An asteroid slammed down and did away with all the dinosaurs, paving the way for such developments as the human race, capitalism, and posting on the internet: it’s the story we all know and love. Yet if things had shaken out differently—if the asteroid had stayed in its place, and the dinosaurs allowed to proceed with their business—what would things have looked like?


Would the earth be a pristine, unsmogged paradise, or would the dinosaurs have somehow evolved into even more rapacious profiteers/industrialists, wrecking the world with their dinosaur refineries and dinosaur dark money? The latter scenario being totally implausible, what’s a likely answer to the question of what our world would look like if that asteroid never hit it? To find out, for this week’s Giz Asks we reached out to a number of geologists, paleontologists, and evolutionary biologists.

Principal Lecturer, Vertebrate Paleontology, University of Maryland



Assuming the asteroid misses Earth, and all the other physical changes that happen later in the Cenozoic (especially the motion of the continents and the change of the climates) happen as they do in the real timeline, here’s what I think might happen. In our world only the toothless group of birds (Aves) survived among the dinosaurs (remember, all birds are dinosaurs, but not all dinosaurs are birds). In this case, all the dinosaurs (toothed birds, tyrannosaurs, horned dinosaurs, duckbills, clubtailed ankylosaurs, etc., etc., etc.) that were around at 66 million years ago are around at 65 million years ago.


That doesn’t mean that all these groups of dinosaurs are still around at 0 million years ago (aka the alternate “Now”). After all, plenty of groups of mammals that arose in the early Cenozoic have long since vanished in the real world. And—this is important—zero (zilch, nada, nichts, null, etc.) species of dinosaur that lived in the real world Cretaceous will be present by the Alternate Now!!! No Tyrannosaurus rex living today; no Triceratops prorsus, no Edmontosaurus annectens. They would all have vanished millions upon millions of years ago. Individual dinosaur species lasted less than a million years back in the Cretaceous; they wouldn’t suddenly last for all time to come afterwards. Just like there is a lot of evolution from Purgatorius unio (one of the first members of the primate lineage, which lived right after the Cretaceous) to Homo sapiens, there would be a lot of evolution among Alternate Cenozoic dinosaurs. The Alternate Cenozoic dinosaurs would face the same challenges that Real World Cenozoic large mammals faced. At around 35 million years ago the world would get colder and drier (due to changes in ocean circulation as South America and Australia move farther away from Antarctica); the rainforests that used to range from the tropics for the Rocky Mountain West and Germany and the like shrink back. Scrubland (drier forests separated by fields of grass) takes their place. In our world herbivorous mammals begin to adapt to these conditions, evolving grinding grazing teeth and broad snouts to deal with the drier, grittier fodder. The same might be true of Alternate Cenozoic dinosaurs: there are some groups of dinosaur with grazing teeth and snouts in the real Mesozoic, so it wouldn’t be hard for them to do so again. (Different from mammals, though, is that dinosaurs use and throw away teeth because they get new teeth their entire life; mammals, with only baby teeth and adult ones, have to evolve tall crowned complex molars and ever-growing incisors and so forth to counteract the grittiness of the scrublands). Later still (around 7 million years) even the scrublands die back to be replaced by the open prairies/steppes/savannas/pampas/other local words for grasslands that dominate the world ever since, which exaggerated the grazing traits. (It also resulted in the rise of vast herds of herbivores and pack hunting in mammals: however, it looks like dinosaurs were already doing that in the Real World Cretaceous). But the greatest change is to come: the Pleistocene Ice Ages. Starting at 2.588 million years ago the Earth shifts to a much colder state. As the mammals did in our history, this means that the dinosaurs in the northern regions would have to evolve shaggier fur (well, feathers in the case of dinosaurs) in order to survive. One possibility is that dinosaurs of the Alternate middle-and-later Cenozoic might be smaller than the Mesozoic and early Cenozoic ones. That is because the reduction of carbon dioxide in the atmosphere might result in a decrease in the amount of lush vegetation (in favor of scrubbier, grassier biomes). That said, there were some pretty big grazing mammals (the Columbian mammoth and the steppe mammoth, for instance), so perhaps the dinosaurs could adapt and retain giant size. But it wasn’t just the dinosaurs that got clobbered by the asteroid. It was a mass extinction, and by definition a mass extinction affects all kinds of organisms in all the environments. So all the groups that we lost in the real world would have survived: pterosaurs in the skies; ammonoids and rudist reef clams and mosasaurs and plesiosaurs in the seas; and a diverse bunch of mammals (and others) on land. All of these would have to deal with the new conditions. No doubt some of these would go extinct over time, but others would thrive and survive.


