So what does some of this earliest life look like? Around 3.4 billion years ago, we have what are known as stromatolites. These are, this is a cross-section of a sedimentary column, and you can see the different layers there. And each one of these layers represented the outer edge of one of these deposits where mats of photosynthetic cyanobacteria were growing. These were prokaryotes, they're what some people used to call blue-green algae but they're not algae. They're cyanobacteria. And the Earth's record preserves these sedimentary structures so that we know that life was existing at this time. And how do we know that this was caused by, by algae? Because we see them in certain few places at the present day. This is at Shark Bay on the west coast of Australia. And you can see these stromatolites, these mushroom-shaped, pillars, where the algae is growing, collecting sediment and building up these sedimentary structures as they grow old. In Saratoga Springs, in upstate New York, we see a, a bedding plane here, now sheered off, and you can see the pillar structure and the dome-like structure to the stromatolites, now highly weathered. So, as this cyanobacteria is growing, and the filaments are trapping sediment as the waves wash up on it, the sediment is trapped, and the cyanobacteria have to grow around that sediment, and so it traps that sediment in this structure, and then that's how these things build up over time. If we look at what early life might have looked like, it might have looked something like this. After the cyanobacteria made their debut on planet Earth, we see other spectacular fossil evidence of single-cell organisms. In the Gunflint Chert in, near the Canadian-US border around two billion years ago, we have what, at the time, was the first PreCambrian microfossils that were found in 1953. We now have many more sites where single-cell organisms have been fossilized. And this is what these structures look like. All kind of variety of structures, probably algae, or other kind of, bacteria or organisms of an uncertain, construct. But it's clear that these were organic substances. They left chemical fossils that indicate the presence of organic processes going on in these cherts. And it was around this time that the first eukaryotic cell formed. And what has been suggested how these eukaryotic cells formed, is by incorporating an bacteria and into an archaea organism through a process called genome fusion or endosymbiosis. So we don't know how the first eukaryotic cell formed, but we have a hypothesis that's held up to some evidence that's been presented, not all of it. But what has been believed to have happened is a process called genome fusion. This is where you might have had cyanobacteria that were photosynthetic, brought these chloroplasts to the inside of eukaryotic cells. And so, what might have happened is that an archaea, absorbed the chloroplast. The chloroplast gave energy to the cell, and in return, the cell provided protection for the, cyanobacteria. And so this is what has been proposed to have created the first eukaryotic cell. Now this was a significant step in evolution because these eukaryotic cells then were able to undergo sexual reproduction in the mixing of the chromosomes during reproduction. So this sexual reproduction then is a big evolutionary advantage for the eukaryotes. It increases the genetic variations so there's more variety within the species, and this greater variety will mean then a greater potential for evolutionary change. As species become isolated, those bits, differences in genetic variation can become dominant and eventually develop a distinct new species. Here's the oldest eukaryotic cell, probably a giant bacterium or a colony around 2.1 billion years ago. So to summarize today's lecture, we talked about the different types of organisms, the prokaryotes, the eukaryotes, and the metazoans, and how they are related to the three different domains, the bacteria, the archaea, and the eukaryotes. We talked about extremophiles and their importance in possibly seeding the first bit of life on Earth in such hostile environments. We talked about the first sedimentary structures, the stromatolites, that we believed were produced by cyanobacteria. And we can see them in some situations now. And then finally, we discussed the concept that eukaryotes could form from the symbiotic relationship between two different types of prokaryotic cells.
