Pharyngula

I like this hypothesis

But we have to be clear that it is only a hypothesis at this point. I was reading about domestication syndrome (DS) — selecting animals for domestication has a whole collection of secondary traits that come along for the ride, in addition to tameness. We are selecting for animals that tolerate the presence of humans, but in addition, we get these other traits, like floppy ears, patchy coat color, shortened faces, etc.; the best known work in this area is by Belyaev (YouTube documentary to get you up to speed) who selected silver foxes for domesticity, and got friendly foxes who also had all these other differences from their wilder brethren. Similar changes have been seen in rats and mink, so it seems to be a mammalian characteristic that all these differences are somehow linked. Here’s a handy list of the changes in domestication syndrome.

List of traits modified in the “domestication syndrome” in mammals

class="table-left table-vtop">Trait class="table-center table-vtop">Animal species class="table-center table-vtop">Location/source
Depigmentation (especially white patches, brown regions) Mouse, rat, guinea pig, rabbit, dog, cat, fox, mink, ferret, pig, reindeer, sheep, goat, cattle, horse, camel, alpaca, and
guanaco
Cranial and trunk
Floppy ears Rabbit, dog, fox, pig, sheep, goat, cattle, and donkey Cranial
Reduced ears Rat, dog, cat, ferret, camel, alpaca, and guanaco Cranial
Shorter muzzles Mouse, dog, cat, fox, pig, sheep, goat, and cattle Cranial
Smaller teeth Mouse, dog, and pig Cranial
Docility All domesticated species Cranial
Smaller brain or cranial capacity Rat, guinea pig, gerbil, rabbit, pig, sheep, goat, cattle, yak, llama, camel, horse, donkey, ferret, cat, dog, and mink Cranial
Reproductive cycles (more frequent estrous cycles) Mouse, rat, gerbil, dog, cat, fox, goat, and guanaco Cranial and trunk (HPG axis)
Neotenous (juvenile) behavior Mouse, dog, fox, and bonobo Cranial
Curly tails Dog, fox, and pig Trunk

(Hah, reduced brain size. I have a cat, I believe it.)

We have a very good idea of the proximate cause of tameness: the animals have reduced adrenal glands, which means their stress response is reduced, they’re generally less fearful, and they are more open, in early life at least, to socialization. But why can’t genetic mutations that reduce the size of the adrenal gland occur without also changing the floppiness of the ears? There isn’t an obvious physiological link between the two, or other traits in that list.

One idea is that there is a Genetic Regulatory Network (GRN). A GRN is a set of genes that mutually regulate each other’s expression, and may be controlled by the same set of signals. Imagine a lazily wired house in which the lights in the kitchen and the living room are on the same circuit, so you use one switch to turn them both on and off. Or perhaps you’ve cleverly wired in a simple motion sensor, so that when you trip the living room light, the changing shadows concidentally trigger the kitchen light too. Everything is tangled together in interacting patterns of connectivity, so you often get unexpected results from single inputs. The mammalian GRN works, though, so it’s been easier to keep it for a few tens of millions of years, rather than rewiring everything and risking breaking something.

More evidence that there’s a network involved is the fact that these domestication changes can happen incredibly rapidly — Belyaev was getting distinctive behaviors with only decades of selective breeding. What that means is that we’re not dealing with the sudden emergence of mutations of large effect, but with many subtle variations of multiple genes that are being brought together by recombination. This also makes sense. Rather than gross changes that change the entire GRN, what you are doing is tapping into small differences in a number of genes that individually have little or no effect, but together modify the target organ. So in order to change the size of an adrenal gland, you gather together an existing mutation that makes a tiny change in the size while also making ears floppier, and another one that also makes a tiny change in size while also shortening the snout, and another that makes a tiny change while modifying pigment cells.

That’s a very nice general explanation, but in order to advance our understanding we need something a little more specific. What genes? What links all these traits together?

