With humans and their ancestors, until we had direct genetic analysis, the base assumption was that any traits that we shared with other Homo, Neanderthals or australopithecines (and these would have largely been overt physical and physiological ones from the shape of bones) came about through common ancestry.
The boom in genetic analysis allowed two things - to see genetic identity such as unique genetic markers, which was previously invisible for much of fossil remains, and then it allowed us to see them in a chronological scale, i.e. that Gene X came into the human sequence at a specific point in time, rather than having to be assumed to be there from the beginning.
It does complicate the way we map interaction between human lineages, and by what we mean as a species. The reliance on fossil remains also reinforced the view of the human past as being rare, remote individuals. This has influenced a view of human ancestors as being very circumscribed in space and time, and in much of the world only bumping together at the edges in their distribution. That has also made it easier to think that the hominids were endogamous (within-group mating).
Genetic palaeontology is proving those assumptions are largely unfounded, and also changing the nature of how we see ourselves as humans separate to our hominid cousins, so Neanderthal-Denisovan (a group entirely differentiated by genetic markers) babies are a Big Deal.
In the 1980s I went to a talk by a leading palaeoanthropologist talking about the latest discoveries, and the baseline ‘null’ position was that while we had about a dozen named human ancestors, and perhaps twice that many cousins there was almost zero evidence of them co-occurring, because of the limited fossil record and the fuzziness in their dating. That meant that at any one time in any one part of the planet there was seen to be only a single human species on the ground, as there is now. Forty years on its hard to maintain that position.
Anything between about 19.5% and 50% oxygen is safe to breath indefinately.
There might be a problem however if you have a “non-Tesla armormores vehicle” as an attempt to keep you safe from preditors. The higher levels of oxygen would increase the flamability of things the spark plugs could cause an explosion. For this reason OSHA standards define safe lower and upper limits to the concentration of oxygen in the atmosphere to which workers are exposed as a miimum of 19.5% and a maximum of 23.5% (you would “probably” be OK a little outside that range but the level of risk would rise the further outside it you are).
I’ve heard that atmospheric oxygen might have been 35% back in the Carboniferous, when all those trees were being converted into coal, but not in the Cretaceous. IIRC, someone figured out that 35% would be about the limit because higher than that and green wood would burn from any source of ignition.
It’s the mega-bugs that I would be most afraid of. Dog-sized centipedes and spiders with funnel-trap-doors the size of garbage can lids. And don’t get me started on ant-lions. For each dino out there, there are x1000 super-bugs.
Scorpion-ish critters the size of cows. Those would be no picnic. Or rather you’d be their picnic. Pronto.
I doubt Hollywood’s entire historical output of scary aliens and fictional earthly creepy crawlies adds up to what you’d find in 5 square miles of Pleistocene forest & swamp.
Possibly on the dinos. Try lice the size of crawfish. In fact there was a short story about a time-traveling hunter who shot a brontosaurus and then didn’t wait long enough before going near the carcass.
Not much larger. The main thing keeping insects from evolving to larger size is their inefficient breathing method. Increased atmospheric oxygen can make up for this, so that there were larger ones only when the oxy levels were higher.