Neanderthals had bigger craniums than homo sapiens (and thus bigger brains), so it does indeed seem like size isn't the biggest factor. Granted, that size difference is much smaller than elephant vs giraffe haha.
What I remember being taught in school is that Neanderthals were more intelligent than Cro-Magnons in many ways, but were ultimately less successful (evolutionarily speaking) because they were less social.
But isn't all that essentially cultural knowledge?
If you compare the art and technology of Homo sapiens 50,000 years ago vs 50 years ago... it's clear that 50,000 years ago gives no inkling of Homo sapiens' intelligence.
So how could we possibly infer that Neanderthals were less intelligent? How could we possibly separate their intelligence from their state of cultural and technology evolution?
Edit: indeed, a quick Google search reveals that the idea that Neanderthals were less intelligent than humans has no real evidence, that it's now considered to be an outdated/discredited view. So wherever you seem to be getting your facts and conclusions from, appears to be wrong.
I've read that Neanderthals were actually more intelligent, but that they were less social than Sapiens. Sapiens ganged up on the more solitary Neanderthals and overwhelmed them through violence or breeding.
Interestingly enough, homo sapiens was also really slow to adopt to change for a very very long time. Basic toolkit was unchanged for something like 1M years.
Then cam the Tonga event, homo sapiens population is reduced to less than a thousand individuals (perhaps much less), and suddenly we got creative.
Innovation wasn't a thing before that. It has been everything since.
I was under the impression that the evidence for the Tonga event being the driver of a human bottleneck has weakened in recent years. Is that incorrect?
This particular failing of Neanderthals resulted more from their extremely small effective population size, i.e. spiralling inbreeding, which eventually lead to total population collapse.
There's not a lot of evidence to support that, though there is some. They could have been wiped out by disease, as opposed to any superior intelligence on the part of us sapiens.
I mean most anthropologists now believe that Neanderthals were at least as intelligent as homo sapiens, and most of us do have some Neanderthal genes...
“Intelligence” isn’t synonymous with “evolutionary fitness.” There are plenty of reasons why the genes of a more intelligent species might have had less reproductive success than the genes of a less intelligent species.
Indeed, I think Steven Jay Gould said that the jury is still out on whether intelligence has proved a successful trait for humans, given that we have been around for barely eye blink on evolutionary time scales.
Given that we know next to nothing about intelligence at this level, no one can say that some gene doesn't have any effect on intelligence, especially if it has any kind of effect on the brain.
IIRC, the vast majority of Neanderthal genes that have persisted in to Modern human populations relate to immune system function, but I haven't read anything about depression or addiction-related genes being in that set. Is this a new discovery, or have I just missed it?
Many, many genes can contribute in some (usually very small) way to depression/addiction/other psychiatric disorders, and a single gene can contribute risk to a variety of disorders. So it's not as clear cut of a thing to identify as whether a gene is involved in the immune system. In fact immune system genes have come up towards the top of genome wide association studies for conditions like Schizophrenia. So it doesn't even have a clean relationship with "genes expressed in the brain".
All that is just to say that there is evidence that some Neanderthal mutations are linked to depression risk, but I wouldn't go so far as to characterize any of the involved genes as depression-related. Maybe that's just semantics though.
More specifically here is a relevant snippet from a study summary [1]:
"These analyses supported links to traits such as mood disorders, depression, obesity and different types of keratosis. The most significant associations were for mood disorders and depression, for which Neanderthal SNPs can explain up to ~1% of the risk."
IIRC there was also a (less strong) association with nicotine addiction. Not sure about addiction more generally.
