As most bacteria evolve to a less deatly version of itself to promote spreading, why hasn’t TB evolved to a more harmless bacteria? It’s so old, so there was plenty of time. Killing the host seems pointless in a evolutionary sense.
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In: Biology
Keep in mind evolution doesn’t have “points” or “make sense”, it just goes with what works best, now. There are plenty of really, really, REALLY dumb adaptations out there.
In the case of TB specifically though, why would it need to change? Untreated TB infections can take 5-15 years to kill someone, almost every moment of which the person is hacking up particles to spread the illness around. A decade of active spreading feels like a pretty sweet score for a bacterium to get out of it’s host.
TB is a relatively harmless bacterium. Can it kill? Sure. But most people you know don’t die from it, because it doesn’t harm most people. Projected over mankind, it’s not bad at all. It evolved to be harmless to most people, mildly harmful to some people for a while, and deadly to a relative few.
The main factor is that they don’t kill many hosts and most importantly they kill slowly. If the flu virus killed us immediately, the first village that got it would have been wiped out and that would have been the end of it. Instead the flu sticks around a week, during which we spread it to others. TB goes the extreme way: it lets you live for years without harming you much, so you can spread it. Eventually you may become weak enough to die.
As long as this strategy works, TB will do it.
Killing the host *before the bacteria can spread* is an evolutionary dead end. Killing the host after the bacteria has spread is perfectly fine, because “live just barely long enough to breed” is the only result that evolution encourages.
Im not sure your original supposition is in fact correct. What you are seeing is a constant competition between organisations rather than a rule of evolution.
Im not an expert. Id appreciate those with specific evolutionary theory / knowledge add to the conversation.
I doesn’t need to. Killing the host is relative to how long it takes to grow versus its host. Its reproductive cycle is so slow that there’s no selective pressure for it to do less damage.
Microbes typically evolve to be less virulent when they are very aggressive and fast growing in their host. TB grows at the same rate that human cells grow under optimum conditions.
It doesn’t have to evolve into a harmless bacteria. Its very successful at living and spreading. It takes years for TB to kill someone. Its able to spread to hundreds of new hosts in that time. All it needs to do is spread before the host dies. Consider that this actually could be a less deadly version of what it once was millennia ago.
Your premise is false. Most bacteria *don’t* evolve to a less harmful version. That is an evolutionary path that occasionally happens, when the context suits it to be a favorable mutation. Other bacteria evolve to become *more* harmful; but the vast majority of the time, bacteria evolution makes it neither less nor more harmful to its hosts or its environment.
It has, while it kills a few( relative to the people it infects) its spreads quickly, about 25% of the worlds population has latent TB
TB has evolved closer to harmlessness. It replicates extremely slowly and the human immune response is to basically lock it up inside an egg with a bunch of food. Doesn’t kill it outright but it is a hell of a band aid since it will take decades for them bust out.
And that’s why it takes decades for TB to actually kill you once contracted. Even once it is in its active stage you can live with it for as long as 15 years if left untreated. That’s a hell of a lot of coughing and sneezing to spread the shit around.
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