Not Even The Smallest Are Spared Extinction

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Birds do it, bees do it. Even bacteria do it. Bet you weren’t expecting that. I’m talking about going extinct, kicking the bucket, becoming an ex species. Up until now, research on bacterial history has been difficult because bacteria do not fossilize well. And you would think bacteria, which conquered the globe long before we arrived, could encounter any new hardship– well, they could eventually adapt to it if they got in trouble.

But according to a new model based on modern bacterial relationships and some advanced math, death comes even for the smallest, at a rate that claims nearly as many species as are generated. In other words, most of the bacterial lineages that have ever existed– most of them are already extinct. Here to explain how we know that is Stilianos Louca, a post-doctoral fellow in zoology, University of British Columbia’s Center for Biological Diversity in Vancouver. He joins us by Skype. Welcome to Science Friday.

STILIANOS LOUCA: Hello, Ira.

IRA FLATOW: Hi there. So, how do you investigate the history of bacteria without using fossils?

STILIANOS LOUCA: Well, the only thing that’s pretty much left is to look at the bacteria that exist today because that’s the only thing that we have. So we look at phylogenetic trees. Those are mathematical structures that encode, in a very formalized way, the evolutionary relationships between bacteria living today. And so by looking at these relationships with some sophisticated math, over time we can say something about how evolution generated but also destroyed lineages over time.

IRA FLATOW: So, you’re saying your model found that bacteria go extinct almost at the same rate as they speciate, but in general are getting more diverse? Is this surprising?

STILIANOS LOUCA: Well, if you consider it in the whole context of what we know about life, in particular plants and animals, were we know much more about their diversification, it’s actually in line with what we’re seeing there. So that speciation rates are just a little bit above the extinction rates– that has been observed for plants and animals. It was just widely believed that that is not the case for bacteria.

So, to be fair, that was mostly speculation. It was based on intuition. There was very little data to really answer that question. So for the longest time, we’ve just been in the dark about what’s going on with bacteria.

IRA FLATOW: So why didn’t they think bacteria went extinct?

STILIANOS LOUCA: Two things mainly separate bacteria apart from other larger organisms in that regard. First, their population sizes are immense. Some bacteria population sizes in the order of billions or trillions of living cells. That is much larger than any other species on this planet. So the high population sizes make their extinction unlikely, at least within what we– within the context of what we know about other, larger organisms.

The second reason people thought bacteria should not go extinct is that they have mostly global dispersal ranges. So you can find the same lineage all over the world, and so the chance that a lineage goes extinct just because a specific environment changes– let’s say there was a flood somewhere, or a volcanic eruption, you would expect bacteria are much more able to persist on a global scale simply because they also have representatives on the other side of the planet, so to speak.

And then the third reason was that they quickly adapt. You mentioned that in your introduction. They evolve very rapidly. So one would expect that perhaps they’re resistant to any kind of change.

IRA FLATOW: Yeah, but at the same time, bacteria were not subject to those great mass extinctions we’ve seen in other kingdoms over time. And even though bacteria sometimes rely on plants or the animal hosts, why would they have been immune to these great extinctions?

STILIANOS LOUCA: You know, that was probably one of the biggest surprises for us when we saw that, and we were scratching our head about this for long. We made sure this is a strong, robust pattern. Yeah, it turns out they are not affected by those mass extinction events that affected the larger organisms. Even though many bacteria, but not all, rely on hosts, it could actually be that they’re not very host-specific, that the same lineage could survive in many different host species. They might not even be very closely related host species. And so if one host goes extinct, the same lineage could actually persist in another host species.

The other reason is that it could be the same lineage that we think is so specific actually also has representatives that are living outside of hosts. In fact, a very prominent example is E. coli, which is a very well-known mammal gut commensal. It’s a human pathogen– or some strains of it. But the fact is that there are also E. coli strains that are perfectly fine with living in the soil. We just don’t hear about it too much. So if all mammals were for some reason to go extinct, the e cola, as a lineage, could actually survive just because it has strains that live in the soil.

IRA FLATOW: Interesting. And you keep using the word lineage where I would use the word species. Why are you using that and not species?

STILIANOS LOUCA: I’m using the word lineage simply because our common definition of species, as we know it for plants and animals, or sexually reproducing organisms in general, does not really apply for bacteria. And so while historically people have been occasionally using the word species for bacteria, we now know very well it’s not as easy to define species for bacteria. Bacteria don’t reproduce sexually, and therefore it’s hard to define two species just based on reproductive isolation.

