For nearly 180 million years, dinosaurs dominated life on land, while the skies and seas were ruled by their relatives, pterosaurs and marine reptiles, respectively. This Age of Reptiles came to a catastrophic end 66 million years ago when a 15km-wide asteroid collided with Earth and wiped out approximately 75 per cent of all species. But imagine that didn’t happen – what would Earth look like today if dinosaurs, and their close cousins, still called it home?
Ever since dinosaurs were formally discovered (and scientifically described) in the early 19th century, they’ve been at the forefront of public imagination and influenced pop culture in a way that no other group of animals – living or extinct – ever has done. From the moment we first learn about these ‘terrible lizards’ as children, we can’t help pondering what it would have been like to live alongside them.
The Jurassic Park franchise has undoubtedly shaped our ideas of what a human-dinosaur world would look like and, for most, has served as a warning about why cultivating such a world would be a phenomenally bad idea. However, if we remove humans from the equation for the time being and imagine a world in which non-avian dinosaurs survived the mass extinction at the end of the Cretaceous Period, what would that look like?
Would dinosaurs have become extinct eventually?
It has long been speculated that dinosaurs were on the decline prior to their asteroid-induced destruction 66 million years ago. Some have even gone as far to say that, even if the asteroid had not made impact, they were inevitably doomed.
Mike Benton, a professor of vertebrate palaeontology at the University of Bristol, has previously said “cooling climates” may have eventually consigned dinosaurs to extinction. However, recent research has suggested the opposite – that dinosaurs were in their prime the moment the rug of life was pulled from under them.
In 2025, a study examining dinosaur diversity in North America during the End-Cretaceous found that multiple communities were flourishing and evolving alongside each other in ‘geographic provinces’ separated by slight differences in vegetation, temperature and other natural factors.
If dinosaurs were in their prime during the End-Cretaceous, as this study and other recent research suggests, then it’s reasonable to assume they’d have not only survived the following periods – the Palaeocene (66–56 million years ago) and the Eocene (56–34 million years ago) – but thrived during them. These periods, like the Cretaceous, were warm, with lush forests worldwide, even at the poles.
The biggest impact on dinosaur diversity in the last 66 million years may have taken place during the Palaeocene-Eocene Thermal Maximum (PETM) – a period of rapid warming 56 million years ago that saw average global temperatures rise by 8°C.
This event massively impacted mammal diversity and triggered dwarfing in many large-bodied groups. It’s likely the PETM would have had a similar impact on dinosaurs, forcing Late Cretaceous giants such as T.rex, Triceratops and Alamosaurus to shrink in size. Pterosaurs would have likely been subject to the same fate too, with biplane-sized species such as Quetzocoatlus disappearing from the skies.
While dinosaurs may have gotten through this period relatively unscathed, though dwarfed, their ocean-faring cousins would have probably struggled and several groups may have faced extinction as a result. The effects of the PETM were felt much more strongly in the oceans where sea surface temperatures, in some places, exceeded 36°C – the same temperature as a warm bath.
This temperature spike is believed to have wiped out nearly 55 per cent of deep-dwelling foraminifera species, causing major disruptions in marine food webs and leading to significant turnovers of marine animals. The mosasaurs and plesiosaurs that survived the End-Cretaceous mass extinction in our hypothetical scenario would likely have faced severe ecological pressure during this time, potentially leading to irreversible decline.
There’s also a possibility the onset of the ice age may have impacted dinosaur diversity. That said, considering how adaptable they were and the fact they survived many periods of climatic instability during their 180-million-year-long reign, it’s likely they’d have been just fine. The species that lived further north would have been covered head to toe in fuzz and feathers, similar to the ‘woolly’ mammals that thrived in our timeline.
Would mammals have still evolved?
In short, yes. Mammals had not only evolved prior to the End-Cretaceous, but had already diversified into many of the groups we’re familiar with today, including primates. The first true mammals emerged in the Late Triassic, approximately 225 million years ago, and a relatively short time after the earliest fossil evidence of dinosaurs (243 million years ago).
However, just because mammals were already around during the time of the dinosaurs doesn’t mean their evolutionary fate in our hypothetical scenario would be the same as it was in reality. In fact, if dinosaurs survived the End-Cretaceous it’s likely that there would have been no Age of Mammals at all; instead, mammals would have likely continued living in the shadows of dinosaurs, their diversity plateauing or maybe even decreasing all together.
The biggest barrier to mammals in our hypothetical scenario is, of course, dinosaurs. Prior to the End-Cretaceous, dinosaurs dominated most terrestrial ecosystems, leaving very few niches for mammals to exploit.
The most successful mammals during this time were small-bodied, nocturnal animals that scurried through the undergrowth or burrowed into the earth. With this in mind, it’s likely mammals would have continued living in these environments, evolving into increasingly specialised species whose main objective was to avoid the gaze of hungry dinosaurs.
In time, dinosaurs may have evolved to occupy these specialised niches too. So far, palaeontologists have only found evidence of one burrowing dinosaur, Oryctodromeus, but there could have been several more, potentially dozens had they survived the End-Cretaceous and lived deep into the following periods.
