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Science fiction and fantasy stories are full of alternate worlds populated by races of beings that look mostly human but kind of different. But is that at all realistic?

When the dominant life forms emerged on our world, they were nothing like humans; dinosaurs ruled the earth for millennia. And mammals didn't out-compete them on anything resembling a level playing field; there were no cavemen hunting the dinosaurs to extinction. A cosmic one-in-four-billion freak accident dropped an asteroid on the planet and plunged it so deep into nuclear winter that anything the size of a dinosaur simply didn't have enough food chain to support itself.

So, assuming a planet with generally Earth-like conditions, where life arises, but it never gets a giant doomsday rock dropped on it 3.95 billion years in, what would the picture of life be like 65 million years later? How would the rise of humanoid life to dominance that sci-fi takes for granted be affected by the lack of a past incredibly unlikely catastrophe that wiped out the original, decidedly inhuman, dominant life forms?

Some specific points to keep in mind, as basic examples of the things that might have been different:

  • Would mammals have ever come about?
  • Would sentience or sapience as we know it have ever come about? (The dinosaurs did just fine without it.)
  • If sentient or sapient life exists, would abundant fossil fuels exist to enable them to achieve the Industrial Age?
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  • $\begingroup$ Earth has been subject to multiple mass extinction events, including far more dramatic ones than the one at the end of the Cretaceous that killed off most of the Dinosaurs. Dinosaurs were (and the survivors still are) generally seen as sentient (having minds able to react to perceptions), but not sapient ('thinking like us' in some vague way we can't quite agree on) which is what you probably mean, and should definitely clarify. You also seem to be assuming that your "Earthlike" planet would have exact or near exact duplication of Earth's biological history to that point. $\endgroup$ – smithkm Dec 1 '14 at 23:00
  • $\begingroup$ @smithkm: The whole "sentient vs. sapient" distinction is a bit fuzzy to me. What I meant, specifically, was "having minds capable of self-awareness and abstract logical and philosophical reasoning." (For example, the ability to ask what-if questions about the consequences of past events having taken a different course.) ;) $\endgroup$ – Mason Wheeler Dec 1 '14 at 23:39
  • $\begingroup$ Both terms are fuzzy in general but if you mean 'the kind of thinking that is specific to a "person"' then sapient is probably what you are after rather than sentient. Depending on the exact way you 'defuzz' it, sentience may be common to most animals with a central nervous system. $\endgroup$ – smithkm Dec 1 '14 at 23:49
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    $\begingroup$ @Mason Wheeler there are surviving dinosaurs - we call them birds. And some birds are pretty smart. $\endgroup$ – Peter M. - stands for Monica Dec 3 '14 at 23:40
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    $\begingroup$ Polly want an opposable thumb...then Polly get all the cracker. $\endgroup$ – Oldcat Dec 4 '14 at 0:52
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Would mammals have ever come about?

Yes. Mammals did exist at the same time as Dinosaurs. Humans? or anything resembling a human ancestor? No.

Would sentience as we know it have ever come about? (The dinosaurs did just fine without it.)

It is possible yes. Maybe the Dinosaurs would have eventually had sentience, maybe they did and were wiped out while still in a "hunter/gatherer" level of society and tech. Humans have been around 200,000 years, that isn't much compared to the 65,000,000 years before that. So it is actually very likely that sentience would have come along, probably much sooner than it did.

If sentient life exists, would abundant fossil fuels exist to enable them to achieve the Industrial Age?

Why not? Petroleum comes from "large quantities of dead organisms, usually zooplankton and algae" The extinction of the dinosaurs had very little really to do with current oil reserves.