The continued presence of large dinosaurs on land and of mosasaurs and plesiosaurs at sea would probably keep mammals in check, just as they did in the Mesozoic. So no origin for large mammalian carnivores, no big hoofed mammals, no whales: those niches are occupied. But small-bodied mammals could still thrive, as they did for their long history in the shadow of the dinosaurs. Rodents and rabbits, anteaters and armadillos, bats and weasels, moles and shrews, possums and koalas, could all probably arise. But no horses, deer, giraffes, kangaroos, big cats, bears, or whales. One group that probably could still arise would actually be primates. There doesn’t appear to be any dinosaur or other Mesozoic group doing the primate niche in the Mesozoic (the closest the dinosaurs got to that is parrots). So you might arguably get Alternate Primates evolving in that world. And when the grasslands spread, some of them adapt to those conditions to become Alternate Hominids. And as in our time line, these Alternate humans might have started as lower on the food chain, but evolution of greater intelligence, tool use, coordination, and so forth, means that they can survive and thrive in this world. (And also like our timeline, these Alternate humans might contribute to the extinction of the big animals in the world; just this time it is dinosaurs and so on, not mammals).

“No Tyrannosaurus rex living today; no Triceratops prorsus, no Edmontosaurus annectens. They would all have vanished millions upon millions of years ago.”

Professor, Biology, New York University, known for his scientific contributions on causes of mass extinctions of life

Sixty-six million years ago, the dinosaurs were experiencing continued success. Climates were changing, but slowly enough to allow the dinosaurs to adapt to the altered conditions. The extinction of the dinosaurs was sudden, catastrophic. And total. What would have happened if the asteroid had not hit the planet? The mammal cohorts of the dinosaurs were small, furry nocturnal creatures. They lived in the shadow of the giant reptiles for more than 135 million years. Without the catastrophic asteroid impact and removal of the dinosaurs from the scene, it is unlikely that the inconspicuous mammals would have ever evolved into the large and diverse animals that fill the world today. Without the asteroid strike, the dinosaurs probably would have continued evolving into new and varied forms as climates changed. One evolutionary path might have led to a reptile with grasping hands and a large brain, filling the niche in which we humans have evolved.


“Without the catastrophic asteroid impact and removal of the dinosaurs from the scene, it is unlikely that the inconspicuous mammals would have ever evolved into the large and diverse animals that fill the world today.”

Professor, Paleontology, University of Chicago



Well, the answer in short is we humans probably never would have evolved to ask such a question. To understand this, I would start by saying that catastrophic extinctions of the magnitude unleashed by that impact change the dynamic and course of evolution in such a profound way that it would never play out as it would undisturbed. That is true even if the asteroid hit earth in a different place, one must realize. That it hit an extremely carbon-rich shelf in shallow waters is thought to be about a 13% probability, as compared to the much large chance it had to crash into open ocean and eventually impact the basalt bottom of the sea floor. In the latter case, the extinction would have been muted, and it’s possible some “non-avian” dinosaurs (i.e., other than birds) might well have survived. This brings up the bigger nature of evolution on a global scale over millions of years as a contingent history. It is a unique history, such that one event of a consequential scale changes everything that follows. How could one think that the first hominin that stood up on its hind legs some 7 million years ago—nearly 60 million years after a dinosaur-killing accident—would have evolved again if dinosaurs were left unmolested? Not a chance.


“Well, the answer in short is we humans probably never would have evolved to ask such a question.”

PhD Candidate, Ecology and Evolutionary Biology at Brown University, and biomechanist/paleontologist



First off, I wouldn’t be doing my duty as a paleontologist if I didn’t hit you with an obligatory “the asteroid didn’t get them all; dinosaurs are still alive!” We now have loads of evidence that birds—everything from peacocks to penguins to the turkey in your sandwich—are living, breathing dinosaurs. (Well, not so much living for the turkey, I guess.) So, even though it’d be really cool to see what changes 65 million more years of evolution would bring to the dinosaurs we know and love, like the horned ceratopsians and long-necked sauropods, I don’t think “we’d still have dinosaurs!” is the most interesting part of all this. I mean, we already have some! Instead, I think it’s fun to speculate about the other prehistoric critters that might still be roaming the Earth. What about flying reptiles like pterodactyls, and swimming reptiles like plesiosaurs and mosasaurs? All of these groups also became extinct around the time that the asteroid hit, and unlike dinosaurs, they don’t have any living descendants. Without the asteroid, who knows? Chances are they’d still be around.


