Very good argument. Thing is, what gaps are there? We have everything from single celled organisms through to fish, amphibians, reptiles, etc. We even have monotremes and marsupials that were around before placental mammals. So we don't have an example of every single organism that existed. We do have phylogeny that shows a clear route of evolution. We have found so many of the primate ancestors that we can trace a dead end that branches off of our own direct tree. Hominids that stopped evolving and just became extinct.
Answer me this. We can find old skeletons like Lucy that aren't exactly like us now, but we can't find a skeleton that old that matches ours now. That is a clear indication that we did not exist when Lucy did. Lucy came before us and evolution chose for enough different traits to come up with us much much later. Sure, monkeys exist now, but did chimps roam the planet when Lucy did? Chimps are not on our branch of the tree, so we did not directly evolve from them. We do share a common ancestor with them though. Why do creationuts think that every living creature had to evolve from each other? We didn't even evolve from our dead end hominids, but from the ones that managed to live on.
Enough minor changes in the organism's genetics will eventually change it to a different species. We aren't like Lucy anymore, and Lucy wasn't like her ancestors. These changes can happen a few different ways as an animal adapts to changes. Part of a group can be cut off from the original group for some reason. Then only the traits in the each group will help them adapt this way or that.
If one group of animals can't cope with a severe change in environment then they gradually become a minority with many of its similar species becoming extinct, and that gives the organisms that thrive in the changed environment to suddenly thrive and adapt and evolve.
Finally we get us and we are very adaptable, even without having to have physical traits selected for in many cases. It was a matter of time. Unless another species is getting more and more upright with bigger and bigger brains, then we may be the only species that are this adaptable in the mammal form. Our form is very successful, so for other species to become as successful as us, then they would have to start evolving like we did and up with a similar form. We don't have any severe changes in environment going on to start selecting for adaptable traits in other species though. Instead we are encroaching on environments and other species' numbers are dwindling. They won't have a diverse gene pool to help them evolve. They instead die off.
To see a big evolutionary change we'd need to dwindle in numbers and give the other life forms a chance to explode in numbers and mutate and adapt to freed up areas and evolve.
Look at marsupials in Australia. They had virtually no competition and all sorts of marsupials evolved. Some are quite similar the placental forms.
The placentals introduced to australia showed why we don't see the same diversity of marsupials elsewhere. The cats and bunnies caused mass marsupial extinctions by taking over food sources and living space. The placental form has advantages over the marsupial form.