So lineage refers to a certain cluster of closely related bacteria, and that somehow intuitively corresponds to what we know of species in plants and animals. But it’s not really the same. So that’s why I’m using lineage.

IRA FLATOW: OK. I don’t want you to take this next question the wrong way, but what does it matter if bacteria go extinct?

STILIANOS LOUCA: Well, it’s in the same category of questions as how many stars and galaxies are there in the universe? How do galaxies form or die? How has what’s been come to be? But the only difference is that bacteria are right here on Earth with us and we estimate that there are more bacteria living today on Earth than there are stars in the universe.

IRA FLATOW: Wait a minute. Wait, wait, wait. Wait a minute. Say that again. There are more bacteria than there are stars in the universe?

STILIANOS LOUCA: Yes. Based on our current estimates of how many stars there are in the universe, there are much fewer stars. So I know it’s baffling, but bacteria are really tiny. Bacteria occupy virtually every environment on Earth, even environments where plants and animals couldn’t survive. So they’re most– they are the most ancient, the most widespread, and the most ubiquitous form of life on Earth, actually. We just don’t see it. But just as we can’t see atoms but they’re still here, we can’t see bacteria, but they’re still all around us.

IRA FLATOW: So they’re a pretty hardy– pretty hardy– I don’t want to say species, but is that why we think that we could find them living, say, on other planets like Mars? They might have a history. We could find remnants of their past, fossilized history?

STILIANOS LOUCA: You know, with what we know so far it could be. Even though– even on Earth, bacteria are very hard to find fossils of, the fossilization properties are very different from those of other organisms. So it’s like looking for a needle in a haystack. But hey, in principle, it’s possible.

But that brings me to an interesting aspect, namely life on planets in general. And so we know that Earth has been very, very strongly shaped by bacterial evolution over the last few billion years. I mean, it was the invention of different types of metabolisms by bacteria over time that led to some very dramatic changes in Earth’s atmosphere. For example, the accumulation of oxygen about 2 and 1/2 billion years ago. That was because a certain type of bacteria invented oxygenic photosynthesis.

So just getting an understanding for how that type of life– which, as I mentioned, is one of the most important parts of life on Earth. If we understand how that aspect of life evolves on planetary timescales, it might actually tell us something about living planets in general. Planets with life in general. How do they change over time if there is life on them? How does life on planets change over time when it has occured on our planet? So it gives us a very broad context to understanding what makes a planet alive, and when it’s alive, how does it change over time?

IRA FLATOW: Which implies to me that if we were to have another living planet, let’s just use Mars, you would have to have bacteria on it.

STILIANOS LOUCA: Well, we probably wouldn’t call them bacteria because they would be very different from bacteria on Earth, most likely. But in terms of general properties, probably a similar level of complexity, similar size, perhaps. That type of life, we would most likely– that would be the most likely form of life that we would find.

IRA FLATOW: Could we bring, on purpose, bacteria to another planet to start life there, to colonize it, to terraform it?

STILIANOS LOUCA: Well, you could bring them. I would estimate that the probability that they would establish and grow on their own is very low because, I mean, the form of life that we have today on Earth has adapted over time to the type of planet that we have here, the chemistry of our atmosphere, the temperatures and so on. And so if we were to take this present form of life to another planet where temperatures are much more extreme, the gases in the atmosphere are very different, in all likelihood, they would just die off. So it’s not as easy.

IRA FLATOW: OK, what about the archaea? I know they’re not bacteria classified.

STILIANOS LOUCA: The archaea?

IRA FLATOW: Yeah, would they survive?

STILIANOS LOUCA: Well, in terms of this question, they would probably face the same difficulties as bacteria. So archaea are very similar to bacteria in many different regards in terms of their genome structure and cell structure, especially when compared to other organisms, such as plants and animals. So I think there wouldn’t be much of a difference for archaea as bacteria.

IRA FLATOW: You know, these things evolve. I don’t have to tell you. You never know. [LAUGHS]

STILIANOS LOUCA: Yeah, you never know. And I mean, bacteria have shown an incredible skill to adapt over history. So with the right conditions, the right help, it might be possible. But I think it’s very speculative at this point.

IRA FLATOW: We’d love to find out, wouldn’t we?

STILIANOS LOUCA: Yeah.

IRA FLATOW: Thank you very much for taking the time to be with us. Dr. Stilianos Louca, post-doctoral fellow at the University of British Columbia Center for Biological Diversity. Of course, that’s in Vancouver.

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