Wilkins and his colleagues have suggested an obvious starting point: it’s all neural crest. Neural crest cells (NCCs) are an early population of migrating cells that infiltrate many tissues in the embryo — they form pigment cells, contribute to craniofacial cartilages, supporting cells for the nervous system, and just generally are found in precisely the places where we see the effects of domestication. So one reasonable hypothesis is that when you’re selecting for domestication, you’re actually selecting for reduced adrenal glands, which is most easily achieved by selecting for retarded or reduced or misdirected NCC migration or increased NCC apoptosis (multiple possible causes!), which has multiple effects.

nceffects

In a nutshell, we suggest that initial selection for tameness leads to reduction of neural-crest-derived tissues of behavioral relevance, via multiple preexisting genetic variants that affect neural crest cell numbers at the final sites, and that this neural crest hypofunction produces, as an unselected byproduct, the morphological changes in pigmentation, jaws, teeth, ears, etc. exhibited in the DS. The hypothesized neural crest cell deficits in the DS could be produced via three routes: reduced numbers of original NCC formed, lesser migratory capabilities of NCC and consequently lower numbers at the final sites, or decreased proliferation of these cells at those sites. We suspect, however, that migration defects are particularly important. In this view, the characteristic DS phenotypes shown in parts of the body that are relatively distant from the sites of NCC origination, such as the face, limb extremities, tail, and belly midline, reflect lower probabilities of NCC reaching those sites in the requisite numbers. The stochastic, individual-to-individual variability in these pigmentation patterns is consistent with this idea.

They document all the phenotypic changes associated with domestication, and strongly correlate them with neural crest mechanisms. It’s a mostly convincing case … my major reservation is that because NCCs are ubiquitous and contribute to so many tissues, it’s a little bit like pointing at a dog and predicting that its features are a product of cells. It’s a very general hypothesis. But then they also discuss experiments, such as neural crest ablations or genetic neurocristopathies that directly modify the same processes involved in domestication syndrome. So it is a bit helpful to narrow the field from “all cells” to “this unique set of cells”.

I have a similar reservation about their list of genes that are candidates for the GRN — they list a lot of very familiar genes (PAX and SOX families, GDNF, RTKs) that are all broadly influential transcription factors and signaling molecules. Again, it helps to have a list of candidates, it’s a starting point, but in an interacting network, I’d be more interested in a summary of connections between them than in scattered points in the genome.

You need a diagram to summarize this hypothesis, and here it is, featuring the important distinction between selected and unselected traits.

ncsummary

I do have one question that wasn’t discussed in the paper, and would be interesting to answer with better genetic data. We talk about domestication syndrome as if it all goes one way: wild predator becomes more tolerant of humans. But it seems to me that it’s a two-way process of selection, and humans also had to be less stressed out and tolerant of sharing a space with an animal that would like to eat them, or compete with them for resources. Are humans self-domesticated apes? Were we selected for reduced neural crest input? If we figured out the changes in genes involved in domestication, it would be cool to look at dogs and cats and foxes, and then turn the lens around and ask if we experienced similar changes in our evolution.


Wilkins AS, Wrangham, RW and Fitch WT (2014) The “Domestication Syndrome” in Mammals: A Unified Explanation Based on Neural Crest Cell Behavior and Genetics. Genetics 197(3):795-808.

Comments

  1. #1 Jim Thomerson
    July 25, 2014

    That sounds like a reasonable hypothesis. I think there is a large anecdotal literature on dogs and cats making humans more domesticated.

  2. #2 ian
    July 26, 2014

    Are Humans a domesticated animal? Seems pretty likely. I’ve seen this question addressed somewhere before. I’ll search the internets later.

  3. #3 eyesoars
    St Paul, MN
    July 26, 2014

    ISTR some research a few years ago that claimed that Neandertals never tamed dogs, while modern humans did, and hypothesized that this may have been one of the reasons modern humans won out over Neandertals in Europe ~25-30k years ago.

