I don't know if that statement is technically true but it's certainly misleading. Perhaps "all the members of the new mammal orders" were under 10kg, but not all mammals during the early Eocene were under 10kg.
Cambaytherium and Coryphodon, for example, lived during the early Eocene and were roughly pig sized and cow sized, respectively.[1,2]
In other words, that carefully phrased statement, if true, is a great example of the unscientific spin I was talking about.
> fossil evidence of crocodiles at the poles. That likely makes most of the temperate to equatorial zones uninhabitable for humans
We also see fossil evidence of large mammals in India. In the early Eocene temperatures differed less between the tropics and the poles and between winter and summer.
1: https://www.thoughtco.com/coryphodon-peaked-tooth-1093184
I'm suggesting nothing of the kind, but directly quoting from the wikipedia page you linked. Please stop putting words in my mouth.
I will note that the Wiki page on Coryphodon says: "These changes in size are thought to be linked to global climate change, with the size minimum in the Coryphodon lineage occurring shortly after Paleocene-Eocene boundary". Which appears to indicate clear evolutionary pressure to evolve smaller.
The pressure on mammals appears to have been consistently toward smaller size, seen in newly evolved species and the evolution of existing species. We already know that heat penalises larger mammals harder than the small, so that does not seem inconsistent.
That was your statement, not Wikipedia. What was that based on if not the incorrect implication about the size of mammals?
Nonetheless, I've already edited my post to make it clear that I was talking about the implications of the statement from Wikipedia.
> The pressure on mammals appears to have been consistently toward smaller size
If "the size minimum in the Coryphodon lineage occurr[ed] shortly after Paleocene-Eocene boundary" then the decrease occurred before the early Eocene, not during. The temperature actually continued to increase for several million years after the Paleocene-Eocene boundary.[1]
That pressure might have been due to the rapid changes at the Paleocene–Eocene Thermal Maximum, not temperature itself, and after the thermal maximum, size increased again even with the higher temperatures.
1: https://en.wikipedia.org/wiki/Paleocene%E2%80%93Eocene_Therm...
The decrease in size probably had more to do with the chaos wrought by whatever unknown event caused the thermal maximum than with the temperature itself.
To make this all more relevant: I'm not saying that climate change won't be disruptive. It will. I'm just saying we can adapt and endure. And I think we should plan for that, because we clearly aren't going to stop it.
It also "had one of the smallest brain/body ratios of any mammal, living or extinct" so that may not be the best example of how this kind of environment might be fit for humans.
For the Eocene, also according to science, we see fossil evidence of crocodiles at the poles. That likely makes most of the temperate to equatorial zones uninhabitable for humans, whilst being lovely for the "snakes found in the tropics that would require much higher average temperatures to sustain them".
I notice you don't mention that "All the members of the new mammal orders were small, under 10 kg", and "Eocene mammals were only 60% of the size of the primitive Palaeocene mammals that preceded them. They were also smaller than the mammals that followed them".
So whilst it was lovely for species evolved to those conditions, and life as a whole, for large species such as those of the size of say humanity, it seems to have been exceptionally hostile.