Cats already run the internet. They control the furniture. They have trained an entire species of bipedal primates to open tins for them on demand. They have achieved all of this without opposable thumbs.
Now give them thumbs.
The mechanical advantage
A domestic cat weighs on average 4 to 5 kilograms. It can jump roughly five times its own height. It has retractable claws, night vision, hearing that extends to 64 kHz (humans top out around 20 kHz), and reflexes approximately twice as fast as a human's. The only thing preventing cats from complete environmental dominance is that they can't grip things.
Thumbs fix this.
We're not talking about a vestigial nub. We're talking about a proper opposable thumb, the kind that lets you pinch, grip, twist, and manipulate objects. The kind that let humans develop tool use, and from tool use, civilisation. Cats with this capability would start with the obvious applications.
Door handles. Within a week, every cat in the country figures out door handles. Indoor cats are now outdoor cats whenever they feel like it. The entire concept of keeping a cat in a room is over. Baby gates, closed bathroom doors, bedroom doors shut at 3am: all useless. Your cat has unrestricted access to every room in the building, and it has chosen your pillow.
Then taps. A cat that can turn a tap on will absolutely turn a tap on. Whether it can turn a tap off is a different question, and the answer based on all available evidence of cat behaviour is no. It will not. Your water bill triples.
The cupboard problem
Cat owners already know that a sufficiently motivated cat can open some cupboards. Round knobs slow them down. Childproof latches stop them. Thumbs eliminate both barriers.
This means unattended access to food. All food. The fridge has a handle. The treat cupboard has a handle. The bin has a lid that lifts. Every surface in your kitchen becomes accessible to an animal with no concept of portion control and an evolved instinct to eat whenever food is available because in the wild it might not be available tomorrow.
A 2020 study by the Association for Pet Obesity Prevention found that roughly 61% of cats in the United States were already overweight or obese. Give them thumbs and independent access to the fridge, and that number approaches 100% within a month.
The wildlife situation
Domestic cats kill between 1.3 and 4 billion birds in the United States every year, according to a 2013 study published in Nature Communications. In the UK, the Mammal Society estimates that cats bring home 27 million birds annually, and the actual kill count is far higher since cats don't bring everything home.
These are the numbers without thumbs.
Cats with thumbs can use tools. Not immediately. Not consciously. But cats are observational learners. A cat that watches you use a tin opener will attempt to use a tin opener. The results may be mixed at first, but the intent will be there.
More pressingly, thumbs improve grip. A cat that can properly grip a branch can climb higher, hold on longer, and reach nesting sites that are currently inaccessible. Bird populations that rely on height as a defence from predators lose that advantage overnight. Ground-nesting birds were already in trouble. Canopy-nesting birds now join them.
Conservationists would lose their minds. They're already losing their minds about cats. The addition of thumbs turns a serious ecological problem into an extinction-level event for small vertebrates.
The laptop situation
Your laptop is already at risk. Cats sit on keyboards. Cats walk across trackpads. Cats have accidentally sent emails, opened applications, and at least once (documented on Reddit, which counts as historical record) purchased items from Amazon.
A cat with thumbs can type. Slowly, badly, and with no regard for grammar. But it can type. It can also, more importantly, close your laptop while you're using it. It can pick it up. It can carry it to its preferred location, which is whichever spot is least convenient for you.
The cat doesn't want your laptop because it understands technology. It wants your laptop because it is warm, because you are paying attention to it instead of the cat, and because the cat has decided that this object is now the cat's object. Thumbs make possession nine-tenths of the law, and cats already believed that.
Social structure changes
Domestic cats are semi-solitary. Feral cat colonies exist, but they lack the cooperative social structures of pack animals like dogs or wolves. Cats don't hunt together. They don't share food willingly. They tolerate each other's presence in exchange for access to shared resources.
Thumbs could change this. Tool use in primates correlates with social complexity. Animals that can manipulate their environment gain advantages from cooperation that non-tool-users don't. If one cat can open a door but another cat can open a jar, there's an incentive to work together. Or at least to tolerate each other strategically.
I wouldn't overstate this. Cats have had millions of years to develop cooperative behaviour and have consistently declined. Thumbs might change the cost-benefit calculation, but the fundamental temperament of a cat is the fundamental temperament of a cat. They would likely develop tool use independently and then refuse to share the results.
Could we stop them?
We could try. Cat-proof locks exist. We could redesign door handles, cupboard latches, and taps to require a level of dexterity or strength that cat thumbs couldn't manage.
But we'd be playing catch-up. Cats are fast learners when motivated, which they always are when food or territory is involved. Every new lock becomes a puzzle, and cats have nothing but time and an absolute refusal to accept that something is inaccessible.
The pet industry would explode. Thumb-proof feeding stations. Biometric cat flaps. Reinforced bins. Anti-cat cabinet locks rated for opposable grip. You'd spend more on cat containment than on rent.
And the cats would watch you install each new device with that expression they have. The one that says they have already figured out the mechanism and are simply waiting for you to leave.