View Full Version : Hypothesis: human symbiosis with toxoplasmosis as origin of modern humans

03-04-2021, 10:34 AM
Humans seem to be able to derive intellectual advantages from being infected with toxoplasmosis and this could be a sign of a successful symbiosis between both organism (or even worse: cats, humans and the parasite):


("...Toxo-positive subjects could still be demonstrated to be superior to Toxo-negative subjects with respect to response accuracy")


("The RhD-positive, Toxoplasma-infected subjects expressed lower while RhD-negative, Toxoplasma-infected subjects expressed higher intelligence than their Toxoplasma-free peers.)

Now the last study shows that apparently you can only draw real advantage for intelligence of the symbiosis only if you have the right blood type. But IQ is a long term result of many factors and it is not secured that IQ really transfers well into survival advantages. The first study about decision accuracy in a resource related task is hence closer to actually measuring evolutionary fitness.

Toxoplasmosis will alter the balance between dopamine and serotonin in the brain, increasing dopamine so that the intended host (rodents) makes LESS ACCURATE and more risky decisions - and hence gets eaten by cats more often as a result.

Evolution tries to make neurotransmitters balance in a way that you make in the majority of situations the right decisions for survival (not the morally "right" decision by the way) and a parasite that throws this balance off should hence never have a good effects on decision accuracy.

So why does the decision accuracy become better in toxo-positive human subjects?

High dopamine tends to make decisions more impulsive. Low serotonin levels are associated with higher reactive aggression (this is a form of impulsiveness too if you want). High serotonin levels will in contrast make your thinking more psychopathic and maybe more "calculated".

What is even more interesting, is that in some measurements that are done live on the brain (they are so fare limited to only a few areas as the health danger is so high) dopamine and serotonin levels move in exactly opposite directions directly before a decision is taken:


(See minute 30:20, neurotransmitter levels in putamen directly before choice.)

So it seems likely to me that higher amounts of serotonin can counterbalance higher amounts of dopamine to avoid some of the more stupid and impulsive decisions that come from too high dopamine.
Now we know from the study of silver foxes, that self domestication increases serotonin.

The genetic mutation that enabled self domestication could hence (at least initially) have given infected carriers of this gene a much higher survival probability than people who where infected but did not carry the gene for self domestication. (The differences in survival probabilities might however be reduced today if our immune system has in general gotten better to handle the toxoplasmosis infection). This could be the origin of the modern human which is highly self domesticated.

I set this hypothesis up here for contesting or "grilling" if you want. And I am not entirely convinced myself too.

One major problem that is that levels of self-domestication does not map well to today's distribution of the parasite. For example in many parts of Asia neotony seems to be high, but not toxoplasmosis.

The possible explanations I see for this paradox are:
1. Toxoplasmosis was high in this area once too but the cat necessary for that did go extinct. (When did saber-toothed cats did go extinct in Asia for example?).
2. While the process was initially more driven by the parasite (and hence was no true "self"domestication, as a second organism is involved), as soon as high serotonin humans are in the majority it might be mostly driven by social preferences (and hence has turned into true self domestication today). In the areas where there is no or low toxoplasmosis social selection can specialize on eradicating people who are really genetically high in dopamine instead of targeting infected people too. So social selection is more exact and quicker.
3. Eating habits? Besides infecting yourself from the feces of cats you can as well infect yourself by eating other infected animals uncooked. I don't want to have a racist discussion here on eating habits and infections in Asia. I don't see it as a very plausible explanation either. Because if there are no infected cats there are no other infected animals and eating habits don't matter.
4. Another mind altering human parasite is active in Asia, but we have not yet discovered that mind altering effect.

While suggestion 2 reduces the falsifiability of the hypothesis very much (a case of 'true in all circumstances') I see some ways to test it nevertheless - if one focuses on a species where self domestication has not yet progressed as fare as in humans, so that the preferences of the majority are not so important yet. One might create such a species by introducing a genetic alteration (in the neural crest cells) in a species with a similar parasite on purpose - and see if this alteration is positively selected.

There is for example a fish that suffers from a different parasite which reduces serotonin which again gives very bad survival outcomes:

But some species infected by a parasite that increases dopamine would make a better match for experimental purposes.

Any other suggestions for tests which are easier to design :( ?

By the way:
In some eerie way this could even be a successful symbiosis between a certain type of primate brain, a parasite and cats: for some unknown reasons humans massively increased the population of cats and kindly carried them to even the most isolated islands. Is this parasite accidentally - this sounds really crazy now - taking out some genetic material from the cat and incorporating it into the human brain?