Scientists know a great deal about evolution with certainty. The very structure of your arm gives evidence. You have a single large bone (humerus) in the upper arm, two thinner long bones (radius and ulna) in the forearm, several small bones (carpals and metacarpals) at the wrist and multiple bones (phalanges) supporting five digits (fingers). So do whales, birds, lizards, dinosaurs, fossil amphibians and ancient lobefin fishes (all of which are certainly different species). The same basic structure has been modified through time by natural selection and adaptation in the course of evolution. The underlying similarity exists because evolutionary change is constrained by existing anatomy. New species are not “built from scratch” but have changed gradually from a common ancestor. (Donald Prothero, “Evolution: What the Fossils Say and Why It Matters”, pp. 105-107).
There is more evidence for evolution in your own chromosomes. You have forty-six chromosomes in twenty-three pairs. Chimpanzees (and all the great apes) have forty-eight in twenty-four pairs. Why? If humans and chimpanzees share a common ancestor then two chimp chromosomes must have been accidentally fused to form a single human chromosome. We should find that one of our chromosomes has had its two halves pasted together. Simply losing a pair of chromosomes would have been fatal to any primate. Chromosomes have a centromere in the middle and telomeres at either end. A chromosome that had fused would have two extra telomeres stuck together right in the middle and two centromeres instead of one. We have such a chromosome (#2) and its genes are almost an exact match for corresponding genes on chimp chromosomes 12 and 13, so close that scientists have now relabeled them 2A and 2B. This could not occur because of chance (unique DNA sequences are involved) but only because of shared common ancestry. It is proof that one species evolved into two species: chimpanzees and humans. (Kenneth R. Miller, “Only a Theory: Evolution and the Battle for America’s Soul”, pp. 103-110).
Perhaps the clearest example we have of a sequence of transitions from one species to another is the fossil and genetic record of the history of whales. From early terrestrial artiodactyls living 55 million years ago scientists can trace a detailed succession of species, each linked one to another by clear anatomical transitions that can be observed in the fossil record, supported by genetic analysis. One key piece of evidence was the discovery of a unique ankle structure, the “double-pulley” astragalus in early whales, exactly like their land-based hooved ancestors. And some of today’s whales still possess vestigial hind limbs, although they are concealed beneath their blubber. (Prothero, op. cit., pp. 318-322).
I urge you to read these two books as well as Jerry Coyne’s “Why Evolution is True”, Cameron M. Smith’s “The Fact of Evolution” and Alan R. Rogers’s “The Evidence for Evolution.” You will see that evolution is not a mystic belief but is both a proven fact and a powerful theory that not only underlies all of modern biology but is the basis for modern medicine, agriculture, criminal forensics and a dozen other fields as well.