I don’t know about you, but I don’t think I’d want to take a swim at the beach with a bunch of sea monsters hanging around. (If humans had even still evolved and survived, that is!)

“What about flying reptiles like pterodactyls, and swimming reptiles like plesiosaurs and mosasaurs? All of these groups also became extinct around the time that the asteroid hit, and unlike dinosaurs, they don’t have any living descendants. Without the asteroid, who knows? Chances are they’d still be around.”

Research Assistant Professor and Laboratory Head of Vertebrate Paleontology at the University of Hong Kong

Birds are the only living dinosaurs, but without an asteroid impact many more would have made it to the present, including the enantiornithines. They were the most abundant and diverse group of Mesozoic birds and were born feathered and ready-to-fly. Without an asteroid at least some of the non-avian dinosaurs would have probably survived to the present. Imagine seeing Triceratops, Diplodocus and T.rex in the flesh! But how about other life on Earth? What we see today descended from the original survivors of the asteroid, and some groups benefited greatly from the event over the long-term. The decline of non-avian dinosaurs allowed mammals to become much more abundant and diverse later on. It is probably fair to say that we might not be here without the asteroid! Extinctions cause a lot of destruction, but they also present opportunities for new life to evolve, and so are really important to the evolution of life.


“Extinctions cause a lot of destruction, but they also present opportunities for new life to evolve, and so are really important to the evolution of life.”

Professor and Canada Research Chair of Dinosaur Palaeobiology at the University of Alberta



In the early 1980s, Dr. Dale Russell from the National Museums of Canada undertook a thought experiment that led to the creation of a ‘dinosauroid.’ This was a model (done by R. Sequin) and was an extension of a project they were doing with reconstructing the skull and skeleton of Stenonychosaurus (now better known as Troodon). Troodon and its kin are the largest-brained dinosaurs known, with brain sizes equivalent to about six times the size of a modern crocodile with the same body weight. All dinosaurs were clearly not small-brained animals. This was a very ‘sophisticated’ dinosaur, with forward-facing eyes for binocular vision, long legs for running, raptorial second digits on the foot like Velociraptor, feathers covering its body (known from Chinese relatives) and many other highly specialized characteristics that clearly showed that they (and other Late Cretaceous dinosaurs) were far more ‘advanced’ than earlier dinosaurs. Like all other animals, dinosaurs were changing over time, and the ‘newer models’ were showing improvements over all previous models. If non-avian dinosaurs had not been wiped out by the asteroid, would the trends of increased brain-size, more sophisticated, faster locomotion, and so on have continued until Troodon evolved into a dinosauroid? The concept of a large-brained dinosaur that was humanoid in appearance had a huge impact on the public, and sensationalized stories appeared in the tabloids of lost races of dinosauroids taking over the world. Dale Russell had made his point: if dinosaurs had continued the evolve, assuming the non-avian forms had not been wiped out by the asteroid, then they would have continued to evolve and change. And who knows, they may have produced highly sophisticated, large-brained, socially complex animals similar to humans.


It is unlikely, of course, that they would have become anything so human-like as the dinosauroid suggested. But no matter what they changed into, it is likely that dinosaurs would have maintained their dominance of the world, and mammals would have remained small, insignificant creatures scurrying through the underbrush. The asteroid cleared the playing field and gave mammals the chance to take over. Or did it? Birds are the direct descendants of dinosaurs, and are officially classified as part of the Dinosauria. That means that there are more than 10,000 living species of dinosaurs, which is more than double the number of living species of mammals. What will happen the next time an asteroid smashes into our planet.

“It is likely that dinosaurs would have maintained their dominance of the world, and mammals would have remained small, insignificant creatures scurrying through the underbrush.”

Professor, Biology and Geology, Macalester College



It’s fun to speculate that dinosaurs in their larger bodied, toothed, scalier form wouldn’t have gone extinct. They weren’t in any serious decline before this event as far as we know, and were still distributed all over the world, from the poles to the equator, living in all different sorts of environments, eating all different sorts of foods, and interacting with ancient mammals, birds, and other reptiles. All this leads me to hypothesize that if dinosaurs hadn’t been unlucky on that terrible day 66 million years ago, our world today might still be populated by a diversity of dinosaurs (not just the little feathered ones that we call birds). And who knows? Since mammal diversity didn’t explode until dinosaurs were out of the way, maybe our own lineage would still be scurrying around at the feet of giant dinosaurs. Also, it’s worth noting that dinosaurs thrived on Earth for more than double the amount of time that has passed since they got wiped out (not counting modern birds, which would extend their range until the modern world).


