It’s somehow unsettling that we are the only ones left. Most large mammals come from a genus with many cousins. We’re alone either because we killed the others, or because intelligence actually has a hard time surviving in nature. Neither is really good news for us.
Given the number of genetic bottlenecks we have evidence of in our past, we may all be the same species and the modern human is just the last surviving group of inbred Homos.
Bottlenecks are more likely to be an issue for species with a limited range, population and a slim range of survivable conditions. Intelligence has a way of expanding those ranges by making you more equipped to access otherwise inaccessible resources. Think of elephants navigating between watering holes during droughts, crows using tools to fish out hard to get food, or orcas using sophisticated hunting techniques to get at prey they otherwise could not.
Intelligence adds flexibility, and flexibility is especially useful under pressure.
Alas, I can't find it now... there was a book that I read (Bear / Brin / Benford as the author I believe... maybe Baxter... one of the great Bs I'm sure of) about the matrilineal line from a small shrew like animal in the time of the dinosaurs through neanderthals to modern times... and beyond.
Part of the "and beyond" was the theme that intelligence and our brain is an extremely expensive organ to maintain (our brain consumes about 20% of the calories that we consume a day https://www.ncbi.nlm.nih.gov/pmc/articles/PMC124895/ ). After some catastrophe that reduces the energy budget that animals can use... well, intelligence is one of the things to go.
(Late edit: It was Baxter - https://en.wikipedia.org/wiki/Evolution_(Baxter_novel) )
Intelligence does have an incredibly hard time surviving in nature. Humans are hyper-optimized for it. We are far weaker than chimps, and burn 30% more calories. We are slow reproducing k-strategists, with infants that are helpless for years.
That said, Humans are planet level apex predators, and universal omnivores that consume everything whales to yeast and synthetic foods made from elemental materials.
Over the course of our existence, our capability for growth has vastly overshadowed any downside from intra-species hostility. We are also the most cooperative and social animal that has ever existed.
This is a pretty strong position to be in, and certainly better than just about any other animal sharing the plant.
I'm not saying that this necessarily happened, only that the burden of proof for your theory should be higher.
Not to say that there weren't interspecies genocides, but we interbred with them and then outcompeted, for resources, the pure species into extinction generally speaking. Which was inevitable, parallel to how homo sapien lineages also die out from simply being outcompeted.
It is likely that the Neanderthal genome was outcompeted down to a percentage at which it was and is competitive. 50-50% Neanderthal-Modern Human? No dice, Darwin. It had to be reduced to a few percentage points, at which point individuals started being competitive. Was the last pure Neanderthal pair murdered, did they die from starvation, or did they just refuse to have kids? It could have been any of those, but the general procedure was likely defined by resource competition.
And Neanderthal isn't the only alien-homo genome mixed with that of modern humans.
> No evidence of Neanderthal mitochondrial DNA has been found in modern humans.[30][31][32]
> This suggests that successful Neanderthal admixture happened in pairings with Neanderthal males and modern human females
Of course it also might mean that human males and Neanderthal females can’t produce viable offspring
Consider a serial 'wife-stealing' human culture. One generation of human males leaps upon a small population of Neanderthals, killing the men and stealing the women. The next generation, a new group of half-neanderthal males leaves the colony in search of wives, finds a human group, kills the men, and steals the women. The children of this generation will be 1/4 Neanderthal with no Neanderthal mitochondrial DNA. Imagine that the first colony gets wiped out, as most ancient communities were. There you go.
So either it didn’t happen at all or if it did it happened in on a much smaller scale than the opposite (Neanderthal male and human female)?
Also this scenario is completely unsubstantiated conjecture so hardly worth discussing.