If humanity went extinct, what species would replace us?S

Could another species someday replace us as Earth's dominant intelligent life and, if so, which species? To answer that question, we need to understand just what intelligence really is. And why, from an evolutionary standpoint, an organism would even want it.

Once we understand those things, we can start speculating about what the next intelligent, human-like species will be on Earth.

Image by jörg röse-oberreich/Shutterstock

What do we mean by intelligence anyway?

What makes intelligence uniquely human? This isn't an easy question to answer, as plenty of animal species show remarkable levels of what we would think of as intelligence, particularly our fellow primates, cetaceans like whales and dolphins, and even elephants.

Let's run through some of the potential criteria for human intelligence. How about something crucial, like our use of tools and technology? Humans don't have a monopoly on this - birds can use tools in a very basic way as part of their foraging, and even incredibly simple organisms like cephalopods are known to cover themselves in coconut shells for camouflage. But it's chimpanzees that really approach human levels of tool use - they've shown signs of flexible tool use, and a 2007 study revealed chimpanzees sharpened their sticks to use as spears, which is the first systematic use of a weapon ever observed outside humans.

Then there's self-consciousness. The classical test for this is the mirror test, in which an animal's face is marked while it's asleep, and then it's brought to a mirror when it's awake. If the creature recognizes the face in the mirror as its own, it'll start grooming the strange new mark, which is a sign of self-awareness. We've found evidence of this in the great apes, cetaceans, and an elephant. Recent research on macaques has shown evidence of even more complex forms of animal self-awareness, as we recently reported.

Well then, how about language? Surely we've got a monopoly on that. Well, there are a number of seemingly very complex forms of animal communications, such as whale songs, bee dances, and the color change communication of the Caribbean Reef squid. Then there have been the attempts to teach primates the basics of human language, often through sign language, although it's still hotly debated how much these primates truly understand about what they're "saying."

There's also been research into the language and reasoning capabilities of mimicking birds like parrots - the most famous of which, an African Grey named Alex, was supposedly able to use words creatively and had the intelligence of a five year old human by the time he died. Our understanding of animal language remains in its infancy, and what conclusions we can draw remain highly controversial. But we can't definitively say that language is the exclusive province of human beings.

Even mathematics isn't something humans can call entirely their own. Elephants have performed simple arithmetic, monkeys can count, ants show a remarkable grasp of quantitative values, and chimps have an exceptional memory for remembering numbers - one that can even outstrip humans.

So where does that leave us? We're treading dangerously close to having to define human intelligence in much the same way former Supreme Court Justice Potter Stewart defined pornography - we'll know it when we see it. What we might be able to say is that humans represent an intellectual extreme in terms of our tool use, self-consciousness, strategic thinking, linguistic capabilities, and mathematical prowess, and that this combination (along with other factors) has allowed us unmatched scientific understanding and cultural sophistication.

Even that can be picked apart - defining intelligence is fiendishly difficult, particularly if you're trying to not be too biased towards humans - but let's just say we want to know what it would take to create another species that was intelligent in pretty much the same way humans are.

What would it take for a new intelligent species to evolve?

First of all, humans would almost certainly need to be extinct - or, at the very least, to have completely abandoned the planet - for any other intelligent species to have much of a chance. We already occupy the environmental niche that would produce a human-like intelligence (more on that in a moment), and besides it's hard to imagine humanity passively sitting back and watching an entirely new intelligent species emerge.

Of course, it would take millions of years (or, at the absolute least, hundreds of thousands) for such a species to evolve, so it's not like any single generation of humankind would observe such a rise. But we've already reached a point in our development where we have the power to control and reshape much of the planet, and part of that domination likely means - at least subconsciously - that we will prevent the rise of a second intelligent species by holding our preferred environment in relative stasis, preventing new, intelligence-breeding niches from opening up.

That, of course, is just speculation, and indeed that's part of what makes this subject tricky. Only one human-like intelligence has ever evolved - although our hominid relatives like the Neanderthals certainly got close, and perhaps under slightly different conditions it would be the Neanderthals that dominated the world today. In any event, we're working with a very small sample size, and we can only guess what evolutionary pathways a second intelligent species might take.

But let's simplify the matter and assume that humans are out of the picture, leaving all other species behind. That's not a hugely likely scenario - a pandemic could do it, although it's a bit unlikely that such a genocidal virus would never cross over to any other species - but it at least leaves all the best candidates for human-like intelligence, such as chimpanzees and the other great apes, still around.

We'll get them to a moment. But before we move on, we really must consider this spellbinding passage from Richard Dawkins's The Ancestor's Tale, in which the great scientist imagines a post-human world that becomes dominated by rodentkind:

If nuclear war destroys humanity and most of the rest of life, a good bet for survival in the short term, and for evolutionary ancestry in the long term, is rats. I have a post-Armageddon vision. We and all other large animals are gone. Rodents emerge as the ultimate post-human scavengers. They gnaw their way through New York, London and Tokyo, digesting spilled larders, ghost supermarkets and human corpses and turning them into new generations of rats and mice, whose racing populations explode out of the cities and into the countryside.