    If so, it seems that a comparison of the Neandertal vs. human genomes might provide evidence, if we can obtain suitable historic genomic information.

  4. #4 G
    July 27, 2014

    Re. Neanderthals: If I’m not mistaken, they also had larger brains, by approx. 100 – 200 cc.

    The causality might work in both directions: Some humans had reciprocal domestication with other animals, then the brain size of the humans decreased slightly, and the relationships with the other animals had selection benefits for the humans. For example domestic dogs who sound the alarm when hostile animals were near, might give their humans more time to prepare to fight off the hostile animals.

    It would be very interesting to find out what other changes in the brain occur when brain size shrinks slightly. I would guess that regions specializing in detecting and dealing with threats shrunk more than average, in relationship to the mutual defense benefits of humans and other animals living together. That would be one of a number of changes, and others were also favorable to mutual domestication.

  5. #5 Pete A
    July 27, 2014

    Anthropomorphism is a trait that varies widely between individual homo sapiens, but even on average, it is remarkably strong. The dedication of people working in animal welfare centres provides irrefutable evidence for the top-end strength and power of this human trait.

    Conversely, many animals are in welfare centres because their owners had little or no compassion and empathy for their pets — either due to the owner lacking anthropomorphism or having learnt to hate humans and animals that refuse to be subservient to them. Some humans and some domesticated animals develop behaviours that are extremely dangerous and beyond our current means to safely rehabilitate them i.e. they are unable to be domesticated.

    Let’s face it, do we own our pets or do they actually own us? Pets much more readily accept a new owner than humans will accept a new family or a new boss.

    While I like the hypothesis presented in this article, I can’t help feeling that there are much simpler explanations. E.g. the individual spices equivalent of anthropomorphism combined with the biological advantage of having this trait would better satisfy Occam’s razor.

  6. #6 Lambert Strether
    http://www.nakedcapitalism.com
    July 29, 2014

    I’m going to link to this for Naked Capitalism’s daily Links feature tomorrow at 7AM, so you might want to fix that bad table markup.

  7. […] I like this hypothesis Pharyngula. On tameness. […]

  8. #8 Larry Headlund
    July 29, 2014

    You have an error in your column header definition:
    <th valign="top" align="left" scope="col" rowspan="1" colspan="1" id="th-1"
    class=”table-left table-vtop”>Trait
    Should be
    Trait

    and similarly for the other table headers.

  9. #9 Noni Mausa
    July 30, 2014

    Another point, perhaps not related but possibly so, might be to ask why human beings, among all animals on the planet, have an obsessive and nearly universal interest in keeping other animals, from scorpions to dromedaries, even when those animals are so far from useful as to be better described as pernicious?

    Might this propensity be related to two other very useful human traits: the willingness to bond to and collaborate with strangers, and the strong urge to foster children, even if they are not of the same tribe or culture?

    This latter trait would be necessary in a species where growing to adulthood took nearly half of the likely lifetime of the child’s parents — orphans would be far from rare. And the former trait seems like it would be an extension of that.

    Once humans started fostering stray children as an imperative, not just a rarely indulged whim, could kittens, puppies, gibbons, colts, sloths and orphaned bear cubs be far behind?

    Noni

  10. #10 Craig Thomas
    August 4, 2014

    Before looking at Neanderthals v. other Homo species to look for signs of human domestication, wouldn’t it make sense to look at where anatomically modern humans originated (Africa) and how much domestication occurred there (not much).
    Virtually all animals were domesticated in the middle-east or nearby lands. Cattle, sheep, goats, dogs, cats, horses.
    No. To me it seems that domestication was a technology that succeeded (by a long way) modern human features.
    How do we know Neanderthals didn’t domesticate dogs, btw?

