History of Life on Earth - Online Tutor, Practice Problems & Exam Prep

The history of life on Earth is shaped by plate tectonics and continental drift, leading to biogeography, where species evolve differently due to geographic separation. The fossil record, though biased, reveals ancient life, with radiocarbon dating helping to determine ages. Mass extinctions, like that of the dinosaurs, allow for adaptive radiations, where surviving species diversify. Key events include the Cambrian explosion and the emergence of eukaryotes through endosymbiosis, illustrating punctuated equilibrium in evolution, marked by rapid changes followed by stability.

Plate Tectonics and Continental Drift

Hi. In this video, we're going to take a brief look at the history of life on Earth. And to begin that discussion, we need to examine the earth itself. First, we're going to examine the theory known as plate tectonics, which is essentially 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. That crust, 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 resulting in the shifting of the continents over time. So you may or may not be familiar with this image right here, the supercontinent, as it's sometimes called, that used to exist on Earth known as Pangaea. Now, Pangaea was, as I said, a supercontinent, 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 two giant continents, Laurasia and Gondwana, which will, of course, over time break up into the continents that we're familiar with today.

Biogeography, Fossil Record, and Radiocarbon Dating

Now, because of continental drift, the distribution of organisms and ecosystems across the continents has shifted over time as continents break apart. As these continents separate and ecosystems and species become divided, they are influenced by different evolutionary forces and therefore follow different evolutionary pathways. We call the study of the distribution of species and ecosystems in geographic space and across geologic time biogeography. One can see an image trying to represent how certain species were distributed across continents and how, when those continents broke up, those species—the pockets and populations of that species—would be separated. Evolutionary forces would act differently upon them, resulting in different evolutionary courses. Our picture of the ancient world and the life on Earth before our time comes from the fossil record. The problem is that 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 conducive to forming fossils. So, when organism A dies, there is a high chance that its body will be preserved as a fossil. Whereas, organism B lives in an environment where it's very likely that its body will be degraded when it dies, meaning there is a low chance that it would turn into a fossil. Due to this, millions and millions of years later, when humans are digging up fossils, we are much more likely to uncover a fossil of organism A than organism B. Not because there were more of organism A back in the day, but because organism A had a higher chance of dying and turning into a fossil. That example I just mentioned is an oversimplification and only illustrates one way that bias can arise in the fossil record. There are many other ways. However, it's important to note that the fossil record does not provide us with a complete picture of life on Earth. In fact, it offers a very limited snapshot of some of the things that used to exist.

We date fossils using radiocarbon dating, which we discussed in a previous lesson at the beginning of this course. Radiocarbon dating involves comparing the ratio of carbon-12 to carbon-14. The oldest fossil of a living organism, called a stromatolite, still exists today. Here we can see the fossil of a stromatolite, a structure created by photosynthetic cyanobacteria, sometimes called blue-green algae, though they are a type of bacteria, not algae. These stromatolites are formed by the metabolic processes of cyanobacteria as a byproduct of their metabolism, and they are still around today. As seen in a recent photo, these are stromatolites with living cyanobacteria. It's incredible that our oldest fossils of living organisms are actually things that still exist today that we can visit if we desire.

Getting back to radiocarbon dating, the ratio of carbon-12 to carbon-14 is crucial, especially when discussing the concept of half-life, which represents the time it takes for half of a sample to decay. To give a refresher on how this all works, let's consider that there is a small percentage of carbon in the environment that is an isotope called carbon-14. Most carbon is carbon-12, a very stable atom, which is why so many living things are made of it. Carbon-14, however, will decay and transform into a different element. For example, let's assume that 10% of all the carbon in the world is carbon-14. When an organism is alive, like myself, it incorporates carbon-14 regularly. Therefore, approximately 10% of the carbon in my body would be carbon-14. Once I die, carbon-14 incorporation ceases, and over time, such as a million years later, less than 10% of the carbon in my body would be carbon-14. With the known half-life of carbon-14 and the percentage of carbon-14 present in the environment, scientists can approximat

Mass Extinctions and Adaptive Radiation

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 very short period of time. Basically, like, less than 1,000,000 years, which doesn't sound like a lot. But when you consider that life has been around on Earth for 1,000,000,000 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 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 two 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 closes, another door opens situation. Because so many of the species out there die off, it gives a 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 Galápagos 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 Galápagos Islands present many different environments, and the finches on each of these islands 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 experienced different evolutionary pressures and evolved to fill different niches, as evidenced by all these finches on the outside of the circle.