When all the relics of human profligacy are eaten, populations crash again, and the rodents turn on each other, and on the cockroaches scavenging with them. In a period of intense competition, short generations perhaps with radioactivity enhanced mutation-rates boost rapid evolution. With human ships and planes gone, islands become islands again, with local populations isolated save for occasional lucky raftings: ideal conditions for evolutionary divergence.

Within 5 million years, a whole range of new species replace the ones we know. Herds of giant grazing rats are stalked by sabre-toothed predatory rats.* Given enough time, will a species of intelligent, cultivated rats emerge? Will rodent historians and scientists eventually organise careful archaeological digs (gnaws?) through the strata of our long-compacted cities, and reconstruct the peculiar and temporarily tragic circumstances that gave ratkind its big break?

Is that a realistic vision of the future? Unfortunately, it's pretty much impossible to tell. The problem is that we don't know what the probability is that intelligent life will emerge. Humanity might just be a hugely improbable evolutionary accident, with intelligence just being an unlikely solution to certain ecological challenges that has now gotten completely out of control. After all, dinosaurs dominated this planet for 160 million years, and there's no evidence that intelligence was ever a particularly prized evolutionary trait.

Or intelligence might be a very common outgrowth of complex evolution, and any number of other species could make that leap if our current domination of the planet didn't prevent it. As with most things, the truth probably lies somewhere in the middle, but we're not likely to have a good answer to the question unless we can start surveying the prevalence of intelligent life throughout the rest of the universe. So for now, we'll just have to keep guessing.

What would prevent such evolution?

It's much easier to say what couldn't evolve human-like intelligence than what could...at least, not without completely doubling back and restarting its evolutionary path. For instance, we can pretty much rule out any non-animals right off the bat. Plants rely on very low energy yields from photosynthesis, which is part of the reason plants have never evolved mobility - they don't have access to sufficient energy to start moving around. There's no chance that plants could harness the energy needed to support a complex brain. Insects have their own structural problems that would preclude intelligence, as their exoskeletons couldn't possibly be scaled up to the sorts of sizes that would allow for a brain capable of complex cognition.

So what we're really looking at is just the vertebrate species, which even if some are too small to support the hardware of intelligence right now (such as Dawkins's rats) do at least still possess the ability to grow larger with time. So then, we need to consider what sorts of ecological niches would produce circumstances in which intelligence would be a beneficial adaptation. In what instances would being smarter help an organism reach reproductive age?

An animal faces two life-or-death struggles in its quest to reach reproductive age: it mustn't starve to death, and it must avoid being eaten by predators. If both of these tasks are already easy to accomplish, then intelligence is unlikely to serve any evolutionary benefit. If a species has plentiful food resources and faces no significant predatory threats, then there's no point in being smart. After all, a complex brain requires a ton of energy, and there's no point in wasting food on the brain if it's not actually being used for survival.

A situation in which a vertebrate species is faced with suddenly scarce food sources or an increase in dangerous predators is the most obvious way to kick-start the evolution of intelligence. Indeed, the collapse of key lush regions in ancient Africa millions of years ago is likely what drove the divergence of humans from chimpanzees, as our ancestors had to employ more strategic thinking to find the no longer plentiful resources.

But the real driver for the evolution of intelligence might well be a healthy social life. Humans, our fellow primates, cetaceans, and canids like dogs and wolves - all species noted for their intelligence - tend to be social creatures, and the importance of sharing resources, information, and even emotional bonds may well drive the move toward ever greater intelligence.

We can tie this back into the basic life-or-death struggles. Hunting as a group can often be more effective than hunting alone, but this involves more complex strategic thinking, the ability to form unit cohesion, and the social structure necessary to allow the food to be divvied up afterwards. Chimpanzees act as a sort of pale shadow of what we know human hunter-gatherers are capable of, as the apes have displayed cooperation and social rank in their hunting strategies, not to mention more selfish social techniques like deceit and manipulation.

Building a society can also be helpful in warding off predators, as again it shares the responsibility of protection. A solitary hominid hasn't got much chance against a saber-toothed tiger, no matter how advanced its stone tools might be. But a whole group of hominids could take on the tiger without any one member running too much risk of being killed in the defense.

There's still the question of technology, which seems like a key prerequisite for any human-like intelligence. This is where we may have to eliminate dolphins, whales, and other marine animals from the discussion. As their anatomy is streamlined for more aerodynamic movement through the water, that would seem to rule out any chance of evolving an opposable thumb and other traits that would be crucial for sophisticated tool use.

But even that might not be absolutely for definite, which is a recurring theme in any speculative discussion of human evolution. A population of bottlenose dolphins has been observed holding sponges over their beaks as a fishing tool, and these dolphins were also observed passing on the skill to their young. This would seem to indicate the rudiments of a tool use cultural, although it likely doesn't change the fact that marine anatomy - and, indeed, the overall ocean environment - simply is not conducive to the evolution of human-like intelligence.

