Why is the baseline assumption that early humans were not as intelligent as we are now? I've never understood that. It seems like levers and rolling logs would be pretty easy to figure out, or what am I missing?
That isn't the assumption. Modern archaeologists usually assume that ancient people were as intelligent as we are today, or even more so.
What's not assumed is that they had the same thought patterns. People don't derive ideas uniformly from the space of all possible ideas, they tend to think within the constraints and realities of past experiences. If you build a house, it's going to be similar to houses you've seen before. If you paint a painting, it's going to be a painting rather than some other means of expressing yourself with colored pigments.
In other words, ideas are subject to the same kinds of path dependence that technology is. When we see something that's severely anachronistic (outside of it's "normal" place in time), the initial priors are that things like the dating are wrong rather than ab initio invention of a whole suite of different ideas that just happened to be preserved for us.
There's also the assumption that humans in the Americas, prior to the mass arrival of Europeans, were primitive and did not form sophisticated societies like those found in the "Old World". I think this has been otherwise shown, but old assumptions die hard.
The large American civilizations (Inca, Aztec, etc) were essentially bronze-age civilizations. Perhaps Europe would have been so as well had there not been a bronze-age collapse.
My view is on why that is the assumption is because anthropology seems to put a lot of pressure on higher level tasks only being achievable through some mass of individuals. The idea is that ancient people had a struggle to survive and deal with daily tasks, so they must not have had enough time to pursue advanced topics or even care to focus on them.
Probably the lack of similar structures in the area at the time makes this significant. If large arranged stones from 10,000 years ago were common around the great lakes then we'd assume everyone knew how to move large rocks back then.
This assumes we've been comprehensive in unearthing ancient structures that were buried under water and dirt 10K years ago. We mostly find these types of sites when doing new construction. There is likely countless underground ancient structures undiscovered off the coastline, since sea levels rose more than 120 meters at the end of the last glacial period.
Also, I wouldn't assume at all that knowledge would transfer outside of a localized area. It didn't happen like that in the so-called hearths of civilization like Babylon, Egypt, China, etc.
One aspect of this is knowledge is part of intelligence and lifespans influence shared/communal knowledge.
I don't think anyone claims that early humans were not as intelligent as we are now but the weakest link in the knowledge of how to effectively use levers or roll logs, or not (metaphorically) 'reinvent the wheel', would have been the eldest members in the community.
Few things in the field are _ conclusive _ many things are highly suggestive and have few other explainations.
WRT "brain surgey" - the operation in question would be to relieve swelling under the skull that was pressing on the brain - probably caused by a blow.
The remaining skull would show that the hole was cut using an edge, likely extremely sharp flaked stone similar to a modern ceramic edge knife - an original blow would have splintered bone and caused a swelling, the splintering would have been cut away. Other evidence would likely have included signs that the bone restitched itself and "grew back" to a degree, demonstrating the person survived for some years afterwards.
I know nothing about this specific skull, there are others with similar work discussed in journals.
