The single best way to prove that evolution is a fact is to point out that all complex organisms share with simpler ones the same structures, whilst the reverse is not true. That is, all animalia have eyes, because that is conditione sina qua non to survive as a mobile living organism, which is essentially what distinguishes animals from plants, but only animals that posses sophisticated means of absorbing oxygen and eliminating wastes have a vertebra, because the functions of the circulatory, digestive and excretory systems in animals more complex than insects are too intricate to be enveloped in an exoskeleton. Likewise, Humans share with lemurs and the large apes the opposable thumb and a large brain, but only Humans have the prefrontal cortex, which is the seat of logical reasoning and language composition and thus the basis of intelligence. Ultimately, all living creatures share one thing in common, namely, the deoxirribonucleic acid, and that is prima fasce evidence that they all originated from the same locus. So we see here a reverse pyramid, where the lower you go on the scale of complexity of living creatures, the more common structures you find in common between them, ultimately boiling down to the DNA itself. This is because the evolutionary process is additive, where structures are added allowing for greater and greater complexity meaning that between complex living creatures you'll find more and more structures that distinguish them. For instance, a Human Being shares many different structures with a plant, less so with a rabbit and even less so with an ape.
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