A fascinating question to consider is: could some other civilized species have appeared on our planet long before the current race of humans appeared? I refer to the possibility that civilized intelligent life may have arisen on this planet many millions of years ago, such as 10 million years ago, 20 millions years ago, or 30 million years ago.
Most people will immediately dismiss such a possibility, using reasoning such as this: Of course, mankind must be the first civilized species to have appeared on our planet. If there had been some previous civilized species, they would have left many traces of their existence which we would have detected long ago.
Such an argument at first seems very powerful, but it is by no means conclusive, for several reasons I will now list.
Evidence of a previous civilization might have been slowly wiped out by eons of erosion, decay, biological processes, and previous geological upheavals. Life After People was a fascinating recent television series that dealt with the question of how soon the structures of human civilization would break down and disintegrate if man were to suddenly disappear. The answer often given by the series was: surprisingly soon. This television series was constantly stating that large human structures such as skyscrapers and bridges will break down and crumble over centuries or thousands of years, if there is no one around to maintain them. A simple example is a bridge. Given a few centuries with no maintenance, sunlight will cause paint on the bridge to flake and peel, rust will cause the steel to weaken, and the bridge will then collapse. Another example is roads. Given many years of no maintenance, and many years of freezing temperatures, cracks can form in roads, which winter ice will then widen. Given thousands of years with no maintenance, the road will disintegrate.
A rusting remnant of a vanished civilization
Then there are long-term geological processes, which tend to break down and cover up existing structures. Examples are earthquakes, glaciers, sedimentation, volcanoes, asteroid collisions, flooding, and continental drift. Acting over millions of years, such processes may have completely buried evidence of a civilized species that may have existed before our species.
But wouldn't there be many fossils of such a species, if it had existed? Not if the species buried its dead without coffins. When bodies are buried without coffins, the bones will normally disintegrate within a few thousand years. It usually requires a freak geological occurrence (such as being surrounded by tar or amber) for a bone to be preserved for millions of year.
A previous civilized species on our planet may have become extinct before it became industrialized. If you think about the possibility of a civilized species existing on our planet long before the current race of humans, you may think of it having all the trappings of modern industrial civilization. But a civilized species could have arisen on this planet millions of years ago, and become extinct long before reaching the industrial age. Wars, plagues, natural disasters or climate change (such as the onset of an ice age) may have made such a civilized species extinct. If a civilization perished long before reaching the industrial age, it might have left relatively few traces of itself. Cultures such as the ancient Romans and the Egyptians (with their tendency towards gigantic engineering projects) are less common than cultures which leave smaller traces of their existence.
Evidence of a previous civilization may have been quickly wiped out by a planet-wide war or natural catastrophe. There are several possibilities for a planet-wide catastrophe that could have entirely wiped out traces of a previous civilized species. One is a global nuclear war. Imagine nuclear war involving the explosion of, say, 50,000 200-megaton bombs. That might have been sufficient to instantly remove almost all traces of a previous civilized species. If such an event had occurred millions of years ago, we might have no evidence of it today. Another possibility is that an asteroid might have struck the planet, causing a planet-wide upheaval that buried all traces of a previous civilized species. Another possibility is a gigantic volcanic eruption like the one that supposedly occurred out of Yellowstone Park in the United States about 640,000 years ago, burying land 1000 miles away in ten feet of ash. If there had been a particularly bad eruption millions of years ago, it might have buried all evidence of a previous civilization under many meters of dust and ash.
A previous civilized species on our planet may have been wiped out by interstellar visitors, who then removed all traces of it. One very unlikely but interesting possibility is that visitors from another planet may have wiped out a previous civilized species on our planet, and then removed all traces of it. Imagine if millions of years ago some civilized species had arisen on our planet. Such a species may have received interstellar visitors who judged it very harshly. The interstellar visitors may have thought that the civilization on Earth was too evil or too stupid or too much of a threat. The interstellar visitors might then have completely destroyed the civilized species on our planet, and wiped out all traces of it (which could have been done through the use of robots). Such visitors may have thought: “This species is an evolutionary failure, so let's get rid of it and all of its works – we'll clean the slate, and give this planet another chance to produce an intelligent species.”
A previous civilized species on our planet may have migrated to somewhere else, and removed all traces of its earthly existence. Imagine if a civilized species had arisen millions of years ago, and had developed interstellar travel. Such a species might have migrated to other planets. But surely, one may say, such a species would have left behind many monuments on our planet, to commemorate its previous existence on our planet. This would not necessarily be true, however. If such a species were to evolve to some higher state of existence, it might look back with contempt at its original state when it existed on only one planet, thinking of such a state as being utterly primitive. Such a species might have no interest in preserving the monuments of its younger days. Just as you might throw out a scrapbook of your elementary school achievements, a species that had reached some interstellar level might want to erase all evidence of its youthful existence on our planet, and restore Earth to a purely natural state. Thousands of years after developing interstellar travel, such a species might have developed some refined new standard of beauty for architecture, and might then have regarded all of the buildings it had erected on our planet as primitive and ugly, no more worthy of preservation than some grass huts built by a primitive tribe. Such a species might then have decided to remove all of the traces it had left on our planet, and to restore the planet to its natural beauty, leaving it as a kind of giant park.
We probably are the first civilized species to arise on this planet, but in light of all of these possibilities (and others that can be imagined), we really can't be sure whether civilization first arose on our planet several thousand years ago or millions of years ago.