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This content is taken from the University of Reading's online course, Small and Mighty: Introduction to Microbiology. Join the course to learn more.

Skip to 0 minutes and 16 seconds Every single living organism on Earth today evolved from a single living cell. We call this hypothetical cell the Last Universal Common Ancestor, or LUCA. We’re not sure of the exact date or exactly how it first appeared. But it was somewhere around 3.5 to 3.8 billion years ago.

Skip to 0 minutes and 42 seconds It was probably quite a simple cell at first, a bag of enzymes surrounded by a lipid membrane. No one knows for certain. The key thing we know LUCA did have was a genome. It must have contained at least some DNA similar to the DNA found in the cells of every organism on the planet today. DNA is the molecule of heredity. It contains the instruction manual for creating another organism. As LUCA replicated, sometimes mistakes occurred when the DNA was copied. This introduced mutations. Most mutations were probably harmful.

Skip to 1 minute and 26 seconds But some mutations led to differences that meant the daughter cells were better at doing things than their parent.

Skip to 1 minute and 34 seconds Maybe they could replicate faster or live in a new niche. At some point, two separate groups arose that were becoming noticeably different from each other. One group gave rise to what we recognise today as the bacteria, and the other as the archaea.

Skip to 2 minutes and 0 seconds The bacteria and archaea continued to diversify by mutation.

Skip to 2 minutes and 7 seconds Different species arose that were better adapted to particular niches. At some point, an archaeal cell engulfed a bacterial cell. It may have been trying to eat it. But the bacterium somehow survived and found it could replicate inside the archaeal cell. This archaeal cell found its bacterial resident, or endosymbiont, to be beneficial. A third group of organisms had now evolved, what we recognise today as the eukaryotes. The eukaryotes started to diversify by a mutation.

Skip to 2 minutes and 50 seconds Many eukaryotes, including protists and fungi, remained as single cells. But other eukaryotic cells evolved another way of living, in a group of many cells– sometimes trillions of cells– that are all clones of each other. These are the multicellular organisms and include plants, fungi such as mushrooms, and animals. Over the course of evolution, more species have gone extinct than are alive today. The tree of life is rich with millions of different species, the vast majority of which are microbes.

Skip to 3 minutes and 31 seconds Viruses are not made of cells and are considered non-living entities. They are, therefore, not included in the tree of life. It is thought that viruses evolved independently multiple times and that there is at least one– probably hundreds– of species of virus specific to each living organism in the tree.

Tree of life

Did you know all living organisms, including you, evolved from the same, single celled microbe; the Last Universal Common Ancestor (LUCA)? Where do microbes fit within the tree of life? Watch this animation to find out.

After you have watched the animation, move to the next Step to see some microbes in action in our 3D model. Don’t forget to ‘mark this Step as complete’ before you move on.

This animation was created by Chris Lewis, Izzy Bahrin and Raj Bhogal, undergraduate students from the University of Reading’s Typography and Graphic Communication department.


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Small and Mighty: Introduction to Microbiology

University of Reading

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