Evolution of Evolution — Text-only | Flash Special Report
Interview with Kirk Johnson
BOBBIE MIXON: Digging evolution is more than a popular pastime among geoscientists. For some, it's a lifelong pursuit that combines earth science, prehistoric life, ancient plant remains and asteroids. Paleobotanist Kirk Johnson is chief curator and vice president of research and collections at the Denver Museum of Nature and Science. Dr. Johnson, what does Origin of Species mean to our understanding of geology?
DR. KIRK JOHNSON: In Darwin's time, fossils were known to be through fossil--through the rock record, but people didn't know how much time was in the rock record or, really, if the fossils were put there by separate creations or by different means, and what Darwin did was said, look, there's a pattern of natural selection which causes things to change over time, but the fossil record is very episodic. He read Lyell's “Principles of Geology” and really came to a pretty good, um, almost modern understanding of how geology worked with a few obvious things missing, obviously. But, then what he did was he painted life onto that for the first time in an accurate way.
BOBBIE MIXON: You say the fossil record is episodic, but what did we think we knew about how long the Earth was in existence at the time of Darwin that today we know differently?
DR. KIRK JOHNSON: When Darwin was around, we had not yet discovered radioactivity and people were making estimates of the age of the Earth using a variety of reasonable things, things like if the Earth started as a molten ball of lava, how long would it take to cool to its present temperature? So we're using all these sort of, um, sort of mind experiments to guess the age of the Earth and they came up with widely ranging numbers. Lord Calvin said something like 10 million years for the age of the Earth and Darwin said, you know, there's too much evolution in the plant and animal fossil record for that short of a time. It must've been a longer time. So, we really didn't know to even a close approximation, in Darwin's time, how old the Earth was, and it took a number of things. It took the discovery of radioactivity of Adam Curry and then the Manhattan Project really got us looking at how radioactive isotopes decay. That allowed us to date old rocks on this planet but the oldest rocks on the planet aren't as old as the planet so it was the dating of meteorites and the dating of moon rocks that basically sort of closed us in our present estimate of the age of the Earth at 4.567 billion years.
BOBBIE MIXON: Now, your essay says Darwin concluded that species existed longer than it took for geological formations to form. Is that counterintuitive?
DR. KIRK JOHNSON: But realize two things. The deposition of the sediments that become geological formations happens as long as it takes, and species happen as long as they take and they're not related to things, and so the animals that get fossilized are the animals that happen to live and die in a place and a time when a certain formation is being deposited. So the formation is deposited in a really quick period of time and a species takes a long time to evolve and you only sample that species at one point of its species lifespan, and since Darwin knew neither the duration of the species or the duration of the formations, he said, “Look, okay, we see lots of formations where the species don't appear to change that formation, yet species do change over time.” And he really realized the profound fact about the geologic record, which is that formations, that great thick pile of stratographic sections that you see in places like the Grand Canyon, even though they’re just a thick and massive relative to human scale, those great thicknesses were deposited in relatively short periods of time and there's lots of time that went by between the formations. I didn't really come to this conclusion until I went back and read “The Origin of the Species” [sic]a couple of weeks ago and I realized as I was looking at his quotes that he's absolutely right because we've only recently come to the understanding of how brief some formations are. Some of the recent geochronology we've been doing have taken things like the Hell Creek formation, which we used to think was a five or six-million-year duration formation, turns out that Hell Creek last about 1.3 million years. It turns out we now know that dinosaur species change about every million, million and a half years. So here's the case of a formation that lasted 1.3 million years and you don't see too much dinosaur evolution within that formation. That's because the species are changing slower than it takes to deposit that particular formation. So it's both the realization that species have a certain duration and the formations have a certain duration and, in many cases, the formation duration is less that the species duration.
BOBBIE MIXON: Your essay called the K- Boundary “an event that happened one day at the end of the Cretaceous.” Can you tell us what the K-T Boundary is and what you mean by “an event?”
DR. KIRK JOHNSON: The K-T Boundary, the K is short for Cretaceous, T is short of Tertiary, so it's the boundary that happened with the Cretaceous period, which ended 65.5 million years go, and the Tertiary, which is now called the Cenozoic, that began then and the reason it's such an abrupt boundary is it was caused by the impact of a large extraterrestrial body, either a comet or an asteroid, that struck the Yucatan Peninsula in Mexico and made a fantastically large crater. This is probably the largest hole the Earth has seen in the last four billion years. The crater, when it first formed, was 180 kilometers in diameter and 24 kilometers deep. That's three times the depth of the Marianas Trench. Now, that thing filled back in rapidly. The ejecta that were blown out of that crater rained around the planet and rained down and caused basically blast-furnaced temperatures as the ejecta came back into the atmosphere. There was acid rain, there were the attendant earthquakes, tsunamis and plus just the initial blast, and that thing was so, uh, serious in its impact on the planet that it basically knocked down and burned forests around the world. It killed all the big animals, anything bigger than the dog and it happened instantaneously. I mean, asteroids move at 20 kilometers a seconds; comets move at 50 kilometers a second so this thing happened, literally, in the snap of a finger. This great bolide from outer space changed Earth's history in a moment. Now, that's something that Darwin never thought about. He never thought about instant changes. He was thinking about long-term gradual changes and that's an example of something that's come into the can of geology after Darwin.
BOBBIE MIXON: Now, might other cataclysmic events like this one confuse our reading of the geologic record?
DR. KIRK JOHNSON: I think the big surprise of geology, for me, is that geology is continually uncovering major things about the Earth that we didn't know about, and I'm 48 years old and during my life, I've seen the discovery--well, the acceptance of plate tectonics, the realization that asteroids and comets can hit the Earth, the realization that human combustion of fossil fuels can actually change the atmosphere, the realization that weather and climate can cause mountain ranges to grow, all sorts of things that weren't part of how we thought about how the world worked, and so I think the thing about geology, in particular, is that it's our planet and it's very complicated we live here, and we don't really know all the details of how it works. I mean, just the fact that so many things have been discovered in my lifetime suggests that, um, either I'm extremely lucky or we're looking for a bunch of new discoveries in the next 48 years as well.
BOBBIE MIXON: Now, what do you think the next big discovery in geosciences will be, and what will that discovery say about Darwin's original theories?
DR. KIRK JOHNSON: Okay, the next big discovery is always going to be extraordinarily difficult to predict but I can tell you some of the things I'd like to see us discover. Remember, all the good fossils are still in the ground. The discoveries are still being made. One of the most amazing things is that we've only been digging fossils, really, for the last 200 years. There are not that many paleontologists out there. So the volume of the Earth is huge, the number of paleontologists is small. They've been working for so long and basically we are finding new kinds of fossils all the time. The known number of dinosaurs has doubled since 1995. So, really, what it's telling us is that we are beginning to scratch the surface of an incredibly interesting story, the details of which we don't really know, but we do know this: Charles Darwin, in “The Origin of the Species” [sic], laid out a perfectly reasonable hypothesis about how life evolved. He worked with Lyell's basic“Principles of Geology” to understand how the world works and we're still playing out that amazing story and I will tell you, I've been doing this as a career for 25 years and the change I've seen is so phenomenal that it makes me really look forward to the next 25 years.