch upon in this lesson is the idea of punctuated equilibrium. And this is essentially the theory that evolutionary changes occur rapidly, like in the Cambrian explosion. And these rapid, thes events, where there's rapid evolutionary change punctuate long periods of very little change. So basically, if you were to plot the course of evolutionary change on Earth, you'd have long periods were not a lots going on and then boom, spikes of activity and then again, long periods of not a lot going on. And then some major thing causes boom, another burst of activity. So Cambrian Explosion is an example of one of those bursts. But the creation of you Kerasiotes represents another example of that punctuated rapid change, right. We have some biogenesis new life is created and boom, it just explodes and takes off into all these different shapes and forms. So, yes, Thecornerscores of Earth's history is not just a steady progression with steady change but in fact, more chaotic, You know, periods of rest with periods of evolutionary explosive chaos. Basically. All right, that's all I have for this lesson. I'll see you guys next time.