The number of differences between the proto-organisms and current ones is massive. We're talking about taking a few trillion years for that number of changes to occur. Not a couple billion years, but orders of magnitude more than the age of the universe.
When a change happens, it's not some huge specie altering over-night change, it's a minor change. A limb that's now .2% longer, giving a .00002% better chance of survival to those who carry the gene. That means that not only does the gene have to spread through the entire population, it has to avoid getting bred out, since there's no guarantee that the advantage will get passed on. It also has to work with the rest of the anatomy of the carrier, so if that .2% longer limb means that the stress it puts on the muscles that have to control it overload those muscles, then it doesn't work. Then there's also the fact that a lot of changes demand more than one change in order for them to be effective. If a creature evolves a light-sensitive spot on it's ear, but that spot is covered by hair, then it's useless, unless there is another change that causes the hair to not grow there. Remember that evolution is not triggered by environment, but rather reinforced by it. It's like having a billion-sided die with a unique color on each side. Evolution is like rolling that die on a felt table that is one of those colors and every time it matches the color of the felt, you get to change one side to match the color of the felt, but if it doesn't then 1 out a thousand times 1000 random sides gets changed to a color that is not the color of the felt, giving the chance that the color will be dropped. A lot of people think that evolution happens as if the color of that felt drives the chance that the right color will come up. The changes that evolution demand are random. Environment can help to filter out those random changes to keep the good ones and lose the bad ones, but it has NOTHING to do with what changes happen. All of this and more works together to become a process that is so slow that the time it takes for the changes required to get from a mammoth to an African elephant is huge and the time it takes for us to go from a sponge-like creature to a human is longer than the age of the universe. Then let's talk about the fossil record - long periods of massively slow, punctuated short bursts of massive change. Poeple like to try to say that the environment changed, but that doesn't create more change, it just changes the filtering process that the environment provides.