system
Given the following post, respond as if you're commenting on Reddit:
human
Why did every single species in the Homo genus except for Homo sapiens sapiens go extinct?

From what I can gather Homo neanderthalensis for were intelligent enough to make tools out of bone and rock for example, so why couldn't they evolve alongside what would eventually become modern humans? Did our species' "merge"?
gpt
To a first approximation, every species that has ever lived is now extinct, so the unusual thing is that H. sapiens is still around.  

Species don't last all that long, geologically. The average is a couple of million years, but much shorter periods are not unusual.  Most of human ancestors and cousins probably did last around that long.  Homo sapiens is only a couple hundred thousand years old, and Neanderthals probably lasted about twice that long before going extinct.  So neanderthals were arguably more successful than sapiens so far; we have another 200,000 years to show we can do better.  

What's more, for the majority of H. sapiens species' lifespan, multiple Homo species did coexist.  Neanderthals and sapiens coexisted for about 150,000 of the 200,000 years sapiens has been around; probably so did Denisovans, and "hobbits" were around for a fair bit of that too.  So it's only the most recent quarter of our species lifespan that we've been alone.  

H. sapiens and H. neandertalis did interbreed, but the species didn't merge.  Successful interbreeding was extremely rare, on the order of one or two successful interbreeding per generations (interbreeding per se was probably rare, but the hybrids were probably subfertile at best -- male hybrids were probably completely sterile).  That's nowhere near enough to merge the species.  

No one knows why Neanderthals went extinct, or why sapiens succeeded them.  There may well have been interspecies conflict, but one most important point was the Neanderthals and sapiens needed different ecosystems, and climate change led to Neanderthals running out of the denser forest they needed for their ambush-type hunting, with sapiens preferring the lighter forests and the half-open savannahs that were left behind.  Quite possibly it was a combination -- in previous climate changes, Neanderthals were able to cope with the new environment reasonably well, but this time there was a smaller, faster, better-adapted cousin already there, competing for food and resources, and that may have been enough to push them over the edge.