Timeline of Life on Earth

Now let's take a quick look, and I really mean a brief look at the course of life on Earth. 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 going to make two big divisions in the course of Earth's history. What I'm going to cover is 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 evolutionary diversification. 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, a 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 processes evolved that oxygen became abundant in the atmosphere. And in fact, way back in the 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 course of history since the dinosaurs until now. A 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 two separate life forms fuse together to create a new life form. So what is endosymbiont theory? Well, remember, we had our big giant membranous cell that engulfed an 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 two living symbiotically. Additionally, multicellular organisms appeared during this time. Another really important development in the course of life on Earth.

Now in the Phanerozoic era, we've had a couple of 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. The Cambrian Explosion created a great diversity of life. Also, during the Paleozoic era, we have the terrestrialization 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. First 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 kind of like one door closes, another door opens.

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, 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 is 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. The 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.

Here’s what students ask on this topic:

What is the theory of plate tectonics and how does it relate to the history of life on Earth?

The theory of plate tectonics posits that Earth's crust is divided into large, solid plates that float on the semi-fluid mantle beneath. These plates move over time, causing continental drift. This movement has significantly influenced the history of life on Earth by altering habitats and climates, leading to the geographic separation of species. This separation, in turn, has driven evolutionary processes, resulting in the diverse distribution of organisms we see today. For example, the breakup of the supercontinent Pangaea led to the isolation of species, which then evolved independently, contributing to biodiversity.

How does radiocarbon dating work and what is its significance in studying the history of life on Earth?

Radiocarbon dating measures the decay of carbon-14 (C-14) to estimate the age of organic materials. Living organisms constantly incorporate C-14, but when they die, the C-14 begins to decay into nitrogen-14 (N-14) at a known rate, called the half-life (about 5,730 years). By comparing the ratio of C-14 to carbon-12 (C-12) in a sample, scientists can estimate its age. This method is crucial for dating fossils and understanding the timeline of life on Earth, providing insights into when specific organisms lived and how they evolved over time.

What are mass extinctions and how have they impacted the evolution of life on Earth?

Mass extinctions are events where a significant percentage of Earth's species die out in a relatively short period. There have been five major mass extinctions, each reshaping life by eliminating dominant species and allowing others to thrive. For instance, the extinction of the dinosaurs, likely caused by an asteroid impact, paved the way for mammals to become the dominant terrestrial animals. These events often lead to adaptive radiations, where surviving species rapidly diversify to fill ecological niches left vacant by extinct species, driving evolutionary change.

What is the Cambrian Explosion and why is it significant in the history of life on Earth?

The Cambrian Explosion, occurring around 541 million years ago, was a period of rapid evolutionary diversification where most major animal phyla appeared. This event is significant because it marks the transition from simple, mostly unicellular life forms to complex, multicellular organisms. The Cambrian Explosion set the stage for the development of diverse life forms, including the ancestors of many modern species. It highlights the dynamic nature of evolution, where significant changes can occur in relatively short geological periods.

What is biogeography and how does it help us understand the distribution of species?

Biogeography is the study of the distribution of species and ecosystems across geographic space and through geological time. It helps us understand how species have spread and evolved in different regions, influenced by factors like continental drift, climate changes, and ecological interactions. By examining the geographic distribution of fossils and living organisms, biogeographers can infer historical connections between landmasses and the evolutionary pathways of species, providing insights into how life on Earth has diversified and adapted to various environments.