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.

Geologic Record

We now want to take a look at the history of life on Earth, and here we're going to take a look at all of that history at once. Now in your book, you're going to see all sorts of different eras and periods, and like anything, how much detail you need to know. That's up to your class and your professor, so check your notes. But usually, when we're looking, especially at this sort of deep time here, you don't need to know all those details. So we're going to hit the highlights here.

And then for the stuff that you're more likely to be responsible for, we'll go into more detail later on. All right. So here we're talking about the geologic record. The geologic record is the history of the Earth told through geology, but we can think of it here as the history of life as told through fossils. All right.

So to start, we just want to say that the Earth is about 4.5×109 years old. So a little bit older than 4,500,000,000 years. Now my brain doesn't think in billions very well. Right? I don't have a good reference for that number.

So instead of just giving you the dates, we're also going to lay things out. We're going to visualize it here as a calendar year. So I'm going to lay down a calendar year here, and we're going to imagine that January 1st over there, that's equal to 4,500,000,000 years ago. That's sort of day 1 for planet Earth. And then December 31st, that's equal to today.

And this will help us sort of see how these things relate to each other. All right. So we think that life started about 4,000,000,000 years ago, but we don't have fossils from that old. Our first fossils are from about 3,500,000,000 years ago. Now that comes out to about March 25th on this calendar year here.

So it's almost a quarter of the way through the year before we get any fossils at all. Now these first fossils are stromatolites, and these are these sort of fossil layered rocks that are formed by prokaryotes, we're going to say. And they look like structures that we can find on Earth that are still made today. All right. So again, life starts about 4,000,000,000 years ago, but our first fossils are not until 3,500,000,000 years ago.

Now the next thing we want to call out is called the Great Oxygenation Event. Life on Earth, well Earth itself, the atmosphere didn't have oxygen in it to start. Right? All the oxygen on Earth was tied up in molecules like carbon dioxide and water. But from 2,700,000,000 to 2,400,000,000 years ago, we get our first photosynthesis.

And these were prokaryotes doing photosynthesis, and a byproduct of photosynthesis is oxygen. So this increases oxygen levels drastically. Now this happens kind of very slowly where all the oxygen kind of gets sequestered and doesn't change anything, and then it kind of crosses a threshold and kind of happens all at once. And that all at once happens at around June 19th. That's that 2.4×109 years ago.

And this is a big deal because the atmosphere goes from no oxygen in it to have oxygen in this sort of single digits of percentages. Now today, we have 21% oxygen in our atmosphere. So it's not at the levels it is today, but it's still a big deal because anaerobes, things that don't use oxygen, well oxygen is usually poisonous to them. So this is a huge pressure on the things that are alive, and frankly a lot of things probably didn't survive. And life changes quite drastically once oxygen is available.

Alright. Well, the next thing that we want to call out here are the first eukaryotes. Those more complex cells, things with nuclei and organelles, they show up at 1,800,000,000 years ago. Now 1,800,000,000 years on our timeline here, that's equal to about August 8. Now I don't know about you, when I think about life, I'm often thinking about eukaryotes.

Those don't come around till halfway through our calendar year here, halfway through the history of Earth. Alright, now eukaryotes come around through endosymbiosis, and endosymbiosis is when prokaryotes are engulfed by these larger cells, and they became the mitochondria and chloroplasts that we see in eukaryotes today. All right. The next thing we want to call out on this timeline are the first multicellular eukaryotes, because again, when I think of life, I'm usually thinking of multicellular eukaryotes. Well, they don't come around till about 1,300,000,000 years ago.

That's equal to September 18th. We're starting to get pretty late in this calendar year when we've get our first multicellular organisms. Now these still aren't plants or animals. This is a form of red algae that first is that first multicellular eukaryote. All right.

So, September 18th, that's right there. Again, pretty far through our year here. Well, the next thing that we want to talk about, well, I'm actually to do this one, I'm actually just gonna move myself over on the screen, so we can see it better here. It's the Cambrian explosion. All right.

The Cambrian explosion happens about half 500,000,000 years ago here, 535 to 525,000,000 years ago. And this comes out on our timeline to November 19th. And the Cambrian explosion, we're going to say was this rapid diversification of life forms, especially animal life forms. Now animals existed before this, but you might not recognize them as animals. They were more things like sponges.

Now in the Cambrian explosion, sort of all of a sudden in the fossil record, we see all sorts of different types of animals that you would recognize as animals. Now it's a long time ago, you'd probably say they look kind of like weird animals, but you'd probably use the word animal in describing them. All right, the last thing we want to put on here is the colonization of land. What you probably didn't realize here, at least for the eukaryotes, everything that we've been talking about so far has happened in the ocean. Now there probably were prokaryotes on land before this, but it's not until 500,000,000 years ago, and on this calendar that's November 21st, that eukaryotes, the plants and animals come out of the water and colonize land.

