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Listen to part of a lecture in an Earth Science class. (male professor) OK, when we talk about changes that have taken place on this planet over geologic time, the formation and movement of continents, the rise and fall of oceans, climate changes, we tend to focus on inorganic physical processes, things like volcanoes and plate tectonics. And we don’t pay much attention to the influence of biology. Uh, yes, Sara? (female student) But almost every day we hear about how humans are affecting the environment? (professor) Oh, absolutely right, if we look at news reports about, say, the rainforest destruction and all types of pollution, it’s easy to see how humans have made their presence felt, especially in the last century or two. But for geologists that’s just a blink of an eye. And what we’re going to do today, uh, we’re gonna go back much earlier in Earth’s geologic history and find some examples that aren’t exactly everyday topics of discussion. OK, we know that in the earliest years of Earth history, the atmosphere contained almost no oxygen, whereas it’s over twenty percent of the atmosphere today. The only plausible explanation for this huge increase is photosynthesis, no doubt taking place largely due to microbes called cyanobacteria. Cyanobacteria are tiny microorganisms that produce oxygen using sunlight as well as water and carbon dioxide. This all started about two and a half billion years ago when they began to replace the more primitive microbes that didn’t produce oxygen and was able to thrive without it. And eventually, cyanobacteria as well as some other oxygen-producing organisms began raising oxygen levels to what we see today. So, the oxygen in the atmosphere, uh, the fact we have so much of it, well, what’s responsible for that? We can say, life. So, then, many years after the appearance of cyanobacteria, about two billion years actually, plants began to appear on land. Uh, how did that change the land? (female student) Well, soil, plant roots break up rock to help make soil. And when they die, they add nutrients back into the soil. (professor) Good, not only plants, microorganisms, fungi, and later on, animals like worms, all helped build up a layer of topsoil on the land, which, of course, encouraged more plant life. And, by the way, what does that mean for the atmosphere, for the climate? (female student) Well, land plants take water from below the surface and they return it to the air and that process cools the air. (professor) Yes, at least in that local area, and how about forests, the leaf canopy, the shade provided in that local area by the leaf canopy has a cooling effect as well, right? OK, let’s turn the question around. How would the planet look, geologically, if life no longer existed on it? Much different? (male student) Well, there’d be more, like, landslides, erosion, um, hillsides and river banks are stable when they’re covered with plant life, but without that, um, eventually a lot of the soil would just be stripped away. (professor) Yes, uh, especially on hillsides. So, in place of the rounded hilltops we’re used to seeing, there’d be lots of rough bedrock surfaces. Uh, think of a jagged eroded mountain top you might see in a desert, uh, a place with almost no vegetation. That’s not to say that rounded hilltops would be impossible in the landscape of a desert or even another planet, just that they’d be a lot less frequent. Another landscape feature that would be seen a lot less frequently? Well, rivers that meander, move along with curves, uh, lots of S shapes. Rivers develop those S shapes partly because of the strength of their banks. But when river banks are weakened because there’s no vegetation to hold the soil there, then the soil gets eroded away. And you’d generally see a river that’s straighter but often with what’s called river braiding, multiple channels with small, usually temporary islands forming in between them.