Recent comments in /f/askscience

Eomycota t1_jdr86zr wrote

Reply to comment by beefydeadeyes in How did humans 10000 Years ago care about their Teeth? by Takaharu7

Found those two from a quick search. I know there are other research on this topic out there, just can't remember how I found them.

https://link.springer.com/referenceworkentry/10.1007/978-3-319-16999-6_2352-1

https://scholar.google.com/scholar?start=10&q=life+expectancy+hunter+gatherer&hl=fr&as_sdt=0,5#d=gs_qabs&t=1679844806366&u=%23p%3DMLcekP_DW7QJ

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dukesdj t1_jdr6owx wrote

Reply to comment by TerminationClause in Around 550 million years ago the earth's magnetic field almost collapsed, but then strengthened a few million years later. Scientists say this may have been due to the formation of the inner core. But why exactly would that cause the magnetic field to get stronger? by somethingX

In the words of one of my colleagues "99% of the universe is fluids, the remaining 1% is just details". Fluid dynamics is everywhere, it is actually harder to think of things that do not involve fluid dynamics than otherwise. I extend this not just to geophysical and astrophysical fluid dynamics but all of physics, engineering, biology, medicine, chemistry, and probably more.

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TerminationClause t1_jdr251s wrote

Reply to comment by dukesdj in Around 550 million years ago the earth's magnetic field almost collapsed, but then strengthened a few million years later. Scientists say this may have been due to the formation of the inner core. But why exactly would that cause the magnetic field to get stronger? by somethingX

So, forgive me if I misunderstand, I'm still on my first cup of coffee... but all those years I spent learning about fluid dynamics (off-handedly learning about chaos theory) can actually be applied to the inner and outer core as well as the mantle? Not only that, but that we understand it as well as you seem to? I really should have gone to college for that stuff instead of just reading it on the crapper.

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dukesdj t1_jdqzjci wrote

Reply to comment by CrustalTrudger in Around 550 million years ago the earth's magnetic field almost collapsed, but then strengthened a few million years later. Scientists say this may have been due to the formation of the inner core. But why exactly would that cause the magnetic field to get stronger? by somethingX

That quoted line summarises it quite nicely really. Convection is great for dynamo action as not only does it provide the kinds of turbulent motion that is great for inducing magnetic field, but it acts as an energy source. Prior to any freezing of species one would imagine the fluid to be well mixed and essentially only a single phase homogeneous fluid. For a single phase single composition fluid convective instability sets in under the Schwarzschild criterion which essentially says that the instability sets in when the temperature gradient is larger than the adiabatic gradient (the temperature gradient at fixed entropy, we fix the entropy as temperature is a function of density and pressure and so mathematically the gradient is a partial derivative). In physical terms the way I think of this is there is an amount of heat a static fluid can transport through conduction, but if the amount of heat the system is trying to push through the fluid is above this amount then instability sets in, the convection then transports the heat by physically moving it.

If the fluid also has compositional gradient then we fall under the Ledoux criterion for convective instability and it is easier for convection to set in. This is known as compositional convection or double-diffusive convection. Mathematically the compositional gradient is subtracted from the adiabatic thus lowering the actual temperature gradient required to onset the convective instability. The best way to physically understand this is through parcel arguments which really require figures so instead I will refer to Pascal Garauds excellent lecture notes which are more related to astro than geo. The result will be more efficient heat transport and more energy available for dynamo action.

Another process is two phase fluids which is the freezing out of material. This I know a lot less about as I am more concerned with starts than terrestrial planets but I am in the same boat as the rest of the fluids community in this regard as even the hydrodynamic (no magnetic field) problem has only recently begun to be properly worked on. In simplistic terms it can be thought of in a similar way to compositional convection in the sense that it provides an extra source of energy that can power the dynamo.

The bottom line is, convection is a natural way of producing a dynamo and stronger convection will lead to a stronger dynamo. Thus any mechanism which can aid convection or act as a source of energy/entropy will be beneficial for dynamo.

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h3rbi74 t1_jdqxx3q wrote

Reply to comment by iamamuttonhead in Why does tetanus vaccination lasts longer than influenza vaccination? by E-C-A

Very excited to see if the Valley Fever vaccine is successful! Especially because if it is, one can hope that vaccines against blastomycosis and histoplasmosis aren’t far behind, and that’s what I’m more likely to see where I currently live. (Also advancing science and human medicine and etc, but dogs with systemic fungal disease are so sad and challenging to treat!)

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iayork t1_jdqx141 wrote

Reply to comment by jawshoeaw in Why does tetanus vaccination lasts longer than influenza vaccination? by E-C-A

> if you’re not regularly exposed to the organism in question, the antibodies fade faster, sometimes much faster than in communities where the organism is endemic.

Do you have a recent reference for this? My impression is that that was a good working hypothesis, but it hasn’t held up very well to data - in particular, I think that measles vaccine immunity holds up well even in regions where measles is essentially eradicated.

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iayork t1_jdqwpp2 wrote

Reply to Humans experienced a massive population expansion in a very small amount of time. What are the evolutionary consequences and benefits of such an event, massive popular of a species in a small amount of time? by bent_over_life

TL;DR: rapidly expanding populations have faster adaptive evolution, and that’s what is seen in humans.

> Human populations have increased vastly in numbers during the past 50,000 years or more (1). In theory, more people means more new adaptive mutations (2). Hence, human population growth should have increased in the rate of adaptive substitutions: an acceleration of new positively selected alleles. … In such a transient, large population, size increases the rate and effectiveness of adaptive responses. For example, natural insect populations often produce effective monogenic resistance to pesticides, whereas small laboratory populations under similar selection develop less effective polygenic adaptations (5). Chemostat experiments on Escherichia coli show a continued response to selection (6), with continuous and repeatable responses in large populations but variable and episodic responses in small populations (7). These results are explained by a model in which smaller population size limits the rate of adaptive evolution (8). A population that suddenly increases in size has the potential for rapid adaptive change. The best analogy to recent human evolution may be the rapid evolution of domesticates such as maize (9, 10).

—Recent acceleration of human adaptive evolution

The author of that paper has a blog post giving more background and explanation: Our new paper on why human evolution accelerated. His summary there:

> Our evolution has recently accelerated by around 100-fold. And that's exactly what we would expect from the enormous growth of our population.

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