One of the things that you need to get out of your head about evolution is the assumption that there is a purpose. Everything evolves as a side-effect. Something that has evolved successfully enough to survive any given environment may stay more or less the same for millions of years or, if the right combination of things occur, a new species may emerge. For a variety of reasons.
Genes change, causing physical or other biological changes. If those changes manifest themselves is positive ways in the critters they affect, then said animal may slowly change into some other species as it starts specializing in eating a new type of food or living in another environment. Longer fur might mean it can live further north or south. A more effective respiration system might allow it to live in hotter, dryer climates. The legs may have gotten longer due to genetic change and suddenly the critter can run faster and cover more ground.
Humans may well have succeeded when they climbed out of the trees because the leg and lung changes that occurred made us great runners. A good human runner now cannot outrun a deer, but it can outlast a deer, and chase it down in a couple of hours by wearing it out. Then you just walk up to the thing and hit it over the head with a rock.
Most changes, however, are detrimental and kill off the recipient of the gene change. Or offer no major advantage. The critter may still be on the way to being a new species if only because the feather colors change or it starts eating by climbing up trees instead of walking down them.
Don't forget that all this takes place over an incredibly and unhuman amount of time. We can't think in terms of millions of years. Our imagination can't handle such numbers well. Let along billions of years. But you have to factor time in with the other forces of change.
We have no trouble finding genes in humans that not only match genes in yeast, but that perform the same function in both. We can match genes in other critters as well. And plants. And sponges and sea urchins. So some of our genetic material is still similar to other species. While other material has changes drastically. Thats why putting people in bread dough is of little use.
If we didn't have evidence for this stuff, it would be way to incredible to even consider. But we don't even need the fossil record to provide sufficient evidence to show that evolution has and does occur.
You mentioned in the other thread that you sort of understood viruses or bacteria (I forget which you were talking about) evolving, but that they were still viruses after they were done. It works in big things too. We evolved from older species of apes. And we're still apes. So not all big changes cause a fish to become a salamander. Though that did happen. Other fish became us. We know that because the nerve in fish that used to go from the brain to the gills, and past the fish heart on the way, now controls swallowing in humans and other mammals. And it still goes past the heart on the way. Except the heart is not in a direct line any more, so it has to leave our brain, go down around our heart, and return to our throat. In giraffes, instead of going about a foot, it goes 14 feet. Evolution doesn't have to be efficient, it only has to work.
Not that efficiency isn't better. It's just that there is no actual planning involved, so silliness like that is commonplace.
Sometimes it has repercussions. Like the human back, which was worked fine in our ape ancestors as we ran around in trees. When we stood upright, our back went along for the ride but didn't like it. Thats why at leat 25% of us suffer from serious back problems in our life. Because our back is a weak link in our skeleton. An error typical of evolution. And one I would hope a god making us would have avoided.
Evolution via the genetic change process helps some critters survive. Others die because of it. There are no guarantees. Stuff happens. Sometimes good, sometimes bad.
Life is a crapshoot. So far we humans are winning. If you don't count the backstabbing that we've evolved to do so well.