Where multiple species are competing for the same food in prime habitat, one will likely come out on top, and the others disappear.
Those species which do survive find ways to make it on land that others won't use.
I should probably also mention that there's DNA evidence of interbreeding between humans and Neanderthals & Denisovans.
So, in that sense, those old creatures did not fully disappear, they just joined, all in the family.
DeoVindiceSicSemperTyrannis: "...if everything is always survival of the fittest, then how does one explain symbiotic relationships?"
"Survival of the fittest" doesn't have to mean constant competition, and there are lots of examples in nature of species which help each other out.
Hard to say how such relationships started, but if you are a large African buffalo, and you wake up from a nap to find a bird pecking the ticks off your back, maybe you're still tired enough to let it continue, and even realize it's doing you a favor?
That's a laugh. Would you like a list of species that eat cottontail rabbits, or mice, or mosquitoes, or alfalfa, or minnows? Many species can compete for the same food and succeed.