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-center table-vtop">Animal species
|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
|Cranial and trunk
|Rabbit, dog, fox, pig, sheep, goat, cattle, and donkey
|Rat, dog, cat, ferret, camel, alpaca, and guanaco
|Mouse, dog, cat, fox, pig, sheep, goat, and cattle
|Mouse, dog, and pig
|All domesticated species
|Smaller brain or cranial capacity
|Rat, guinea pig, gerbil, rabbit, pig, sheep, goat, cattle, yak, llama, camel, horse, donkey, ferret, cat, dog, and mink
|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
|Dog, fox, and pig
(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.
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.
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.
That sounds like a reasonable hypothesis. I think there is a large anecdotal literature on dogs and cats making humans more domesticated.
Are Humans a domesticated animal? Seems pretty likely. I've seen this question addressed somewhere before. I'll search the internets later.
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.
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.
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.
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.
You have an error in your column header definition:
<th valign="top" align="left" scope="col" rowspan="1" colspan="1" id="th-1"
and similarly for the other table headers.
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?
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?