sethoflagos

I was fine with you up to this final point. But if moderate 'darkness' yielded sufficient advantage to balance African level UVB flux over the somewhat lower fluxes of elsewhere, then where is the driver for 'very dark'? 'Very dark' is at least in modern times a minority characteristic in Africa both locally and as a whole.

The fact that the male generative organ is particularly well designed for the removal of previously deposited fluids from the vicinity of the female generative organ is a pretty good guide to the comings and goings of our ancestors. Conspicuously different to the chimp strategy and seems weighted towards gaining maximum reproductive advantage from an infrequent opportunity. Make of that what you will. Mind you, the shenanigans that went on half a million years ago are not necessarily a good guide to current practice.

No matter had poorly you perform at any skill. if you practise it diligently in small increments, and demand perfection at each increment, you will gradually improve that skill. People typically fail to achieve this principally through two means: 1) They expect to achieve great advances with little work. 2) They prepare for failure with lame excuses, and then gladly accept the fulfilment of their expectation. Success requires a determination to succeed and a willingness to devote whatever time it takes to achieve that goal. Often that time can be measured in years.

Over the last few hours, I've been experimenting with the Wolfram free calculator. I hope you guys appreciate what a tremendous advantage this facility provides compared to what previous generations had access to. It solves in seconds stuff that used to take me weeks to work through by hand. But you know that don't you?

Why do you guys have so much difficulty in embracing the concept of multiple evolutionary drivers? There's no doubt that probably around the time we lost most of our body hair, MC1R became very strongly conserved under strong selective pressure to darken our pale ancestral (chimp-like) skin. There can be little doubt that the strong UVR protection provided by eumelanin producing melanocytes played a part in that. Possibly a dominant part. And it still remains highly conserved, almost invariant in most African populations. But there are a number of points to address here: 1) Active MC1R does not necessarily turn the individual jet black. It covers very nearly the whole gamut from a golden fawn upwards (and that's leaving aside albinism). This is at least partly under environmental control. 2) Susceptibility to mortality in adulthood from skin cancer has never seemed sufficiently strong a driver to explain the conservation of MC1R. The strength of that conservation has more than a hint that eg sexual selection played a role too. 3) You seem happy to accept the idea that MC1R ceases to be conserved (quite quickly apparently) in a slightly lower UV intense region as southern Asia, yet reluctant to acknowledge the many African populations, particularly those of Central and Western Africa where rain forest shielded them from UV far more effectively than a few degrees of northern latitude and for a considerably longer period of time without MC1R degradation. 4) And why doesn't MC1R reestablish its monopoly in high UV areas elsewhere in the world? 5) You can suspect the contrary as much as you like, but your utter dismissal of thermoregulation as a significant evolutionary player in the development of our species speaks volumes. Google 'cursorial hunting' then tell me that heat stress hasn't been a factor in who lived and who died.

Many thanks! I did eventually get some joy out of Wolfram, and the results actually simplify very nicely (or at least better than expected.) But it's good to have a longhand approach available for checking purposes. Seth

It's of the order 10W per degree difference between skin temperature and environment. The shade temperature typically for the African equatorial belt is rarely above 32 C. Usually a bit less. A resting human can typically get their metabolic thermal output down to around 70W. These ballpark figures indicate that radiative cooling could conceivably be a significant survival factor in a crisis. The skin pigment to emissivity relationship at the relevant frequencies (around 10 microns), I've no idea. I don't claim that it's the full picture, but I do see a certain amount of evidence around me and would be interested to see some proper research done on this. On an African population in particular. Rather than have it inferred from studies of other populations, which is what we see so often.

'There is no single gene for lighter skin' does not mean there are none, it means that there are many. I think you know this, The paper you quote does not deny it. And indeed goes on to state that that particular gene can only account for a fraction (27% was it?) of skin tone variation in European populations. So other controls must be involved, mustn't they? But we weren't discussing the skin tone of European populations were we? So I don't really see its relevance. Nor really do I see the point that you're trying to make with it. And when you say 'this particular gene........gives our skin its pigment', who exactly is the 'us'?

Please don't try hinting that I'm attempting to mislead. There are many genes involved. You don't mention KITLG or ASIP for example, though these seem to have diverged a little since they left Africa and could have been used to support your position (whatever that is). But you also ignore MC1R, SLC24A2, TYR, HERC2, OCA2, and ATRN among others. No one is trying to say that Africans aren't typically darker on average than Europeans. Just that our common ancestors will have been genetically diverse, and will not have conformed in general to a particular stereotype.

