[giardini]

Humans and gorillas? What about our ancestors and those of pigs? I found this hypothesis fascinating:

"A chimp-pig hybrid origin for humans?"

http://phys.org/news/2013-07-chimp-pig-hybrid-humans.html

from the article:

" I asked McCarthy if he could give a date estimate for the hybridization event, he said that there are a couple broad possibilities: (1) It might be that hybridization between pigs and apes produced the earliest hominids millions of years ago and that subsequent mating within this hybrid swarm eventually led to the various hominid types and to modern humans; (2) separate crosses between pigs and apes could have produced separate hominids (and there's even a creepy possibility that hybridization might even still be occurring in regions where Sus and Pan still seem to come into contact, like Southern Sudan)."

[Steuard]

That's one of the most ridiculous things I've ever read. We've done genetic studies on humans and apes, and we share some huge fraction of our genome with them (99%+, right?).

It's quite telling that in this era of abundant genetic data, this guy bases all of his arguments on anatomical similarities and says nothing about genetics. It's all but impossible that a link like what he's suggesting would have been overlooked if pigs had made any significant contribution to the human genome. (Just for example, we've got all sorts of estimates of species divergence dates among primates based on genetic data that should have given nonsensical results if there had been massive influxes of pig DNA in the middle of that history.)

[grey413]

The champion of the theory attributes the lack of genomic similarity to repeated back-crossing to one of the parent populations. To put it in other words, the droplet of initial hybridization got diluted in the larger gene pool, but the novel genes (and thus traits) remained and underwent selection.

That being said, diluted gene contribution is not the same thing as no gene contribution. To even begin accepting such a theory I would require direct genetic evidence showing considerable horizontal gene transfer between porcine and hominid lines.

[Steuard]

I saw that claim in the article, yes. I'll admit that I don't know enough about genetics to make strong claims here, but it doesn't sound remotely plausible to me. He wants to simultaneously claim that 1) hominids got so much porcine DNA that they have lots of substantial, recognizable anatomical features as a result, and 2) hominids have so little porcine DNA that it's all but invisible in our genetic code.

I'm not going to say that's impossible, but it sounds like one heck of a stretch. Even just sitting here thinking about it, most genetic inheritance happens one full chromosome at a time. We clearly don't have any full pig chromosomes, so to make this theory work you'd have to have a whole lot of lucky recombination events (chromosomal crossover, etc.) that preserved only the precise genes involved in all these "distinctive pig traits" and got rid of the rest. So what's the selective effect that selects extraordinarily strongly for this random selection of pig-like anatomical traits but against all of the other pig genes that would have usually been linked to them?

In short, this is a very extraordinary claim, and it requires equally extraordinary evidence, especially given how remarkably consistent the known genetic evidence has proven to be.

[ghkbrew]

In general I agree with you, it's an extraordinary claim with less than extraordinary proof to put it mildy. However I thought I'd make one small correction on your genetics.

> most genetic inheritance happens one full chromosome at a time.

is actually not true. Sperm and eggs have only one copy of each chromosome instead of two like most cells in the body. However, that one chromosome is a pretty good mixture of the versions received from each parent due to recombination events that occur randomly during maturation of those cell types. Linkage between nearby genes does exist, but it's not nearly so strong as you seem to imply. Even in a single generation inheritance of two genes on either end of the same chromosome is nearly uncorrelated.

[Steuard]

Thanks! I didn't realize that recombination was so frequent. I'm glad to have a better sense of that now.

[Houshalter]

I don't see why it's that implausible. You are descended from almost every single human being alive 1,000 years ago. Can you find any significant amount of DNA contributed from only a single one of your ancestors?

What he is suggesting is that a single hybrid made it's way back into the hominid population. It had children with other hominids, and those children would have had half as much pig DNA. They had children with even less, and so on. After awhile there would be almost nothing left of the pig ancestor. The only genes that would survive such dilution would be ones that were significantly selected for.

[Steuard]

Right, so he's claiming that the genes responsible for this really pretty random list of traits ("protruding, cartilaginous mucous noses"? "Prostate encircles urethra"? "Alcoholism"?) were so strongly selected for that they became 100% ubiquitous in the human genome. Meanwhile, every other trace of pig ancestry got diluted away to nothing.

I understand the idea of dilution, I just don't understand why he thinks that so many random things would survive it to become defining features of the human species. Remember, he's not just saying that 2% of living humans carry Genghis Khan's Y-chromosome or have red hair or something, he's saying that all of these traits became completely universal.

[spullara]

"You are descended from almost every single human being alive 1,000 years ago." Um. No.

