How Did We Get Dinosaurs?
The history of life on Earth is amazing, and much more has happened than we can possibly cover in this course. We will, however, give you a very short sketch of a few of the key events. You might watch the following short narrated video first (Evolutionary Process).
Video: Evolutionary Process (3:20)
Sometime about 3.8 billion years ago, life appears. And it starts coming up. There's new kinds appear. There's evolution going on. There's splitting of the types of things that are alive. And each of these lines represents a lineage. It's a living type, something that's alive in the world.
Now we know that about 570 million years ago, as oxygen is rising, as big critters are appearing, as shells appear, that there's this great diversification giving rise to lots of different kinds of life, which we call the Cambrian explosion. We find more and more types coming up. Each new living thing can give rise to other ones. And there's always some extinction going on, and so some of these lines don't come up to the today.
But there's quite a variety of life. Land plants appear. Land animals appear. And then we see this hideous mass extinction at the end of the Paleozoic, when most of the things on Earth died, probably because a really hot stagnant ocean is belching out poison gases that are really nasty. There's still a little debate on that.
We know that just a few things managed to get through that and that they then are spreading, giving rise to new species in the new world. Because all the big critters were gone, there's room now for diversification. You get dinosaurs appearing and the other interesting things.
The dinosaurs are actually doing very well. They're flourishing. They are the big critters. Mammals are around, but they can't beat the dinosaurs. And then the meteorite does, so you get the great mass extinction at the end of the Mesozoic. And just a few kinds, again, succeed in getting through.
Those few kinds that manage to get through again give rise to diversity. There's space for splitting, for new types to appear. And so over 10s of millions of years, we see the rise of diversity and the things coming up to today. And so the world goes back to being a very rich, very diverse place, again with some level of natural extinction, but with a lot of splitting going on, a lot of types getting up to today.
Now, a couple of things that you may want to notice. One is that there's different types, that you come over here and you'd see something that might be mammals, or it be some subset of mammals like bears, and they will seem to be very different from some other type. And that difference arises because the split that gave rise to those types is actually way the heck down here.
And you can follow one type up to here, and you can follow the other type on up over here. And so because they came so far down in time that they split, you'll find that they look like very different types today. The other thing to notice, there's a reasonable chance that today, as we see marked right there, today may be the next mass extinction if we don't change our behavior.
We don’t really know exactly how or when life started, but the geological record includes evidence of life almost as soon as conditions developed that could support life as we know it, perhaps as early as 4 billion years ago. Life mostly remained in the sea for much of Earth’s history, because life is water-based and tends to dry out on land. Before the start of the Paleozoic about 530 million years ago, most living things remained small and slow-moving, probably because oxygen remained scarce in the atmosphere. We get our energy by “burning” food with oxygen to release energy, and without oxygen, we couldn’t release energy rapidly from our food. The history of oxygen in the atmosphere is fascinating, with fluctuations caused by changes in tectonics and climate and evolution, and those changes affecting biodiversity; some of the important work was done by Penn Staters, but again, the details are for another course.
After oxygen finally became consistently common in the atmosphere, with plenty of dissolved oxygen in the ocean, large and active creatures evolved “quickly”, over 10 million years or so. Many of these had shells, which are preserved well as fossils, so the fossil record of life became much more interesting. The 10 million years of this “Cambrian explosion” at the start of the Paleozoic are not exactly an explosion, but after 3.5 billion years of small, rare fossils, a large increase in just 10 million years seems fast.
Through the Paleozoic, life spread onto land, large and agile animals evolved, trees evolved, and more. As life became more diverse, some species went extinct while new ones evolved. But, at least twice during the Paleozoic, and at the end of the Paleozoic, and once during the Mesozoic there were “mass extinctions”, when a lot of species went extinct in a short time, reducing biodiversity. Then, over millions or tens of millions of years, new diversity evolved.
We are not going to make you learn the details of these older mass extinctions (we’ll cover the one that killed the dinosaurs soon…). You should know that these older mass extinctions were primarily caused by climate change, and especially by extreme warmth from release of carbon dioxide from really large flood basalt volcanic eruptions (we visited this subject way back in Module 3 with Hawaii). The heat from the extra carbon dioxide reduced the dissolved oxygen in the ocean, and the ocean may have been overfertilized by rapid weathering of the new volcanic rocks, leading to “dead zones” and release of poison gas (hydrogen sulfide). The worst of these extinctions, at the end of the Paleozoic, involved temperatures too high for large animals to survive in the tropics on land and in the ocean, and may have killed 90% of the species on Earth at the time.
(We will revisit this issue of climate change and carbon dioxide next time, in Module 12. The release of carbon dioxide that drove these extinctions was as large or larger than what humans might do if we burn all the fossil fuels, but was much slower than we might do, giving prehistoric life more time to adapt. And, we do know that the modern rise in carbon dioxide is being driven entirely by humans, not by volcanoes.)
After each mass extinction, many “jobs” (often called ecological niches) opened up because the plants and animals that had been doing those jobs were all dead. The new species that then evolved often filled the old jobs, but in interesting new ways. The earliest dinosaurs evolved as diverse life returned after the worst extinction that ended the Paleozoic.
The dinosaurs were the dominant large animals on Earth for over 100 million years. Many were quite small, but some were gigantic. They included large plant-eaters and large meat-eaters. Some spent at least part of their time flying or gliding, and others swam.
Virtual Field Trip: Dinosaurs: Where They Lived, and How They Died
Virtual Field Trip: Dinosaurs: What They Stepped In
Want to see more?
Here is an optional vTrip you might also want to explore! (No, this won't be on the quiz!)
Dinosaur National Monument - Provided by USGS