Professor, Biology, The George Washington University, who has collected dinosaurs around the world for more than 40 years



Historical inquiry into “what if” is always dicey, but in the spirit of fun we can make some grounded speculations based on what happened after the extinction event 66 million years ago. Of course, dinosaurs did not go extinct during that event because birds are dinosaurs, so let’s get that out of the way. I’ll also start off agreeing that the asteroid is the best explanation for the extinction event, the timing is precisely right and dinosaurs had already sailed through a previous period of heavy vulcanism at the end of the Triassic. The main signal we have is that mammals and birds diversified shortly after the extinction event (there is a bit of controversy in that calibrated molecular trees suggest the radiations started before the extinction event) and the range of sizes increased upward, suggesting that they were replacing ecological roles the non-avian dinosaurs filled and the latter would have continued to fill them had the asteroid missed. I’ll go out on a limb and say that non-avian dinosaurs would be doing just fine now, and would have continued adapting and diversifying the last 66 million years. On the face of it the fossil record shows a decrease in diversity during the last ~10 million years of the Mesozoic, but I don’t think the fossil record of the late Mesozoic is sampled well enough to demonstrate a trend leading inexorably to extinction. If you look at the best-sampled end-Cretaceous ecosystem, preserved in the Hell Creek Formation, dinosaurs may not have been extremely diverse but they were still doing just fine. Non-avian dinosaurs occupied a wide array of ecosystems during the Mesozoic, including some in the Arctic and Antarctica, and all other things being equal (which of course were not) they would have continued to do just fine in modern ecosystems, with some caveats. Grasses were not around then (there is a single report of grass phytoliths from the end of the Mesozoic in India), but there is no reason to think the herbivores couldn’t adapt to eating and digesting them. Presumably some of them would have evolved to become cursorial, like some mammals did, when grasslands took over in the Neogene. The big question is whether some other group would have evolved to displace them, but it isn’t obvious that one could do so given that nearly all large, terrestrial herbivores and carnivores today are mammals, and for reasons still not understood mammals remained at a small size (usually very small) when they were co-evolving with non-avian dinosaurs for over 150 million years during the Mesozoic. The evolution of non-avian dinosaurs during the Cenozoic would have been constrained by the same continental movements that gave us the endemic faunas of Australia and Madagascar, so there would likely have been convergent evolution of dinosaur groups like with placental mammals and Australian marsupials. And it is just as interesting to look at other groups that might not have gone extinct then. A weird group of small crocodilian relatives called notosuchians, some of which were herbivores, were incredibly diverse in terrestrial ecosystems in southern continents, and I would love to see them running around now. The seas would now be full of mosasaur and plesiosaur descendants, which would be great to see on ocean cruises except we may not have whales around. Pterodactyls would still be flying around, but on the down side birds may not be so diverse.


“The seas would now be full of mosasaur and plesiosaur descendants, which would be great to see on ocean cruises except we may not have whales around.”

Assistant Professor of Geology & Geophysics and Assistant Curator of Vertebrate Paleontology & Vertebrate Zoology at Yale University and Yale Peabody Museum of Natural History