Some dinosaurs may have also gone down the evolutionary route that early primates did, using opposable thumbs to swing through trees and enhanced colour vision to find nutritious fruits. Like early primates, these frugivorous dinosaurs may have started forming large, social groups and, from there, gotten smarter and smarter.
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Would dinosaurs have continued evolving?
It’s a common misconception that evolution works towards an optimal, predetermined goal before slowing down and coming to a halt when a group of animals reaches its prime.
Instead, evolution happens constantly and is a combination of random, unpredictable events, such as genetic mutations, and non-random, deterministic processes, such as natural selection. This means dinosaurs would have continued evolving up until the present day, some groups doing so at different rates to other groups, depending on the environments they lived in and the varying ecological pressures they were put under.
Some palaeontologists have worked on this area of speculative evolution and, based on evolutionary trends observed during the Late Cretaceous, have hypothesised what some groups of dinosaurs may have eventually looked like had they not become extinct.
A particularly good example is tyrannosaurs – the group that includes the most infamous dinosaur of all time, T.rex. This group of large-bodied, hypercarnivorous theropods is known for having comically small arms, which get increasingly smaller in later species. Since these arms are believed to have played little role in hunting, some think tyrannosaurs may have eventually lost them entirely (like snakes did with their legs), resembling something like a shark but on land.
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Another group that would probably look very different today is the long-necked sauropods. This group comprises the largest animals to ever walk the Earth, though it’s unlikely they’d have remained so large. It’s thought sauropods may have increased in size to accommodate larger and larger guts, as the plants they ate (such as ferns and gymnosperms) were hard to digest and low in nutrients, meaning they needed to eat an awful lot of them.
As less nutritious gymnosperms made way for more nutritious and easily digestible angiosperms in the Late Cretaceous and following periods, sauropods and other dinosaurs wouldn’t have needed such large guts and may have decreased in size as a result. Indeed, by the End-Cretaceous the largest sauropods, the titanosaurs, were already long gone.
Would humans have evolved at all?
It’s unlikely humans would have evolved in a world ruled by dinosaurs, but that’s not to say that there would have been no human-like animals.
As a group, primates appeared 66 million years ago, meaning nearly all of their evolutionary history took place in a world devoid of dinosaurs. If dinosaurs survived and became part of the primate story, this would change things dramatically, right?
Since most primate evolution took place in the trees – a niche where mammals were able to compete with dinosaurs during the Mesozoic – it’s reasonable to assume that many significant evolutionary events that took place in the absence of the dinosaurs would have also occurred in their presence. There would have no doubt been a lot of unrecognisable primates in our hypothetical scenario, but it’s conceivable that species resembling lemurs, monkeys and perhaps even apes could have evolved.
The moment these primates attempted to leave the safety of the trees behind, they’d have run into some problems. With dinosaurs still occupying most terrestrial ecosystems, including the grasslands our ancestors ultimately evolved in, there wouldn’t have been an opportunity for large, upright walking apes to establish themselves.
Instead, some primates may have experimented with life underground, evolving powerful front limbs and strong claws for digging burrows. Others may have sought safety in the shallows and followed in the footsteps of today’s otters, beavers, platypuses and capybaras.
No matter which environments they adapted to in our hypothetical scenario, primates have the building blocks for high intelligence and might eventually have evolved into smart, human-like animals given enough time… and perhaps a localised extinction of large, predatory dinosaurs.
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Would there be highly intelligent dinosaurs?
Most dinosaurs had small brains relative to their massive bodies, but some demonstrated relatively high brain-to-body ratios. While this isn’t the best measure of intelligence, it does give us a loose idea of an animal’s cognitive abilities.
Stenonychosaurus, a genus belonging to a bird-like group known as the troodontids, is widely considered the ‘smartest’ dinosaur, with the largest brain-to-body ratio of any dinosaur so far discovered. At 2.5m in length and weighing in at just over 35kg, Stenonychosaurus was about the same size as a German Shepherd, though probably not as smart as one.
Still, Stenonychosaurus is the subject of a popular – but highly speculative – theory about how dinosaurs may have evolved had they not become extinct. In 1982, former National Museum of Canada curator Dale A Russell imagined a possible evolutionary path for Stenonychosaurus, one where it evolved into intelligent animals with bipedal, human-like body plans.
Russell noted there had been a steady increase in the brain-to-body ratio among dinosaurs and, had this trend continued, the ancestors of Stenonychosaurus may have developed brain cases measuring 1,100 cm3, comparable to that of humans. While this theory has been met with criticism from other palaeontologists, Russell’s so-called ‘dinosauroid’ has featured widely in pop culture – there’s even a model of one in Dorchester’s Dinosaur Museum.
Of course, dinosaurs needn’t look like us to be smart. In fact, there are ‘dinosaurs’ alive today, birds, that are just as if not smarter than most mammals. Take crows and other corvids, for example. These birds have been observed fashioning hooks from twigs to extract insects from trees, dropping stones into a tube of water to raise the level and access floating food, and even playing, rolling around in snow.
If these kinds of intelligent ‘dinosaurs’ were able to evolve in a world dominated by mammals, then who knows how smart the smartest dinosaur may have been had the Age of Reptiles continued.
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