Humans have dominated this planet for tens of thousand years. One study showed the arrival of people in Australia about 47,000 years ago caused the extinction of marsupials such as the giant wombat-like Diprotodon, and there have been many other victims, such as the dodo, since then.
The impact we have on the planet has accelerated arguably out of control since the agricultural and industrial revolutions of the 18th and 19th centuries.
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Who knows if we are heading for extinction ourselves as a result of our unsustainable lifestyles, but there’s no question it would leave a massive void. But were that to happen, would any other species fill the space we have left?
Let’s have a look at some of the candidates.
Which species would dominate the world after humans?
You might think that other great apes, such as gorillas or chimpanzees, would be in pole position. They are the next most intelligent terrestrial mammal after us, and crucially have dextrous digits, including the all important opposable thumb, which enables chimps to manipulate objects into tools.
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There is an argument that any species that came to dominate the Earth as humans have done would need to be able to manufacture things. And it would also need to create fire to be able to cook food which would enable it to increase its calorie intake and so, over time, grow a bigger brain.
But, in reality, other great apes are not likely candidates for succeeding humans. Their language is limited to unsophisticated grunts (in the case of gorillas) and pant hoots and other vocalisations (in the case of chimps) and is nothing like as complex as that which humans have developed.
The complete absence of dextrous digits would also rule out another very intelligent group of animals – whales, dolphins and porpoises. They’d have a very hard time indeed building things with their flippers.
Species such as brown rats have flourished in human societies, and are now found on every continent except for Antarctica. They are highly adaptable and breed quickly, but have no language or a capacity for abstract thought. They might prosper for a while if humans disappeared, but the fact is they are also prey for small predators.
Perhaps they could evolve greater intelligence over time and better motor skills in their front paws, but it seems a long shot.
Some domestic dogs and cats would thrive in the absence of humans. We know that feral dogs can form large stable packs in some cities, while cats also do well in all kinds of environments where they too have gone wild. Again, though, the chances are that over time, other larger predators such as wolves or leopards would restrict their numbers.
Perhaps we need to lift our eyes away from the ground to find the real candidates. Some of the most intelligent and adaptable animals on the planet are corvids – crows, ravens, jackdaws, magpies and so on – and, to a lesser extent, parrots.
Carrion crows in Japan, for example, have learned to drop walnuts onto roads so that cars drive over them and crack them open. Not only that, they are bright enough to drop the nuts at pedestrian crossings, so that they can collect the prize when it’s safe to do so.
New Caledonian crows can make and use tools and even select the correct tool for a task in advance of carrying it out. They can also understand cause and effect relationships, as demonstrated by this study where they reacted differently to an event depending on whether they could see what was causing it or not.
Meanwhile, the renowned parrot researcher Irene Pepperberg has demonstrated that parrots (most notably, an African grey parrot called Alex) can use and understand language.
So, crows and parrots already have an intelligence roughly akin to great apes (apart from humans), but the question is whether this could further develop, over millions of years, in the absence of humans.
Could they perhaps evolve to use their feet like we use our hands for basic manufacturing? Could they cooperate as human societies do, with individuals taking on different tasks within the community? It does seem unlikely, but who knows?
Should we allow our imaginations to soar even further, just as Dougal Dixon did in his book, After Man: A Zoology of the Future. He dreamt up a menagerie of strange, new creatures that included birds that had evolved heads like flowers to attract insects in order to eat them. This article on BBC Future envisaged trees evolving to walk or to kill animals with toxic fumes and poisonous fungi floating in mid-air and killing and engulfing anything they bump into.
Is the most likely though pessimistic scenario that humans destroy ourselves in some nuclear, chemical or biological apocalypse or we are killed off by a massive asteroid strike which also kills off, or substantially reduces, most other mammals and birds?
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In that situation, it’s likely that small, adaptable mammals, birds and insects survive and, over millions of years, one of them treads an evolutionary route towards greater consciousness and intelligence.
In that case, we can barely begin to imagine what will take over the world. But it’s interesting to try.