The first thing I should say here is a disclaimer: no matter what I think about events that might have been likely to occur in the absence of the K/Pg impact, if the history of life has shown us anything, it’s how important chance has been in shaping the face of the planet. A small shift here or there—a volcanic eruption, the redirection of ocean currents—could begin a chain reaction whose ultimate conclusion would have a profound effect on Earth and its inhabitants. That said, I can make some guesses on the basis of what has happened in the past. We know this: once legs evolved and vertebrates got onto land, we swiftly spread over the surface of the planet. The initial winners, composing the first dominant vertebrate empire, were actually our own distant relatives: early members of the mammal line, like the bizarre sail-backed Dimetrodon. Just like modern mammals, our scaly, toothy forebears had a monopoly on the large-bodied ecological niches of the day: big plant-eaters, big omnivores, big carnivores. From about 330 to 250 million years ago, they dominated the surface of the Earth and excluded other groups. It would take the largest extinction ever, at the end of the Permian period, to shake things up enough that reptiles could rise. And the first reptiles to become dominant were not in fact dinosaurs, but distant relatives of crocodiles. Another extinction event—at the end of the Triassic, 200 million years ago—would again destroy enough of life’s web for regime change to occur; only then did dinosaurs emerge as unambiguous victors. Therefore, dominion over the Earth really seems like a lottery: whoever happens to bounce back first is able to found an uncontested dynasty until the next cataclysm. Non-bird dinosaurs had been going strong for over 130 million years and showed no signs of decadence when the asteroid hit. Even today there are more than twice as many species of birds as there are of mammals: it is still, in some ways, the age of dinosaurs. Therefore I think it’s likely that, on an unimpacted Earth, dinosaurs would reign on the land as they still do in the air. The oceans would be filled with giant marine reptiles, at least some of which we now think were “warm-blooded” and able to adapt to a cooling world. Tyrannosaurs, horned dinosaurs, long-necked sauropods and the like had been around for many tens of millions of years and might still be around today. Once mammals achieved their “modern” body forms (dogs, cats, elephants, bats, whales, etc.) during the middle of the Cenozoic (the “age of mammals”) about 30 million years ago, those overall forms remained pretty stable. Mammals on unimpacted Earth would still be, for the most part, small and nocturnal—however, we now know that our line had been diversifying for some time. In the Mesozoic, there were swimming mammals, gliding mammals, even predatory mammals that ate small dinosaurs. We would have remained masters of the night, I think (indeed, most mammals today remain nocturnal—monkeys, including humans, squirrels, and some big antelope and other herbivores are exceptions to the rule). On the ground, we might be vying with diminutive land-living crocodile relatives for some measure of the set of niches occupied today by animals like weasels, skunks, and cats. Lizards and amphibians would continue to evolve the way they have since the time of the dinosaurs. Flowers had begun to appear by the end of the time of dinosaurs, but flowering plants may not have achieved the sudden, opportunistic dominance they enjoyed after the K/Pg extinction. So powerful was the impact that it set the world alight, and when the world burned, the primeval forests turned to coal and ashes.


What would have happened as the continents moved and the world cooled? We have no real evidence that mammals are better at adapting to the cold than dinosaurs (birds). Today, however, mammals are extremely successful in temperate and cool parts of the planet, and non-dinosaurian reptiles are not. Perhaps, beginning about 40 million years ago, we would have had our shot, and mammals and dinosaurs would have split the Earth—dragons in the tropics, tigers in the snow. Whales probably couldn’t have happened without mammals’ having achieved terrestrial dominance, and perhaps there would already have been some reptile analogue in the water. Perhaps, though, dinosaurs would simply be too well-poised for adaptive radiation, mammals would not radiate, and there would be lean tyrannosaurs on the equator and wooly tyrannosaurs at the poles. What about us? Would there be beings on the alternate Earth with high intelligence, self-awareness, society and high technology? I’m not sure. In all the many hundreds of millions of years prior to mammalian dominance, despite the sophisticated bodies and brains of Mesozoic reptiles including birds, it didn’t happen. I suspect that there is something specific about the mammalian brain, and the specific mammalian way of forming early attachments (filial and sibling-to-sibling, to start)—of socializing, of caring about each other—that is a prerequisite for the kind of intelligence we see in ourselves. Intelligent, long-lived, tool-using birds with some rudiments of a society and a collective memory—parrots and ravens—have existed for quite some time, but there has been no “runaway” intelligence cascade. On the other hand, maybe we are preventing them from going further. Maybe there is something about being large, dominant, and terrestrial that promotes the development of consciousness. Possibly, simply by virtue of becoming very large, something like a crow or a parrot would come to possess a brain in which the sheer number of neurons were enough to produce a spark of consciousness. They lack hands, but work in my own lab has shown that the avian beak can serve quite well as a surrogate hand. And extinct non-avian dinosaurs like Deinonychus (the “raptor” of Jurassic Park) had very dexterous hands and brains almost as sophisticated as those of modern birds. Once something like a human evolves, all bets are off. Humanity developed art and fire and burned and ate and created and destroyed its way through the world with such swiftness and such efficacy that the progression of life took a sudden and permanent swerve. Maybe something like a person-sized, round-headed, long-clawed sapient dinosaur (probably more bird-shaped than man-shaped, because the dinosaur body has a different history and a different build) would have looked over a tamed world and wondered whether its kind, and its kind alone, had always been destined to strive and to conquer and to ask what might have been.

“Maybe something like a person-sized, round-headed, long-clawed sapient dinosaur... would have looked over a tamed world and wondered whether its kind, and its kind alone, had always been destined to strive and to conquer and to ask what might have been.”

Do you have a burning question for Giz Asks? Email us at tipbox@gizmodo.com.