  11. […] when you tame a canine species (thus reducing the size of its adrenal gland), you automatically get floppy ears, spotted coats, and neoteny. PZ Myers says of genes, “everything is tangled together in interacting patterns of connectivity, […]

  12. […] when you tame a canine species (thus reducing the size of its adrenal gland), you automatically get floppy ears, spotted coats, and neoteny. PZ Myers says of genes, “everything is tangled together in interacting patterns of […]

  13. #13 Richard Dawkins
    August 8, 2014

    EVOLUTION MISTAKES IN WIKI OR PROOF THAT WIKI IS BIASED.
    1. Evolution is the change in the inherited traits of a population of organisms through successive generations. After a population splits into smaller groups, these groups evolve independently and may eventually diversify into new species.

    Key word “may.” There has never been a NEW species appear. EVER

    2. A nested hierarchy of anatomical and genetic similarities, geographical distribution of similar species and the fossil record indicate that all organisms are descended from a common ancestor through a long series of these divergent events, stretching back in a tree of life that has grown over the 3,500 million years of life on Earth.

    ANOTHER LIE ON WIKI no such thing as a common ancestor. Pure conjecture and assumption. PWNED

    3. Evolution is the product of two opposing forces: processes that constantly introduce variation in traits, and processes that make particular variants become more common or rare.

    They fail to mention that there is no proof of such a thing. And the neither of these forces have ever produced a new species or a new organ of any kind. Another mistake in Wiki.

    4. Genes vary within populations, so organisms show heritable differences (variation) in their traits. The main cause of variation is mutation, which changes the sequence of a gene. Altered genes, or alleles, are then inherited by offspring. There can sometimes also be transfer of genes between species.

    HAR HAR HAR HAR HAR check that out. “transfer of genes between the species” The Miracle of Evolution. HAR HAR HAR HAR

    PWNED

    5. Two main processes cause variants to become more common or rare in a population. One is natural selection, which causes traits that aid survival and reproduction to become more common, and traits that hinder survival and reproduction to become more rare.

    They called natural selection a process as if natural selection is looking around and making decisions. They fail to mention that natural selection is merely a theory and has not ever produced a new species. They don’t mention that natural selection is merely death by natural causes. HAR HAR HAR

    PWNED AGAIN

    6. Another cause of evolution is genetic drift, an independent process that produces entirely random changes in how common traits are in a population. Genetic drift comes from the role that chance plays in whether a trait will be passed on to the next generation.

    Wiki fails to say that this is purely made up conjecture and wishful thinking. Looks like the mistakes in Wiki just keep going on and on.

    PWNED ONE MORE TIME — I OWN WIKI

    THE BIG LIE
    7. Evolutionary biologists document the fact that evolution occurs, and also develop and test theories that explain its causes.

    Evolution is not a fact and has neither been observed nor documented. Wiki lied, not true, they don’t have a test to prove that man evolved from monkeys, apes or anything else. Wiki fails to mention that.

    WIKI MISTAKES ON EVOLUTION: MORE HORSESHIT TO FOLLOW

    8. This powerful explanatory and predictive theory has become the central organizing principle of modern biology, directing research and providing a unifying explanation for the history and diversity of life on Earth.

    They fail to mention that all they are using is the fact that there is variation in “kinds” and claiming it is evolution. Looks like when the creationists figure out that the evos are using creation science and calling it evolution they’ll get a real laugh. HAR HAR HAR

    9. Evolution is therefore applied and studied in fields as diverse as ecology, anthropology, conservation biology, paleontology, agriculture, medicine, psychology, philosophy and others.

    HAR HAR HAR HAR what a load of bullshit. Here they are using creation science and calling it evolution. EPIC FAIL

    SUPER BULLSHIT NEXT
    10. Genes vary within populations, so organisms show heritable differences (variation) in their traits. The main cause of variation is mutation, which changes the sequence of a gene. Altered genes or alleles are then inherited by offspring. There can sometimes also be transfer of genes between species.