There is no single gene for lighter skin. Many Africans (my wife's family amongst them) have relatively pale complexions. 'Yellow' in local parlance. And this is not due to some 'back to Africa' travelling of a 'white' gene. It's undoubtedly been added to in various colonial expeditions, but the widespread occurrence of pale skin tones among ancient African lineages (the San peoples being an obvious example) suggest that other than the very blonde features of some Scandinavians (linked to a long history in ice age Europe), the diversity of human skin tone is within the normal variance of African populations. I don't present any academic research papers to support this point of view. Because as far as this particular subject is concerned, I'm not sure there are any that have not been corrupted by an agenda. Sorry to say.

Unfortunately, I do not know using this thing well either, blue89. I was brought up with log tables and slide rules. Wolfram is a mystery to me.

The overwhelming majority of human DNA diversity is amongst populations of African lineage. To try and categorise all that variability into a single phenotype is just plain wrong. Walk down any street in Africa and you will see just how wrong it is.

I think your problems are more semantic than real. If you recognise the term 'reptile' as a valid classification, then the first Amniote was a reptile and we are descended from reptile stock. Though since we are excluded from the definition of 'reptile', that becomes a paraphyletic group and apparently that is a bad thing now. Better to drop the term 'reptile' completely and talk about Series Amniota (https://en.wikipedia.org/wiki/Amniote - based on egg structure) and its split into Synapsida and Sauropsida (based predominantly on the temporal fenestrae, but also a few other features).

I spent a year in the middle of the Gabonese rainforest. Very strange environment. Not sure camouflage is that relevant in the forest providing you're not some luminous primary colour. Sound and smell are far more relevant for locating prey. I put some store in thermoregulation. Forest peoples hardly sweat at all. So it's clearly a significant evolutionary factor for them. But these processes are never simple.

As a relatively pale skinned person who has lived in the African equatorial belt for nearly 20 years, one of the really useful benefits of a dark skin that I envy is the ability to find shade and radiate excess body heat away quickly, especially in very humid conditions where sweating does nothing but dehydrate. In this environment, heat stress and dehydration can kill very quickly. Traditional forest peoples who rarely see direct sunlight are often among the typically darkest-skinned. So I find the solar exposure theories a bit of a simplification. Africa is far, far more varied in ecosystem diversity than is widely realised (Savanna? Which type of savanna?) and the genetic diversity both of living Africans, and our distant African common ancestors must reflect this. To some extent the question is silly. There is no genetic advantage in monochrome uniformity. So arguing about the precise shading of our ancestors tells us far more about our present than our past.

Many, many challenges. From a practical point of view, if steam is the heat transfer medium, it needs to be heated to preferably well in excess of 400 Centigrade to have enough pressure and superheat in it to be processed in a reasonably compact and efficient steam turbine. With an average lithospheric temperature gradient of 25 C per kilometre, that means drilling at least 15 kilometres. Or 3 kilometres beyond the depth of the Kola Superdeep Borehole. (Which took 20 years.) At those sorts of depths the hydrostatic pressure is typically going to be knocking on 5,000 bar. Which means you're going to have to compress your feedwater up to this pressure to get it into the formation. Process piping above 100 bar asks severe questions of design safety. So multiply that by fifty, add in all the aggressive minerals that become fairly soluble in water at that temperature. On the way invent a high performance drill bit that can withstand those sorts of conditions........ And that's sort of why geothermal energy is going to be fairly small scale and localised for some time to come. In passing, making a significant disturbance to the hydrostatic and chemical equilibrium around the Yellowstone magma chamber before we know exactly how that beast operates may not be a great move.

The physics group have redirected me here. All parameters are constant save the Delta t terms. Related but a bit more involved is The divisor is essentially a large positive value with a small 'wiggle', but it would be nice to have an analytic solution. Many thanks for your time. Seth

Isn't opening a parachute essentially the same as throwing trillions of 'little rocks' (or air molecules) towards earth over a lengthy period? No need for them to be 'thrown' any faster than you're falling just so long as you transfer momentum faster than you accumulate it until you're down to a safe descent velocity.

Thanks! Will try it.

Didn't I read somewhere recently that the dominant force preventing two objects from occupying the same space ('touching') was electron degeneracy pressure rather than electrostatic repulsion?

I'm having a bit of a mental block with integrating a trig function that's cropped up in some work I'm doing on acoustics. The bones of it are dr/dt=(simple terms)-(A-Bcos{wt))^2/(C+Dsin(wt)) A, B, C, D and w all constant. Pointers yo a way forward would be much appreciated Seth