So the first plants, animals, and fungi move to, we're going to say here, terrestrial environments. And again, when I think about life, I'm typically thinking about plants and animals living on land. That's not until really late in this year, November 21st. All right. Well, you may be looking at this and you're wondering, well, what about humans?

Well, we can put humans on here. Humans came around about 200,000 years ago. On this calendar, that comes out to December 31st. So that very last day of the year, but not just December 31st, comes out to 11:40 PM. So that New Year's party's already been going on for a long time, right, when humans finally arrive.

But if you're actually interested in, like, recorded human history, that's like the last 5 or 6000 years. On this calendar, that comes out to the last 30 seconds of the year. All right. So well, the Earth is pretty old. Right?

And I think humans are pretty cool, but the takeaway there, we're definitely late to the party. All right, more to come after this. I'll see you there.

History of Life on Earth Example 1

This example tells us that the timeline below shows the four eons of Earth's history, along with the dates from 4,500,000,000 years ago. Now, based on your knowledge of the timing of the major events in the history of life, we need to identify in which eon each of the four events listed below occurred. This says that not all eons may have a corresponding event. Alright. So, as we look at this, we have 1,000,000,000 years ago from 0 to 4.5, and we have these four eons: the Hadean, the Archean, the Proterozoic, and the Phanerozoic.

Now, quite often, you're not going to be responsible for these specific eons, but like anything, you need to know what your professor, your class, expects of you. But it's good to at least be familiar with them, to have seen them, and if you do need to know them, well here they are. Alright, so we have four events here, and I'm going to read them. And I think the first thing to do that I want to do is just put them in order, and then we can align them to the different eons. So A here says the first multicellular organisms appeared.

B is the first definitive fossils are formed. C says that land was colonized by eukaryotes. And D says the Cambrian explosion. Alright. So of those four, which one do you think happened first? Well, the first one that I see here has to be the first definitive fossils. Right? That's our first fossil evidence of life. So I'm just going to put a one here on the line. Now that's not what it asked for as part of the answer, but that's going to help me out to put these things on the timeline.

Alright. So then what do you think comes next? Well, we have the first definitive fossils, and then we are going to have the first multicellular organisms. That's the next event that's listed here. Now that's going to take quite a while, right? To get multicellular organisms. We first have to evolve eukaryotes, and then eukaryotes become multicellular. All that takes a very long time, but out of these four, that is what comes next. Alright. What comes after that? Well, after that is going to be the Cambrian explosion.

Remember, the Cambrian explosion is going to be this rapid diversification of animals, specifically animals, at the beginning of the Cambrian period. And then finally, that leaves us with land was colonized by eukaryotes. That's going to be the last thing that happens in this timeline. Alright. So let's go through this.

The Hadean. So, the Hadean is from the beginning of Earth to 4,000,000,000 years ago. This is a time when Earth was really not hospitable to life. Right? Earth is forming, it's cooling, not a place where things are alive, and we think the first life started right about 4,000,000,000 years ago. But remember when we said those first fossils were formed that we have evidence from? Well, we said that that was actually at about 3,500,000,000 years ago. So it doesn't ask for the exact date here to draw it on the timeline, but I'm just doing that to help myself out. So at 3,500,000 years ago, that's when we get those first definitive fossils. So that is very clearly in the Archean.

Alright. So what do we have next? Well, we said the next thing was the first multicellular organisms appeared. So which of these eons do you think that happened in? Well, remember it takes us a while to go from the first fossils to getting multicellular eukaryotes. And we said that that happens at about 1,300,000,000 years ago, so well into the Proterozoic. So that's where I'm going to put my A. Again, the question doesn't ask us to draw here, but it is just helping me think through this. So the answer here for A is definitely going to be Proterozoic.

Alright. Well, next we said that the Cambrian explosion is the next thing to happen. You remember when in the timeline that one happens? Well, the Cambrian explosion happened right around 535 to 525,000,000 years ago, but the Cambrian is the first period of the Phanerozoic. So the Cambrian is going to be sort of right here on our timeline. That very first part of the Phanerozoic here, that is D. So that means that my answer here is Phanerozoic.

Well, and if you see the timeline, that means that we know what the answer to C is when land was colonized by eukaryotes. But do you remember when that happened? That happened, we said, just after the Cambrian explosion right at about 500,000,000 years or half a billion years ago. So again, the answer to C is also going to be Phanerozoic.

So with that, we answer the questions. Again, a lot of times you're not responsible for these sort of big eon names, but it's good to be familiar with them. And if you are responsible for them, well, here they are.

Each option below lists several of the major events in the history of life on Earth. Which option presents the events in the correct order from the oldest to the most recent?

• A. Endosymbiosis leads to the appearance of eukaryotes → Eukaryotes colonize land → Cambrian explosion.
• B. Prokaryotes form stromatolites → Multicellular organisms appear → First eukaryotes.
• C. Land colonization → Cambrian explosion → Endosymbiosis leads to the appearance of eukaryotes.
• D. Prokaryotes form stromatolites → Cambrian explosion → Eukaryotes colonize land.

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.