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Protists and Evolution

Let's go back to protists and evolution for a moment. Why should protists be interesting for evolution? Well, first of all they have a pivotal role in phylogeny between the bacteria on one end and multicellular organisms at the other end. So they're the ones that made it over the prokaryote/eukaryote. In fact, you could argue that every time protists started to go multicellular, we simply identified them as an animal or a plant or a fungus. In fact, all eukaryotes are protists, or certainly they're protist descendants. So they've got a very pivotal role in evolutionary history.

And also, as I mentioned with respect to biochemistry, protists are useful model systems for laboratory settings. I think you'll see that to some extent, I do both. In terms of clarifying that first point, here's just sort of a rough phylogentic tree I made up with just the critical dates on here. Formation on the earth about 4.5 billion years ago and then we start to see the oldest rock, about 3.8 billion years ago. And almost immediately you start to see indications of life. And these indications are in the form of things like chemical signatures like stable carbon isotope ratios. What we're talking about here is the ratio of carbon-13 to carbon-12. Out of every 100 atoms of carbon in this room, about 99 of them are carbon-12 and one of them is carbon-13. But when organisms photosynthesize, they preferentially pick carbon-12 to fix. So when you look at organic matter, something that is going through the process of photosynthesis or an animal has eaten a plant or something that has photosynthesized, what you see is a depletion in carbon-13. You see even more carbon-12 relative to carbon-13 than 99 to one. And these are some of the signatures that are using very old rocks to indicate that there were living things at that time, the stable carbonisotope ratio.


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