A good friend of mine asked me what evidences I have for my belief in the idea that all living things on earth have common ancestors. This is an excellent question since it's very important to base our judgement about reality on good evidences, or we can easily be led astray by our commonsense, biases, and other extremely appealing ideas that don't match reality.*My specific belief is this: All living things on Earth descend from one or small number of groups of ancestors. (I would like to say one group, but I'm open to the possibility that there are unknown life forms that have independent ancestors, distinct from our group.)
For this particular belief, a very good summary of evidences can be be found here (29+ Evidences for Macroevolution: The Scientific Case for Common Descent.)
Also, a very informative depiction of known life on Earth can be found at the Tree of Life Web Project. Make sure that you read this page about the interpretation of the image.
An implication of the belief is that I am a (mostly distant) relative of bacteria, mushrooms, blue-green algae, jasmine rice, earth worms, monitor lizards, cows, starfish, mangoes, octopuses, fire ants, bees, dogs, monkeys, gorillas, parrots, sharks, triceratops, T-Rexes, the slaves that built the pyramids, Ghenghis Khan, and you, the reader.
Another implication that if there are extra-terrestrial beings (which I really hope there are, although I don't have any evidence for that yet) it's very unlikely for them to look very similar to us. Even if we and ET have the same common ancestors, that ancestors must have come to earth billions of years ago and will have enough time to evolve into us and myriads forms of animals and plants. Similarly, the ET would descend from the common ancestors over the similarly long time period. So, Captain Kirk in Star Trek should not be able to mate with those ET chicks if he is not really, really into inter-planetary bestiality. (Same reasoning applies to Superman and Lois Lane.**)
Yet another implication is that human is not the apex of evolution. There will be other lifeforms in the billions of years ahead if we don't destroy all life first. We are the first Earth lifeform that can destroy all life on Earth using our technology, but we are not necessarily the best lifeform. To think that we are the peak of all possible lifeforms is just extremely arrogant.
Finally, my body is made of atoms from things (living/dead, never-alive) from the past. Therefore, I also believe that when I die, some atoms that used to be me will be recycled into other living things in the future but the atoms won't carry any of my memory there, since my memory is not stored in specific atoms, but in the patterns of how atoms are arranged. This is supported by the fact that, atoms inside my bodies are very likely to have been in bodies of people of the past, yet I don't share their memory. Conversely, with very high probability, atoms that are in my body now weren't the same atoms that were in my body when I was a child, yet I still remember many things from my childhood. This gives me a hope that, in principle, human can preserve identity/memory by capturing the memory patterns and upload them to more permanent substrate and become relatively immortal.
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* The great Richard Feynman summed it up (in this book) thus: "... there are many reasons why you might not understand [an explanation of a scientific theory] ... Finally, there is this possibility: after I tell you something, you just can't believe it. You can't accept it. You don't like it. A little screen comes down and you don't listen anymore. I'm going to describe to you how Nature is - and if you don't like it, that's going to get in the way of your understanding it. It's a problem that [scientists] have learned to deal with: They've learned to realize that whether they like a theory or they don't like a theory is not the essential question. Rather, it is whether or not the theory gives predictions that agree with experiment. It is not a question of whether a theory is philosophically delightful, or easy to understand, or perfectly reasonable from the point of view of common sense. [A scientific theory] describes Nature as absurd from the point of view of common sense. And it agrees fully with experiment. So I hope you can accept Nature as She is - absurd.
I'm going to have fun telling you about this absurdity, because I find it delightful. Please don't turn yourself off because you can't believe Nature is so strange. Just hear me all out, and I hope you'll be as delighted as I am when we're through."
**Larry Niven wrote a very funny essay called "Man of Steel,Woman of Kleenex" on the problems involved when Superman and Lois Lane try to mate.
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