Are you perhaps referring to how animals had convergent evolution with mankind?
I am sort of expecting them to reveal later on as a twist that the animal planet was actually a planet mankind already colonized years before the MC arrived, and the similarities to mankind may not be a coincidence. Perhaps the animals were biologically engineered to evolve to be mankind's heirs, long after we are gone.
That or a classic Planet of the Apes twist, and this is literally Earth many years in the future.
These could explain a lot of things.
Yes, but that aspect of it is fairly comprehensible. And I more or less imagined/expected the same. Cant imagine it happening in reality, however.
There are a bunch of unrealistic presuppositions we'd have to make if a singular japanese astronaut was the only one to be able to make it to this planet. That and the culture and habits of that astronaut are fairly indistuingiushable from a modern japanese man. There might unironically be more mongolians than japanese people by the point we are even capable of interstellar travel, if these ethnic groups even exist at all! I don't see why japan would be uniquely capable and responsible for this mission.
The scope for interstellar travel, especially for a planet we presume is in a far off system, requires far greater technological advancement than we have today and would necessitate culture change due to how long it would take. If we're going off of this, why does he not have tools that allow him to send messages back and computers/ai. In particulat assisting him with communicating with these creatures.
Other issues are related to how advanced the beastkin civilization is. They have written language and trains, but are not at flight or space travel apparently. They're also presented as less intelligent, which would make such things less possible. Post neolithic populations were not substantially less intelligent than us and even they just got to writing.
Clothing being more or less reflective of how we dress despite the fact that they have fur.
Language acquisition is hard and I am relatively unconvinced by him being able to pick up writing in the language based on what was shown so far. The greater range of sounds that beastkin make should make their language partially incomprehensible to humans. Yet he seems to understand them perfectly. I'd expect them to give him something like a picture book to be able to learn it I guess.
Where are the other humans? A species capable of interstellar travel shpuld have been capable of colonizing other planets. We currently may see a permanent outpost on the moon in our lifetime but leaving the oort cloud is not yet possible with current technology. Even if somehow the earth was inhospitable, which is deeply unlikely even after a span of 100000 years, there should still be no shortage of human life around. Surely he wouldnt have come alone as well.
There's honestly so much more I can question about the premise; language, technology, culture, preparedness of the expedition, genetics, social organization, demographics, understanding of physics and chemistry, probability of such a thing happening, how the interactions were handled. I imagine some of this will be addressed but it wont all be satisfyingly explained. Even at the level of the premise there's a detachment from reality.
But it's a manga so it's not that important. Would still like to see care put into these things but it is what it is.