History of Life on Earth - Video Tutorials & Practice Problems
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Plate Tectonics and Continental Drift
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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. We're just interested in how the crust moves around on top of the mantle. So that crust, made of a solid, plate, or made of solid plates floats around on this incredibly hot mantle, the interior. And those plates are 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, the continents rest on. And in some cases there are 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 land mass 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 land masses, 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, 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 Pangaea. Now, Pangaea was, as I said, a super continent, meaning it was a massive land mass made up of all of, Earth's continents smashed together. Now over time, due to continental drift, these land masses 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, Laurasia, as it's called, includes what was 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 Gondwanaland. And of course it is made up of the 2 giant continents, Laurasia and Gondwana, which will, of course, over time break up into the continents that we're familiar with today. Alright. Let's turn the page.
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Biogeography, Fossil Record, and Radiocarbon Dating
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8m
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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 as 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 the study of the distribution of species and ecosystems in geographic space and across geographic or geologic time biogeography. 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, and populations of that species would be separated, evolutionary forces would act differently upon them and we would get, 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, organism a dies, there's a really high chance that it's going to, have its body preserved as a fossil. Whereas, organism B on the other hand, organism B 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 are digging up fossils, we're much more likely to dig up a fossil of organism a than organism b. Not because there were more 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, is an oversimplification and moreover, only illustrates one way that a bias would arise in the fossil record. There are 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, in a previous lesson way back 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 called a stromatolite, is actually still in existence today. So here we can see the fossil of a stromatolite 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, as a byproduct of their metabolism and they're still around today. As you can see right here, these are stromatolites with living cyanobacteria. This photo was taken in the recent past. These things are still around today. So pretty amazing that our oldest fossils of living organisms are actually of things that still exist to this day that you and I can go see if we want, pretty incredible. Now, getting back to radio carbon 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 this all works. So basically, there is a small percentage of carbon out there that is an isotope called carbon 14. Now most carbon is carbon 12, very stable atom, 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 1000000 years from now there would be less than 10% of the carbon in my body would be carbon 14. So, getting back to half life, 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 only half the sample degrades. So here, we have a nice example of half life. All of our samples starts out as these 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, halves. Again, that's how half life works and this is an example of exponential decay. So, important to note that, with each half life you're only losing half your sample. As opposed to losing the same amount each half life. Right? So we go, we lose 4 in the first half life, 2 in the second, as opposed to losing 4 in the first half life and then 4 again in the second half life, here in this example with red dots. So, half life, works a little tricky way, just remember that and you'll be all set. Alright, let's turn the page.
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Mass Extinctions and Adaptive Radiation
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3m
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Now, things haven't always been great for life on Earth. In fact, there have been about 5 mass extinctions, which are periods where the majority of lineages, meaning the majority of different species out there, actually die out in a a very short period of time. Basically, like, less than a 1000000 years, which doesn't sound like a lot. But when you consider that life has been around on Earth for 1,000,000,000 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, 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 there are actually 2 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 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. Alright. Let's flip the page.
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Timeline of Life on Earth
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7m
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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, Archaean, Proterozoic, Paleozoic, Mesozoic, Cenozoic. But I'm actually just gonna make 2 big divisions in the course of Earth's history. What I'm gonna what is called Precambrian time and the Phanerozoic era, which is the modern era in which we live. Now, Precambrian 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 unicellular. In fact, you can see from this figure, multicellular life is quite recent in terms of the billions of years the earth has existed. Also, very important thing to note especially about the early ages of Earth's history, is that 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 far more 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 Precambrian time, that is the formation of the first eukaryotes and they were formed through a process called symbiogenesis. What is symbiogenesis? Symbiogenesis, which you probably know as Endosymbiont theory, is when 2 separate life forms fuse together to create a new life form. So what is endosymbiont theory? Well, remember, we had our big, big giant membranous cell that engulfed itty bitty little prokaryote which eventually became the mitochondria. That was a symbiogenesis event. So, the first eukaryotes formed through symbiogenesis. Through one cell engulfing another and the 2 living symbiotically. Additionally, multicellular organisms appear during this time. Another really important development in the course of life on Earth. Now in the Phanerozoic era, we've had couple pretty major events. In the Paleozoic era, we have the Cambrian explosion which again kind of defines the boundary between Precambrian time and this modern era that we live in. So the Cambrian Explosion, created a great diversity of life. Also, during the Paleozoic era, we have the terrestrials 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 to land. 1st plants, then animals. Then in the Mesozoic era, we have the dominance of gymnosperms and dinosaurs. Gymnosperms are plant organisms like pine trees, for example. 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 Cenozoic 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 kinda like one door closes, another door opens. Now the last idea I wanna 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, these 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 where not a lot's 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 eukaryotes represents another example of that, punctuated rapid change. Right? We have symbiogenesis, new life is created, and boom, it just explodes and takes off into all these different shapes and forms. So, yes, the course 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. Alright. That's all I have for this lesson. I'll see you guys next time.