Evolution bottleneck event leading to color changing humans

Question

How to get humans that can change their skin color?

Approximately 1.6 million years ago, hominids lost their hair and gained dark skin (so says the New York Times). Evidence shows several several population bottlenecks where the number of hominids dropped to less than 100,000. Every population bottleneck is an opportunity for radical and broad transformation in any population.

Let's assume that in one of these early bottlenecks, a very large ground-bound avian predator (think terror bird) was introduced that preyed on early hominids, drastically reducing the hominid population. Since avians almost always hunt by sight, it's advantageous for humans to camouflage themselves. During this bottleneck, a mutation appears that permits a group of hominids to mottle their skin color to match the background much like octopuses (video). While the ability is primitive at first, it conveys sufficient survivability that chromatophores spread to all hominids.

The giant avian predators disappear (extinction from whatever cause) but the color change capability remains in hominids. Sexual selection takes over and those with the prettiest and/or most extensive color range mate more often ensuring that chromatophores stay in the population while also increasing in complexity and color range. Assume that after the giant killer birds go away, that the evolutionary path to get to modern humans resumes.

Now granted, the communication methods of these early hominids will be very different compared to how this all happened on Earth.

Is this a reasonable series of events to explain humans with color changing abilities? Have I missed something?

Answer 1

Cephalopods change their colors using chromatophores. Mammals on the other hand (along with birds) use melanocytes. Chromatophores can do the crazy coloration techniques you are looking for, melanocytes cannot; or, at least they have not.

Cephalopods have chromatophore organs to control their complex color changing patterns. This is a pretty complex piece of evolutionary machinery. I don't know if there is a definite answer on 'rate' of evolution, but this seems too complex to evolve ex nihilo on a scale of hundreds of thousands of years.

Chameleons and other vertebrates which have color changing abilities. They do not have chromatophore organs but instead have use cell signaling to tell each chromatophore cell what colors to make. This is a less complex system, but it is also slower and, more importantly, it is evidently used in chameleons more for temperature management and signalling (as in, mating) than for camouflage. So while this system is easier to evolve, it would be less useful to your people for hiding from killer pterosaurs.

One last problem. How hairy were people back then? If you are putting chromatophores onto, say, a chimpanzee, they aren't going to do much good since there is fur in the way. That is why it is the mammals and birds that lost the vibrant colors: most of their skin can't be seen anyways.

In conclusion: Humans already have a different cell for skin pigment, one that cannot change colors (except as slowly as getting a tan), so the evolutionary advantage of developing chromatophores would be low until a fully developed system of color changing developed. Given that such a system in squids is very complex, and that a less complex system in chameleons is only marginally effective, the evolution of chromatophores seems unlikely.

A more likely response to avian sight-hunting predators is to become nocturnal or stay in forests.

So if you really want color changing humans you need a. humans that evolved from squid, b. humans that evolved from chameleons, c. mammals never lost chromatophores and most mammals (or at least primates) are hairless and have brilliant colors like tropical fish or d. magic.

Answer 2

Avian predators are not that fascinating and can be killed easily by humans, it requires humans to be always in groups of 3 while moving, with spears, tomahawks, and so on. predators that put evolutionary pressure on humans from hunting havn't been seen, except for the fastest pack animals, wolves, lions, who use mostly scent, in fact all mammals have scent disadvantage.

Animals that hunt in packs, (which would be fascinating) would be a cross between a locusts and bees, and swarming crows. Note that there are no flocks of eagles, because there could be, except that they could empty the entire ecosystem and flocking eagles would be so effective that they would starve themselves.

A equally scientifically likely cause of a color change mutation would be sexual attractiveness, communication, and active hunting ability...

There are different types of metachrosis, spiders even have it sometimes from to adapt to plant colors.

The most cool is the chameleon. It has crystals similar to your nails and hair, contained in soft tissue, which can expand and contract and change density, probably platelets or grains of some kind, and depending on the squeeze, they can reflect light like an LCD crystal or petrol, in fact it's the same as a blue/green/red insect, which all have platelets in their shell which gives a metal reflection.

Chameleons and insects can only do bright colors with protein horn material which is arranged in platelets similar in size to light wavelengths, check TEMicro pics for it... they have expanding melanin organules/cells, black to change them black. images here - https://www.google.fr/search?espv=2&biw=1195&bih=723&tbm=isch&sa=1&q=color+chitine+microscope&oq=color+chitine+microscope&gs_l=img.3...2510.4451.0.4700.13.10.0.0.0.0.332.809.1j1j1j1.4.0....0...1c.1.64.img..10.2.357...0i7i30k1.k9QQZn_J9tc

human eye color changes help humans to indicate the direction of prey and to communicate pathways or dangers without talking, and makes it easier for humans to communicate in groups as it avoids danger within a group. communication seems like a more likey cause for a color change.

Humans with color change would be very communicative about mood and for hunting indications and dangers. long distance communication without waving hands about.

So a human with color change would learn to talk, left right, now, later, etc with colors.

The microbiology is difficult. if humans could do like squids they would have seaslug mucus skin because it would be so flexible and spongey. squids have muscle pores containing red,yellow, black white colors. you could say that they at first developed hair which had iridescent colors, like birds, or even feathers, and learnt to communicate by making color patterns with feathers or by turning their hair like petrol reflections or lcd cells and opals... that's oddly enough one of hte few physical pathways for that trait.

So you haev to figure out a way for humans to show and hide freckles with hair or with other freckles that slide over the first freckles. the most likely way is for humans to develop chameleon skin, so essentially their freckles would become highly developed platelets and humans would have actual muscle or electricl signals going to their skin to make colors by changing platelets or freckles density/postion/rotation.

mammals don't have bright colors, birds do, which is an illustration of the unlikelihood of it happening in mammals, even 60mn years of mouse generations didnt make red/blue/green mice.

