The essential nature of the mind of all beings throughout time is the same as Buddha. This is because the essential nature of mind is free of confusion which leads to suffering. That's all. That's essentially what Buddha mind is.
It doesn't matter where or when. We do not have first-hand knowledge about how dinosaurs thought or behaved, but we can draw what are probably very good conclusions based on evidence we have about them, about what they ate, if they lived in groups or not and so forth. We can also guess, that being animals, they struggled to survive. When hunted, they tried to escape. When hunting, they tried to catch their prey. They probably tried to avoid being burned by fire. In some way or another, beings want to be free from suffering. This is universal, regardless of time or place.
Buddhism talks about the 6 reams. What is a "realm"? A realm occurs when the mind arises with conditions. And unobstructed mind (Buddha) will arise with conditions in an clear way and an obstructed mind (unenlightened beings) will arise in a confused and grasping way. Some say that the conditions are also produced by the mind. Others say it is only the experience of conditions that are produced by the mind. Either way, a realm (loka) appears with the activity of the mind, meaning that the mind experiences itself within an external environment.
If the conditions for a certain experience are not there, then that experience will not occur. If a planet cannot support life, then mind cannot arise with an experience of being alive on that planet. Over time, if a planet can support life, if those conditions develop, then mind can arise as the experience of living on that planet. If the conditions only exist for tiny organisms to live in an environment, then mind will only arise with the experience of a tiny organism. It may be attracted by moisture or warmth, not in any conceptual or intellectual way, but attracted just the same, and not attracted to cold or dry.
Over millions of years, this same behavior manifests, regardless of the evolution of the species. So, even though there was no animal realm or human realm or maybe any other realm on earth hundreds of millions of years ago, this does not contradict the teachings. That is because the mind is not limited by time and space. Mind arises with conditions. Do you see the Sun? That is a condition that exists 93 million miles away. In fact, we only see the Sun as it was eight minutes ago. But because we experience it, because solar radiation reaches our planet, mind arises with it.
We only say "human realm' and 'animal realm" and so forth because these are convenient ways of having a greater understanding of the Dharma. But, are playfulness, fear, loneliness, and contentment animal realm traits or human realm traits? Is unquenchable dissatisfaction a human realm trait or a hungry ghost trait? There is a lot of overlapping. A lot of traits are shared, because realms do not exist without being experienced by the mind. This doesn't mean that they are only figments of the human imagination. But it is the mind that creates the experience and the experience is what is roughly categorized as one realm or another.
Are there really hungry ghosts and hell beings and devas and asuras and so forth? I don't know. I can't prove their existence one way or the other. My teacher says there are, so i ask him a lot of questions. But if we only go by "exist" or "not exist" then we are missing the whole point. We are still clinging to the question of 'are the other realms as real as this one?' and they are not any more real than this one.
Reincarnation and rebirth are not the same thing. Rebirth is a clumsy word. A better word would be "approximate spontaneous replication of recent events" but that is too long. "You" are only an approximate replication of the events that manifested as "you" a moment ago. Over time, as those conditions change, "you" change. Before this life time, the circumstances did not arise for you to be who you are today.
It's all about arising conditions. So, as you seem to imply, if the earth did not have those conditions a long time ago, then mind could not arise with the experience of living here at that time.
But minds tend to wander.
Profile Picture: "The Foaming Monk"
The Chinese characters are Fo (buddha) and Ming (bright). The image is of a student of Buddhism, who, imagining himself to be a monk, and not understanding the true meaning of the words takes the sound of the words literally. Likewise, People on web forums sometime seem to be foaming at the mouth. Original painting by P.Volker /used by permission.