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  • $\begingroup$ "So it is actually very likely that sentience would have come along, probably much sooner than it did." I don't agree with this sentence. Dinosaurs had been on the Earth already for a very long time (I believe they "were born" about 230 millions of year ago) I in hundreds of millions of years they didn't make much progress about intelligence. What make you think that 65 million years would have been enough for them? Also note that mammal brain have always been more evoluted than dinosaurs. $\endgroup$ – Bakuriu Dec 1 '14 at 19:38
  • $\begingroup$ @Bakuriu First dinosaurs were around for 135 million years, not hundreds of millions. 2nd I didn't say Dinosaurs would be the sentience, though it is a possibility, without the collision mammals might have made it 10 Million years earlier. Even wikipedia suggests what I did, that some of the dinosaurs might have made it en.wikipedia.org/wiki/Dinosaur_intelligence $\endgroup$ – bowlturner Dec 1 '14 at 19:44
  • $\begingroup$ 135 > 100 million of years to me does mean "million of years". Anyway wikipedia (which, anyway, isn't a very strong source of scientific knowledge) makes a much weaker claim. You used "very likely that sentience would have come along, probably much sooner than it did". This is quite different than might, eventually, have developed human-like intelligence .... Also I believe that it would have been much more advantageous for mammals to develop intelligence anyway: most dinosaurs were already huge and strong, so intelligence wouldn't have added much. Mammals had to be smart to live. $\endgroup$ – Bakuriu Dec 1 '14 at 19:50
  • $\begingroup$ @Bakuriu read my post again. While I think dinosaurs were a possibility of becoming sentient, I never said what type of animal would ultimately achieve this. Without the extinction event it IS likely that sentience would have shown up earlier on earth, but what or who developed it I never said. mammals, birds, dinosaurs. $\endgroup$ – bowlturner Dec 1 '14 at 19:57
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    $\begingroup$ I think @bowlturner has it right here. Dinosaurs lived for 135 million years, but that's like saying that mammals have lived for 80 million years. How long did an individual species live? And how intelligent was it? Did raptors develop rudimentary language to communicate pack hunting techniques? Did oviraptors use rocks to open harder eggs? Or others to open mollusk shells? But even disregarding the dinosaurs, there was a genetic biodiversity that could have been close to favoring intelligence that was wiped away by that event, and we'll never know. $\endgroup$ – IchabodE Dec 1 '14 at 22:43
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Would mammals have ever come about?

Yes, but not in the same way. After the extinction event, mammals diversified to fill many niches which dinosaurs had previously evolved to fill very well. Big therapod and sauropod dinosaurs were the dominant large life forms on the planet, and mammals probably wouldn't have supplanted them in that role without their extinction.

Mammals did exists, however, and would continue to have evolved and prospered in the roles that they filled better than dinosaurs. Rather than elk, wolves, and elephants, think rodents, weasels, and squirrels.

Precursor primates evolved in the cretaceous, so this could have also (potentially) included the monkeys.

Would sentience as we know it have ever come about?

This is a more difficult question to answer. Almost all animals, mammal or dinosaur, get by just fine without sentience. However, certain patterns of behavior can lead to a tool-using creature that can develop advanced technology. These circumstances arose in humans, but could just as easily arise in some sort of dinosaur that might have evolved without humans in the picture.

Alternately, it's possible that mammals similar to higher primates could have evolved. Early primates that were somewhat squirrel-like in shape probably already existed during the time of the dinosaurs, and it's possible they could have evolved into a race of arboreal tool users similar to modern chimpanzees, from which point development into a modern tool using race.

Ultimately, it comes down to whether or not a species would arise for which developing progressively better tools would be both possible and evolutionarily advantageous. We know by looking at the history of our planet that this only happens very rarely. We also know that most dinosaurs had tiny brains, but we don't know how they would have evolved given another 65 million years. It's definitely possible, though.

If sentient life exists, would abundant fossil fuels exist to enable them to achieve the Industrial Age?

Definitely. The extinction event did not produce a significant portion of the world's fossil fuels. Coal, for example, is mainly formed from plants from the Carboniferous period, which came tens of millions of years before the dinosaurs.

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As pointed out, mammals did co-exist with the dino's...mostly as small mice like creatures filling in the tiny night time scavenging niche mostly.

Oil reserves is another yes. Life on Earth began 4 odd billion years ago, but didn't reach multi-cellular life for a good 2 billion years after that. It's possible dinosaurs where responsible for a small segment of oil reserves, but most oil predates dinosaurs by a significant amount. Plausible that an intelligent dinosaur could drill for oil prior to their extinction.

Sentience as we know it...possible, but you need an isolation situation. I attribute a good section of our intelligence to social dependency, a pre-human primate became dependant on others intelligences, putting pressure on the entire species to evolve intelligence, not just the single creature. We also require an extended developmental childhood. It is possible to have an isolated island scenario...sorta the dinosaur world version of madagascar, where an isolated land mass allowed us to develop (I dunno, descended from Lemurs?). Would create an interesting story line of humans attempting to leave the island and discovering the greater dinosaur world.

