A marine civilization would have been possible on Earth

During the evolution process, several species had the opportunity to acquire intelligence. But only humans have benefited from it. Dinosaurs, birds, and most primates have, to a certain extent, developed their brains effectively enough to acquire fundamental qualities such as bipedalism, prehensile hand, and sociability. What, then, prevented them from taking the place of our ancestors?
Dinosaurs: very intelligent predators
The small carnivorous dinosaurs Troodon inequalis, which lived during the Cretaceous (84-66 million years ago) on the territory of North America and Asia, possessed a rather impressive brain. According to estimates by Canadian paleontologists, the ratio of brain mass to the average body mass (encephalisation coefficient) in Troodon was six times higher than that of their congeners.
Adding homeothermy (American researchers have shown that the body temperature of prehistoric lizards ranged between 36 and 38 degrees), bipedalism, the complex method of obtaining food (the Troodon was a predator, but fed also of plants) and the three-toed hand, we can conclude that this little theropod had everything to be transformed into intelligent being.

His intellectual evolution was interrupted either by an asteroid fallen on the planet at the wrong time, or by the extinction that began 40 million years before the total disappearance of the dinosaurs of the planet.
"Some palaeontologists claim that the growth rate of the theropod brain was comparable to that of the Australopithecus – our ancestors. If they had not disappeared, even with the current pace of evolution without any acceleration, they would currently have a brain of 1,100 cubic centimeters and would have been very intelligent, "notes in his work the candidate for biological sciences Stanislav Drobychevski, master lectures at the Anthropology Chair of the Biological Faculty at Lomonosov State University, Moscow.
A civilization of ostriches
The descendants of flying lizards, the birds, have survived to this day and could have evolved to become intelligent beings if they had given up flying. They claimed "world domination" 65 million years ago, when mammals were still small and weak.
However, they had to pay a high price for the conquest of the heavens – the bones of the birds became lighter, and their brains shrank. This necessary saving of weight even led to the abandonment of the parts of DNA called "trash" (parts of the genome that do not encode any protein). In addition, the need to coordinate movements in flight led to increased volume of the cerebellum, and there was no longer room for the development of other parts of this already small brain.
The return to a terrestrial way of life would probably have given the evolution of the birds a new impulse if the predators had not existed. When the danger is absent, the species "relaxes" and the brain becomes simpler. Examples are kiwis, ostriches and dronts, which are not particularly distinguished by their intellectual abilities, even among birds.
Our brothers the monkeys
Why other primates than Homo did not become intelligent? The oropicque, which has disappeared, could have become smarter long before our ancestors, according to some researchers. But even though baboons and gorillas lived very close to humans, they did not take the same path on the path of evolution.
The size of the brain of the first anthropoid apes (African proconsul, Turkanapithecus) was comparable to that of current baboons. With them and with our ancestors, evolution proceeded at the same speed, and the relative area of ​​the associative frontal cortex was exactly the same as in man today.
In addition, certain types of baboons have emerged in the savannah at the same time as Australopithecus, and have better conquered. At least, they left Africa earlier to settle on the planet. But unlike humans, baboons have chosen the path of aggression and strict hierarchy in the pack. The power of the alpha male is virtually unlimited, and in case of insubordination the baboon pulls the fangs – which have, moreover, been abandoned by our ancestors while their living conditions were similar. This social system works very well because the baboons, who reached their ideal still in the Pliocene, have not changed anatomically for several million years.

The gorilla has not changed either because it has no natural enemies in nature. This monkey, unlike other primates, does not use wilderness tools. On the other hand, it is sometimes smarter than our closest relatives – chimpanzees. The size of the brains of the gorillas corresponds to the lowest indicators of modern man and exceeds those of Australopithecus.
Underwater cities of whales and dolphins
A large brain is a pledge of intelligence, and its size can only increase indefinitely in the marine environment. That is why, already 33-23 million years ago, cetaceans had reached the level of the advanced hominids by the absolute size of the brain and its sinuosity. But everything is relative: in terms of the body mass ratio, the dolphin and whale brain is relatively small, and the transformation of the legs into fins does not contribute to a work activity – although there are known case of female dolphins teaching their children to look for food by placing a sea sponge on their nose.
"Fishing for fish and shrimp is not the best intellectual stimulant. As complex as trapping a fish, it's still a trap. There is little room for intelligence. The same can be said of pinnipeds. Nutritional specialization, the transformation of the feet into fins and the body into a bag of fat do not contribute to the development of intelligence, "writes Stanislav Drobychevski.
Sea otters could create a marine civilization. They are very sociable animals, possessing a prehensile hand and, consequently, a rich work activity. For example, to stay hungry, they break sea urchins and shells with pebbles. The only thing that is holding back sea otters at a higher evolution level for now is that they feel too good in their current habitat environment.