1. Photosynthesis in autotrophic organisms is an ancient process, which originated in some of the very first organisms to live on the Earth. As life has evolved, the process of photosynthesis has changed in some ways, but still has remained the staple metabolic process for autotrophic organisms. Considering how widely spread and deeply ingrained this process is in the history of autotrophs, it provides a significant basis for the common ancestry of organisms.
2. Though a large portion of life on Earth today does not go through the process of photosynthesis, pretty much all organisms go through cellular respiration. This process is vital to the survival of every kind of life on Earth and is conducted in essentially the same way in all of them. Considering how identical this facet of life is in every place in which it occurs, it is likely that the first organism to undergo cellular respiration eventually gave way to all the organisms in existence today.
3. The uniformity of the structure and function of DNA in all life means that, in some way, all living things are related. No species on Earth has DNA that is not made up of the same 4 nucleotides which code for the same 20 amino acids to make the proteins which determine the organisms structure and function and maintain its well being over the course of its existence. DNA is all different, but it's made up of the exact same materials and functions in the same way in everything, so all life is at least tied together in this way. DNA utilization is a common feature of every living thing, meaning that they all probably shared a similar origin.
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