How Long Might It Take?

There's also a time factor to consider. The human race has been around in its current form for about 200,000 years, which is less than nothing when compared to the 3.8 billion years life has existed on this planet. And the vast majority of life on this planet has a relatively recent ancestry - multicellular life doesn't start until a billion years ago, and we don't see any complex animals until 550 million years ago.

Why does this matter? Well, for the sake of argument, let's say all life on Earth was wiped out except for the simplest bacteria. Now, it's possible that, in time, a new human-like intelligent species will emerge, but the operative phrase there is "in time." The Earth has been around for about 4.5 billion years, and we've got another 5 billion years before the expanding Sun will destroy the planet. There's no guarantee that five billion years is enough time to get from simple one-celled organisms to intelligent life.

In his classic book Wonderful Life, the late, great Stephen Jay Gould lays down the basic problem:

Since human intelligence arose just a geological second ago, we face the stunning fact that the evolution of self-consciousness required about half of Earth's potential time. Given the errors and uncertainties, the variations of rates and pathways in other runs of the tape, what possible confidence can we have in the eventual origin of our distinctive mental abilities? Run the tape again, and even if the same general pathways emerge, it might take twenty billion years to reach self-consciousness this time - except that the earth would be incinerated billions of years before. Run the tape again, and the first step from prokaryotic to eukaryotic cell might take twelve billion instead of two billion years - and stromatolites, never awarded the time to move on, might be the highest mute witnesses to Armageddon."

Even if the path back to intelligence didn't require starting fresh with one-celled organisms, there's still a time factor to consider. Humans diverged from our closest living relatives, chimpanzees, about six to seven million years ago. We don't actually have a good sense of what our common ancestor looked like, but our knowledge of the hominid fossil record suggests it looked more like a chimp than a human. So then, even if chimps instantly moved into the environmental niche that led our common ancestor to ultimate evolve into humans, that process could take many millions of years.

Sure, the Sun isn't going to explode in the next million of years, but there's any number of calamities that could happen during that time. We're overdue for the eruption of the Yellowstone supervolcano and a devastating asteroid impact on the order of what killed off the dinosaurs. Neither of these are worth worrying about in our lifetimes, but it's quite likely one or both of these would occur in the time it would take for a new intelligent species to evolve.

Such events wouldn't necessarily kill off the larger species, but one need only look at the KT extinction event that killed off the dinosaurs 65 million years ago to see just how devastating a giant impact can be to life on this planet. And a big reason why mammals became the dominant form of life after this event was that they were nowhere near intelligence.

The diminutive size of KT boundary mammals, comparable to that of rats, made it far easier for them to find safe haven in sheltered environments, whereas the larger dinosaurs all died out, for all their prior dominance. Any species that's close to developing human-like intelligence would be unlikely to survive such a cataclysm. And that could mean another sixty or so million years before a new intelligent species might emerge.

So then, which are the best candidates to be our successor species?

At the risk of a circular argument, humans remain the only human-like intelligent species we've ever encountered. As such, the best candidate for our replacement really does have to be our closest living relative, which is the chimpanzee. It is not at all a certainty that chimpanzees would evolve our level of intelligence if we suddenly disappeared - if the chimpanzees never moved into our vacated niches, there's no reason they would undergo that evolution. And anyway, it's possible that their divergence from our most recent common ancestor brought with it some fundamental anatomical or cognitive adaptations that make human-like intelligence incredibly unlikely. Again, we can only guess how common intelligence actually is, because we're only working with a single example.

Still, if we take the widest possible view, pretty much any vertebrate land animal has a shot at evolving intelligence, provided it finds itself in the right environmental niche that favors strategic thinking and it has sufficient time to evolve the size and anatomy needed for a complex brain.

If we're ranking the odds of such an evolutionary pathway, the great apes would seem to have the inside track on this, then, roughly speaking, followed by the larger land mammals that have shown previous signs of intelligence, the rest of the large mammals, the small mammals, and the rest, likely assuming they have the anatomy capable of supporting sophisticated tool use. For that reasons, dolphins, whales, and birds have to remain wild cards in all this.

Ultimately, we can only guess what new forms intelligence might take if evolution were given a second chance to produce a species with our level of cognitive sophistication. But, if we actually do prize intelligence as something worth having, it's probably for the best if we never give this planet a chance to find out.

Further Reading on animal intelligence and other topics raised here

The evolution of intelligence: are humans the only animals with minds? by James H. Fetzer
Handbook of intelligence by Robert J. Sternberg
Wonderful life: the Burgess Shale and the nature of history by Stephen Jay Gould
The Ancestor's Tale by Richard Dawkins
The evolution of intelligence by Robert J. Sternberg and James C. Kaufman

Additional reporting by Robert Gonzalez.