The deep history of life

Published on September 28, 2023   44 min

Other Talks in the Series: Introduction to Evolutionary Biology

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Hello, my name is Andrew Knoll. I am a Professor of Earth Science and Biology at Harvard University. Today I'd like to talk about "The Deep History of Life". Now spoiler alert. What most people call the deep history of life. You will see is actually most of the history of life on our planet. Our familiar world of plants and animals is actually a fairly recent development in our planet's history. For most of its history, the Earth has been an alien place physically. It has been a microbial planet biologically.
Let's begin with a little bit of perspective. Most of you know something about the fossil record. If nothing else, you know that dinosaurs once ruled landscapes on this planet. If you know a little bit more, you may know that long before there were dinosaurs, animals like trilobites, distant relatives of shrimp and crabs, swam in ancient oceans. Now, it turns out that if we could weigh all the organisms alive today, you'd find that only a very small proportion of that weight would be made up of animals, much less than 1%. It's estimated that there's something like 30 tons of bacteria for every ton of animals. If we look at the diagram, that's something called a universal phylogeny. It's a hypothesis of the evolutionary relationships of all organisms, from humans to bacteria based on comparisons of molecular sequences for DNA and proteins. The one thing I want to call your attention to in this diagram is on the right with the star and the arrow that represents animals. It turns out that all the animals that have ever lived, from trilobites to dinosaurs to you reside on one short distal branch of the tree. The same thing is true of plants, but there are many branches that precede the branch that gave rise to animals. The inference from this is that life existed long before animals first appeared, and that that life was mostly microbial. Now if we look at the time line on the right, we can see that the oldest fossil evidence for animals is about 575 million years old. Yet our planet is more than 4.5 billion years old. Which raises the question of what was going on between the origin of the planet and the origin of animals. From the phylogeny, we should estimate that there was life through at least part of this interval and that life was microbial. That then raises a real empirical question if the deep history of life is microbial. Can such tiny and evanescent organisms like bacteria leave a decipherable record in the rock record?