 stromatolites as evidence for the earliest life on Earth. If you make a map of the world where you color-coded by how old the surface is, you get a map like this. And you can see that there are various epics. There is the early Cenozoic and Mesozoic and Paleozoic where the dinosaurs lived, but earlier there's the Proterozoic and that's 0.5 to 2.5 billion years ago. But we're talking about the earliest evidence for life on Earth, so we're going back even further. We're going back into the Archean. Now the Archean is 2.5 to 4 billion years ago. Now there's another epic even earlier than that, 4 to 4.5, that's called the Hedean, but there are no rocks there. There are no surface rocks there. So we have to go back to the Archean and say, what are the oldest Archean rocks? And the oldest Archean rocks on Earth are in two spots in particular. They are in Ishua in Greenland and the Pilbara Cretan in Western Australia. Now so those are the epics, those are the ages of rocks on Earth. But let's talk about stromatolites and we have lots of good evidence from modern stromatolites, stromatolites that are alive today and they are in Shark Bay right next to the Pilbara Cretan. And when you go there, it's a wonderful place to go. I've been there, it's a lovely place and you walk out on the boardwalk and you see these mats, these bulges on the beach. And when you cut them open and look at the cross-section you will see that they are microbial mats and you can see that the green is at the top. These are photosynthetic layers and then they're different layers down beneath them that are mildly photosynthetic but also I think heterotrophic. They're eating the detritus or maybe the dead mats that die and then they fall down and then get eaten by these mats. So it's a very complex ecosystem. When we look at the oldest rocks on Earth we can also see these stromatolites being fossilized. And so these are the oldest macroscopic fossils we have, the earliest evidence of macroscopic life as fossils in the earliest records of rocks on Earth. Here is a plot of the distribution of the stromatolites as a function of time and their diversity as a function of time and this giant one next to this man is 1.8 billion years old but the fossil records show you ones even much earlier, even off the chart here, earlier times 3.4, 3.5, 3.7 billion years old. And so stromatolites of that age 3.4 to 3.7 are the earliest evidence for life on Earth and here are the papers that give us the earliest fossil evidence, the earliest macroscopic evidence for life on Earth and that is 3.4 to about 3.7 billion years ago with these references. But we should point out life therefore had to have existed earlier, even much earlier, because these stromatolites are not simple things. You can't just go like this and have a stromatolite come out of a hydrothermal vent. They are complicated, highly evolved things and so we don't know how long life existed before that. So this is the earliest evidence but it's certainly not the time when life got started on Earth.