Evolutionary pressure by improved communication rather than simple camouglage amd mimickery, and mutation of human pigments to be similar to chameleons, is the mostly likely cause for humans.

because human color blue green red is mostly in the eye, you can say that humans developed colors in their mucus membranes first, eyes, lips, like blue baboons even, and then they developed rgb hairs or even?

Answer 3

Here are a series of bottlenecks that could be used to justify this.

1st bottleneck. One branch of Hominid creatures suffer massive infestation of mites. Infestations are so bad that individuals can die of infections. An evolutionary advantage is given to those with less hair.

2nd bottleneck. An extreme lack of sources of vitamin D occurs at the same time (many generations in length) that the Sun's output of damaging radiation is abnormally high. While darker pigmentation is advantageous to block the damaging radiation, it also blocks creation of vitamin D. A small portion of the population has a trait that allows pigmentation cells to steadily darken in proportion to the intensity of the sunlight, fired off by nerves. The lighter skinned population suffers from skin cancers, or becomes primarily nocturnal, and the darker skinned population suffers from Vitamin D deficiency. The variable skinned people are able to produce Vitamin D in the mornings and evenings, and can hunt on the open plains during daylight hours.

3rd bottleneck The avian predator (which could be a Golden Eagle) starts heavily predating these plains-hunting hominids. A small portion of the population has some limited control of their skin, allowing a "splotchy" appearance. Rather than the tans and browns of current humans, some ability to produce actual color is available, but very rudimentary.

From here, it continues along your path. As these Hominids grow larger, they are no longer suitable prey for the avian (or the avian died out). Fine control of the color changing ability becomes a primary attractant for mates, giving heavy advantage to those with even slightly better ability.

Answer 4

I don't think this is justifiable. The Kingdleon made is a strong one, though I would also through in that i"m not sure sexual selection could justify maintaining such a trait for so long as you described it in any case. Once the bird died the odd skin color would be a handicap, allowing some parts of the body to be vulnerable to skin cancer while still risking vitamin D deficiency, getting the worst of both pigmentation. Now it's true that sort types of 'harmful' traits in males can spread due to the 'handicap principle', but this wouldn't be a good example of a trait to be encouraged via the handicap principle. long term monogamous herd species wouldn't depend on the handicap principle for mate choice anyways, the means for selecting a mate in social herd species is very different and more complex, physical appearance is no longer the sole definer for picking a sex, with intellect and social skills growing more important, limiting the effect that sexual selection can drive physical features towards a harmful extreme.

However, all that is moot for a far more basic reason. There would never be a flying species that hunts humans, were too big. There is an upper bound on how large any flying creature can get, a side effect of the square-cube law and the difficulty of maintaining flight. No flying creature could ever be large enough to pick up grown humans in order to carry their body somewhere safe to eat (it wouldn't be safe to eat the body where killed, surrounded by angry humans who may not approve of your killing them either).

Furthermore, humans are kind of bad prey. Were bony and provide poor meat content and if were in the middle of a human bottle-neck as you suggest were not even common enough to be worth hunting. There is not going to be a creature capable of regularly hunting us, to the frequency to play any noticeable role in our evolution, and which would find us worth hunting.

Even if such a bird did exist, this sort of camouflage wouldn't work for us. Were a social species, we move in herds; and were tall species that stands out from a distance. It would be easy to find humans in the plains and other areas we would be traveling once we were bipedal. We won't be in areas that we could realistically hide from something flying with our upright stance and herd movement, and our instincts would be less to hide and more to defend each other by attacking the bird most likely, since the bird would barely be big enough to attack our young and thus vulnerable to adults attacking it to protect their children.

If you want variable coloration I wouldn't have it evolve at all, make it an 'accident'. Mutations can occur all the time that do rather random things, and as long as those mutations aren't harmful they can potentially stick around, even if they are not beneficial either. If a random mutation happened to cause odd spread of melanin in skin cells appeared during a population bottle neck it could have spread by 'luck' to most of the population without it necessarily being an evolutionary advantage, simply not evolutionary disadvantageous either.

Of course since random skin tone is going to be a mild disadvantage you would need to offer some advantage to counter it, but it need not be a significant one, which offers a few options.

1) the mutation provided some odd protection against UV light similar to what comprehensible said, though if you see my comment there are limits to his suggestion; but I think bit's an interesting idea that may be persuable

2) The mutation causing random skin color provided a completely different advantage. This seems easier to justify. Mutations are not random changes to one or two things, most mutations have many affects, and even many 'harmful' mutations have significant benefits justifying how they managed to spread. Sickle cell anemia, far more harmful then random skin coloration, provides a defense against malaria explaining how it likely managed to spread so rapidly in our gene pool, to give just one of many examples.

Thus I would say that this completely random mutation happened to prove useful for reasons unrelated to the skill change. The mutation also happened to somehow provide some minor defense to some sort of disease or cancer etc. The ladies would notice that children of multi-pigment males live longer and prefer to mate with them, spreading the gene rapidly during the bottle neck where it was relevant. Given enough time if the disease or other thing the random pigmentation mutation protected against was less of a threat I would expect women to stop being attracted to it. However if you set this mutation as occurring during the most recent bottle neck it's possible the preference will still be around, having not had enough time to evolve away. Though it would likely only be prevalent in the appropriate parts of the world where neither Vitamin D deficiency or cancer caused by UV rays were too common. So perhaps it would mostly be prevalent in those about mid way between equator and the north/south poles.