I remember this being posted here a few months ago. There's the question of evidence but also simply the question of prerequisites for technological civilizatio

I remember this being posted here a few months ago. There's the question of evidence but also simply the question of prerequisites for technological civilization. People have mentioned fossil fuel depletion. These do renew on geological timescales, but there's a minimum bound of when they could have started to accumulate. The reason we don't have fossil fuels going back further than the Palezoic isn't because a far-past civilization may have depleted earlier stores, but because the types of organisms that eventually create fossil fuels didn't exist at a large enough scale until the Carboniferous period, which occurred in the late Paleozoic. There are other possible sources of energy, but none as easily accessible as igniting a high-energy combustible material. There's the general evolution of bodily morphology necessary for advanced tool creation. Although I am sometimes struck during all the nature documentaries I watch of animals stuck in rough situations where greater intelligence might have helped, I am far more often struck by animals that would have been helped by grasping hands. Among sea-dwellers, you've got the tentacles of certain cephalapods for combining manual dexterity with high-intelligence and that's pretty much it. Even extremely intelligent whales are quite limited in what they can do with tools because all of their limbs are taken by locomotive purposes, whereas cephalapods often move via jet streams. On land, this requires bipedalism, so when did this first start to show up? There are no bipedal reptiles. Bipedalism in mammals is quite late arriving just because mammals are quite late arriving. It's not entirely limited to apes. Otters can do quite a bit with their hands, raccoons to some extent. Birds unfortunately ended up needing their wings to fly. Earlier dinosaurs did have grasping hands and likely at least a few of them had pretty decent manual dexterity, but these didn't show up until the Jurassic or so. Then there is intelligence, and not only intelligence, but the need to learn. Species that develop cultures and highly-adaptable learned behaviors tend to have both longer gestation periods and longer times to reach sexual maturity and adulthood. This makes them more dependent on parental nurture. These are r-selected species, which again, I don't believe includes any known reptiles. It seems to have become a common strategy with mammals specifically because mammals developed the ability to secrete milk, and in birds via gaining the ability to store food they could regurgitate from their stomachs. This allows young to not have to fend for themselves and they can spend years learning all of the complex behaviors required to do something like create a civilization. Neither of those traits showed up until after the K-Pg extinction. There's the rather obvious constraint that you need to be a highly social species in the first place. Intelligent or not, you can only build a civilization through the coordinated actions of many across large divides of time and space. Social behaviors and division of labor show up in colonial insects and you could even argue they have something akin to civilization, but their small size seems to preclude the level of intelligence necessary for what we would recognize as an advanced technological civilization. Large-scale social behaviors in larger animals has tended to show up for one of two reasons, either because traveling in herds offers protection from predators, or because hunting in packs allows for all to thrive even if only a subset of adults at any one time are capable of acquiring food while other needs to focus on learning, play, or child-rearing. The group intelligence and behavioral sophistication required for herds is not as great as for hunters, so we would probably expect predators to be the ones who might develop technology and civilization. This is again not something we see in reptiles. It shows up in birds and mammals, but also not until after the K-Pg extinction. Some types of dinosaurs very likely hunted in packs and had social communities with division of labor. Once you have those things, you still need a means of communicating and recording highly technical and specialized information. In humans, this required specific mutations in the linguistic center of the brain and in our mouths and throats to produce language, and these only happened in the last million years. That is, of course, not the only way to do it. Certain whales might have sufficiently sophisticated vocalizations that, if they had the means to invent writing, it might be enough. Some of the chemical ways in which certain types of insects communicate could potentially be far more sophisticated than we could imagine, but again, they're insects and limited in what they can do with extremely small brains. Birds also have extremely sophisticated vocalization capabilities, but can't write because of lack of manual dexterity and they don't show up until after the K-Pg extinction. Reptiles don't seem to have particularly sophisticated communications capabilities as far as we can tell. This again leaves open dinosaur capabilities we may not be aware of. All in all, it just strikes me that all of the prerequisites I can think of that have to be present on the biological level show up the earliest we know for sure sometimes in the late Cenozoic after the K-Pg extinction, and well after really, given the need to recover. The very earliest they might have occurred in earlier types of land animals that show the necessary morphological and behavioral traits is the Jurassic. The idea that organisms like this may have actually existed in the Silurian just seems mind-bogglingly unlikely. Forget about the visible effects they may or may not have had on the surface of the Earth or potentially nearby non-Earth bodies. How and why did their entire evolutionary family tree disappear and leave no descendents?