I'm always curious if an egg born creature is allowed the time to develop to the point of intelligence...the dinosaurs had an incredible amount of time to come to intelligence, but didn't go much beyond that of a chicken. Their general design does not require the thought required to use tools...everything is possible, but I think the intelligent dinosaur (to our degree of sentience) isn't very likely.

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  • $\begingroup$ Mammals used a strategy of investing in child care over numbers of offspring. It is unclear that sentience requires this to be so. There is little evidence that the strategy helped mammals compete relative to dinosaurs in the Permian and Triassic, when large therapsid mammals were wiped out by early dinosaurs. $\endgroup$ – Oldcat Dec 3 '14 at 23:39
  • $\begingroup$ @Oldcat "It is unclear that sentience requires this to be so." - my position is that it is required. A developing brain that can develop sentience to human levels and beyond must have a dependent stage to do so as it requires full commitment by the brain to this development period (proof here is found in what early traumatic events do to a child in stunting it's growth, disruption to a child's dependent development stages has extreme negative effects on it's future development) . Purely speculative as we do not have much for evidence on how sentient dinosaurs are. $\endgroup$ – Twelfth Dec 4 '14 at 0:38
  • $\begingroup$ Well, it is based on 1 case study. Certainly non sentient birds and mammals seem to have equivalent success using different strategies. $\endgroup$ – Oldcat Dec 4 '14 at 0:51
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Remember, a mass-extinction event and massive climate change were the only thing that wiped out the Dinosaur's competitors, the Crurotarsans and allowed them to dominate the planet for a long time.

Inhuman is not non-humanoid, for various definitions.

Bipedal? I think a good case would be made for having limbs free to use tools is probable.

On Earth, at least, bilateral symmetry is mostly true. So, two arms (a left and a right) and two legs. Same for two front-facing eyes (for depth perception; especially needful for easy, successful tool-use). Almost all Terrestrial creatures put air-intake and food-intake near the sense organs (probably necessary to see what you're eating, pick out the bones/avoid the thorns). So, you've got a face. Likely, since most things are vertebrates on Earth for a long-time, that the head is above the other limbs. Reproductive organs are near excretion organs for most species, and not near the head, so that goes towards the tail.

Tool use is going to almost require a thumb, and manipulative digits, which is going to give you a hand. You might get away with tentacles/lips, like the Puppeteers. But you'll need something with which to make your tools.

These, IMHO, mostly make a humanoid.

Given that the Earth has had many mass-extinction events - including the one we're living in right now - I don't see why it's far-fetched to believe that mass-extinction events happen on other planets. If it happens once every hundred million years... it happens more often than it has here on Earth. Given deep time, you can do all the things.

Please define 'mammal'. Warm-blooded (which is out of favor scientifically, I've heard) allows animals to handle bigger changes in climate - versus cold-blooded, which allows more animals per-square-foot (costs less energy). Any time you start having climate change events, or varying climates - you're going to select for warm-blooded-ness. As well as fur/hair or feathers; for their insulation properties. So, something sorta like a mammal will probably exist, if your planet has wide-enough climate swings. You may or may not get boobies. You'll probably get live-birth (either egg-eggs stored internally, or eggs without shells/yolks/raw fetuses). You may get immature live-birth (marsupial). r-selection will probably be ill-favored, especially if you're dumping a lot of energy into having brains for your species.

Sapience is a good question. Of course, any space-faring race is going to have achieved it, ipso facto. And those are the ones we would be interacting with. Those that didn't develop it, will just be interesting animals. Chimps are interesting animals. There are probably a lot of interesting animals in the universe. Uplift might be a solution to that.

Fossil fuels rely on lots of plant growth without attendant decay, and being geologically buried... or, perhaps hydrocarbons squeezed from the primordial matter of the planet. That all happened before we got termites and other saprophytes evolved to recycle our carbon better.

Do I think Star Trek is right? F no. We should have crabs, spiders, aquatic insects, octopii, trilateral symmetrical, and a wide variety of other types of aliens. But, we would also have humanoids. Some that might even look kinda close to human.

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  • $\begingroup$ Dinosaurs are far more naturally bipedal than mammals. $\endgroup$ – Oldcat Dec 3 '14 at 23:40

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