[Houshalter]

Yes actually. Here is an explanation why. http://dgmweb.net/Ancillary/OnE/NumberAncestors.html

[maxerickson]

I submitted the article I link in a subthread here:

https://news.ycombinator.com/item?id=7539143

The author posted some replies, one of which points to this article:

http://phenomena.nationalgeographic.com/2013/05/07/charlemag...

That puts it at about 3400 years (the article gets the number from some paper on the topic).

[pcrh]

"You are descended from almost every single human being alive 1,000 years ago."

Not exactly 1,000 years, but more like 120-135,000 years for the male line [1], and 200,000 years for the female line [2].

[1] "Sequencing Y Chromosomes Resolves Discrepancy in Time to Common Ancestor of Males Versus Females " dx.doi.org/10.1126/science.1237619

[2] https://www.princeton.edu/~achaney/tmve/wiki100k/docs/Mitoch...

[Houshalter]

No it's much closer to 1,000 years. Every generation the number of possible ancestors you have doubles. 2 parents, 4 grandparents, 8 great grandparents. Within a few generations back, you have literally millions of ancestors.

[grey413]

Evolutionary geneticists don't trace individual lineages that far out, nor would they need to. The hypothesis under question is that the porcine/proto-hominind cross provided a variety of important, wide-acting traits that all humans share. That means that every human has them, and that every chimpanzee doesn't have them. That would show up in a comparative genomic assay.

[tlg]

"You are descended from almost every single human being alive 1,000 years ago."

If a generation is about 30 years then you will have close to 33 ancestors lines in your family tree over those 1000 years. That makes 2^33 ancestors or 10 billion, that's impressive (well there is probably a large overlap)

[lutusp]

Yes, but remember the "bottleneck" from 70,000 years ago, when the human race was nearly wiped out. At that time, because of a nuclear winter brought on by a volcanic supereruption, there may have been fewer than 10,000 humans in the entire world.

http://en.wikipedia.org/wiki/Toba_catastrophe_theory

So, based on that, we're descended from fewer people in the past than most people realize. Another way to say it is that most of our ancestors were cousins.

[demallien]

That overlap is more than large, it's actually the most important factor in the calculation. You only need to go back about 6 generations (200 years), and there is practically no interchange between geographically separated groups, which means that I probably don't have any genes from some random asian living 1000 years ago (although we both almost certainly share genes that we inherited from some far more ancient common ancestor).

[marshray]

Eventually though, sufficient data will fall from the collisions between example-fed discussion and informed search to deliver an elevated consensus. One particular approach recommended McCarthy is in silico chromosome painting of the human genome with random pig and chimp sequences in an effort to find hotspots of similarity to pig.

http://phys.org/news/2013-07-human-hybrids-closer-theory-evi...

[saalweachter]

Basically, no.

While goats and sheep diverged 8 million years ago, humans/chimps and pigs diverged 80 million years ago. The last time a proto-chimp got with a proto-pig, the T-Rex hadn't evolved yet. While goats and sheep have a difference of 6 chromosomes, chimps and pigs have a difference of 10. McCarthy, despite being a geneticist, makes his absurd claim without even discussing the genetic difficulties.

What little similarity there is between humans and pigs is simply a case of convergent evolution.

[marshray]

I think he's saying relatively modern chimps and pigs may have had a hybridization event.

Another argument was that the morphological distance, or genetic differences besides chromosome number, are just too great. Most of us are familiar with the platypus. A paper published in Nature a few years ago demonstrated that the platypus genome contains both bird and mammal chromosomes, and therefore that the vastly different bird and mammal sex chromosome systems have been successfully bridged by this creature. This example is not offered as any kind of proof. But it does suggest that sometime, long ago, a cross occurred that would have been even more distant than that between a chimpanzee and a pig – one between a otter-like mammal and a duck-like bird. And if such was the case, the hybrids from the cross must have been able to produce offspring (otherwise they would have died out, and the platypus would not exist today).

http://phys.org/news/2013-07-human-hybrids-closer-theory-evi...

[saalweachter]

No.

The platypus is not a bird/reptile hybrid. That is a wild misinterpretation of a genome study which happened a few years back.

http://scienceblogs.com/pharyngula/2008/05/10/the-platypus-g...

[marshray]

<not sarcasm>Should I take from this that phys.org is not a particularly reliable source?</not sarcasm>

[saalweachter]

I am not particularly familiar with phys.org. Briefly scanning it, it looks pretty normal.

Most likely it's just someone stepping outside their wheelhouse. I mean, I trust PZ on biology, but I wouldn't incredulously accept legal advice from him. So, maybe reliable for physics/space, but unreliable for biology is my guess?

[tokenadult]

That hypothesis is completely wrong:

http://scienceblogs.com/pharyngula/2013/07/02/the-mfap-hypot...

[Noishe]

There isn't enough attention to this in my opinion. When you research it, everything lines up so nice.