The obvious candidates are our closest cousins Chimpanzees.They use tools and they wage wars which are the two biggest predictors of higher intelligence. Apart from Chimps, Ants too wage wars and they already manage colonies of billions. Some Ants even collect plant material and use it as fertilizers to farm edible Fungi.
There are other intelligent species like Dolphins, Crows etc.Dolphins have big brains and have complex social interactions with each other. Crows can solve puzzles meant for toddlers.
There are many candidates set to ingerit Earth, but the one with the highest chance as per Oxford biologist Tim Coulson is the Octapus.
In the movie Arrival, when the aliens land on Earth, they are not little green men. They are not even Humanoid. They are seven limbed or tentacled and speak in ink. Which earth animal do they resemble the most? Octapus.
It's not a coincidence, the same pattern can be seen in War of the Worlds by HG Wells, even Cthulhu by H.P. Lovecraft was inspired by Octapus. Why do these great fictions use Octapus as reference to denote alien life?
The Octapus is closest to alien intelligence we have met. It lives in our oceans, it is intelligent, not as intelligent as us Humans but surely on the way. But it's mind built on a completely different way from ours.
Octapus can live for only two years. They have no bones and male octapi die after mating. Still, many researchers and Science fiction authors think they are the ones best placed to build the next civilization.
Let's start with the brain. The brain of an Octapus has 500 million neurons, roughly the same amount of neurons as a dog. Only a third of those neurons are in the brain, the rest are in it's tentacles.
Each tentacle has it's own cluster of neurons, making a mini brain. It can touch, taste and make decisions on it's own.
Imagine our arms could think. How awesome would it be? We build machines that does not rely on a single brain, because it's efficient. The octapus has evolved that way naturally.
Squencing the Octapus genome is like sequencing an alien. It has vastly expanded family of genes involved in wiring up neurons.Octapus can re-edit their neural protiens to keep their brains running smoothly.
In 2015, an octapus named Inky opened the lid of it's tank, slid across the flood and found a drain pipe. It squeezed it's entire body through it and escaped all the way to the ocean. It was never seen again. This is the kind of intelligence we see from characters in Finding Nemo.
In Indonesia, divers have videographed octapi collecting discarded coconut shells, collecting them like bowls and carrying them to build a shelter. In labs, octapus are observed screwing open jars, solve mazes, recognize individual human faces, liking or disliking specific researchers. When they are bored, they spray a jet of water on the researchers.
Despite all the awesomeness, they do have severe limitations that has prevented them from building a civilization to rival ours.
The biggest limitation is that they die young. Most octapi live one to two years. The great pacific octapus live to 5 years. It's not bad luck or weak bodies, It's programmed in their geners. They wander and die after mating to prevent them from competing with their offspring for resources.
Every octapus is an orphan, even the genius among them have to start from scratch. They have to reinvent tool use every generation. A civilization flourish with the knowledge passed down across generations.
The second limitation is the social aspect. Octapus are loners, they fiercely fight each other over territories and are not above eating each other.
The third one is Fire. The biggest backbone of human cicilization and technology is Fire. It is how earliest human civilization kept themselves cool during winters, and how our planes and rockets work now. Octapus being an aquatic species cannot access fire.
These three severly limit the potential of Octapus from building the next great civilization, but theres hope.In time, provided the right conditions a breed or sub-species of octapus can emerge that are socially close to humans, live long and can wander into dry land.
After dinisours died, it took us mammals millions of years of parallel evolution until Homo Sapiens built civilizations, similarly Octapus can evolve in the next million years to create a species to truly rival ours.