    Next they fail to mention that mutations are 100% detrimental. No mutation nor group of mutations has ever produced a new species. Notice that last line? “transfer of genes between the species?”

    11. Extinction is the disappearance of an entire species. Extinction is not an unusual event, as species regularly appear through speciation, and disappear through extinction.

    Wiki lied again. There are NO new species “regularly” appearing to replace those that have gone extinct. As if species are coming and going. FAIL
    EPIC PWN

    12. Nearly all animal and plant species that have lived on earth are now extinct, and extinction appears to be the ultimate fate of all species.

    WOULD YOU LOOK AT THAT? What happened to the coming and going of species? And they are now calling extinction — evolution. HAR HAR HAR

    WIKI FAILS — PWNED

    13. In medicine, this contributes to the spread of antibiotic resistance, as when one bacteria acquires resistance genes it can rapidly transfer them to other species.

    MORE BULLSHIT FROM WIKI Grasse then points that bacteria are considered to have “stabilized a billion years ago!” Grasse regards the “unceasing mutations” to be “merely hereditary fluctuations around a median position; a swing to the right, a swing to the left, but no final evolutionary effect. PWNED

    14. Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the Cambrian explosion.

    MISTAKE the fossils of the Cambrian explosion happened in one instantaneous capture of all the animals in existence. The paleontologists say it shows that all these animals appeared SUDDENLY. The Cambrian explosion proves that there is no common ancestor.

    15. Wiki uses the Peppered Moth Hoax as an example of evolution.

    They fail to mention that it is a debunked hoax. “The peppered moth experiments beautifully demonstrate natural selection?or survival of the fittest?in action, but they do not show evolution in progress, for however the populations may alter in their content of light, intermediate, or dark forms, all the moths remain from beginning to end Biston betularia.”?Harrison Matthews

    University of Chicago evolutionary biologist Jerry Coyne agrees that the peppered moth story, which was ‘the prize horse in our stable’, has to be thrown out. He says the realization gave him the same feeling as when he found out that Santa Claus was not real. HAR HAR HAR HAR HAR HAR HAR HAR HAR POWNED!

    WIKI IS DESTROYED, OWNED POWNED AND BONED

    16. A key process in evolution is speciation, in which a single ancestral species splits and diversifies into multiple new species, and there are several modes through which this occurs.

    FAIL. There is no evidence that such has ever happened. There is no evidence in the fossil record. That mere fact disproves evolution altogether.

    17. The study of evolutionary biology began in the mid-nineteenth century, when research into the fossil record and the diversity of living organisms convinced most scientists that species changed over time.

    ANOTHER FAIL. The scientists say that diversity does not prove evolution. The fossil record shows there is no transitioning from a common ancestor into another species.

    18. During adaptation, some structures may lose their original function and become vestigial structures.

    PWNED There is no such thing as a vestigial structure. It is a logical fallacy to call anything a vestigial structure. Every year they find what one “vestigial” structure after another is actually used for. To claim it is vestigial is argument from ignorance.

    19. Natural selection is the process by which genetic mutations that enhance reproduction become, and remain, more common in successive generations of a population. It has often been called a “self-evident” mechanism

    This is bullshit. WIKI FAILS AGAIN. There are no genetic mutations enhancing reproduction. Another Wiki mistake. har har har har Natural Selection isn’t a process. It’s merely the natural death or murder of weak individuals.

    20. A special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness of an organism to potential mates.

    What nonsense. Did Fido ever consider whether Lady was good looking? No, Fido screws anything he can find, even RinTinTin. Do you think one spider is more attractive than another spider? Sheer idiocy. Proof that scientists are idiots. And Fido will even hump your leg. HAR HAR HAR HAR

    Alleged? HAR HAR HAR HAR HAR HAR HAR HAR HAR HAR

The site is currently under maintenance and will be back shortly. New comments have been disabled during this time, please check back soon.