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the origins of humans lie in a - ahem! - far-fetched hybridisation event?

Or maybe not.

The internet is a wondrous place: a source of information, of amusement, and - alarmingly often - of material that elicits a combination of 'say what?' and <head-desk>. And a hat-tip to PZ Myers for this particular example...

For it has been proposed (by the originator of this particular hypothesis, & further discussed on phys.org) that humans arose as the result of an interspecies hybridisation event, rather than the current model for human evolution that sees chimps and humans sharing a Last Common Ancestor 5-6 million years ago and following their own trajectories since that time.

The event? Hybridisation between great ape (specifically, chimpanzee) & wild boar.

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Now, we know that hybridisation happens; that the 'biological species concept' is not an absolute. But these events are generally between reasonably-closely related species: hybridisation is quite common in the waterfowl, for example. But even in birds, where hybridisation is well-documented, it doesn't appear to cross the lines between one taxonomic order and another. Yet the 'pig+ape' suggestion requires hybridisation between different taxonomic orders - orders that (so fossil & genetic evidence tell us) have been separate for between 79 & 87 million years.

Surprisingly, Dr McCarthy, a geneticist and the proponent of this novel hypothesis, doesn't present much in the way of genetic data to support it. He does agree that, genetically, we are closer to chimpanzees than to any other mammal, but suggests that this is due to back-crossing (with the chimp parent) after the initial hybridisation event:

And why might one suppose that humans are backcross hybrids of the sort just described? Well, the most obvious reason is that humans are highly similar to chimpanzees at the genetic level, closer than they are to any other animal. If we were descended from F1 hybrids without any backcrossing we would be about halfway, genetically speaking, between chimpanzees and whatever organism was the other parent. But we're not. Genetically, we're close to chimpanzees, and yet we have many physical traits that distinguish us from chimpanzees.

Surely the simpler explanation - that we are genetically similar to chimps because we are sister species - is more likely. Especially since at least some of the differences between the two species can be explained by differences in timing of developmental stages (the relative proportions of face & skull, for example), which may be sheeted home to mutations in regulatory portions of the genome. 

In addition, there are major differences both in chromosome number (38 in pigs, 46 in humans) and in the position of various genes on those chromosomes that would make successful gamete production in any hybrid unlikely in the extreme (always supposing the hybrid was actually viable) - PZ discusses this in more length.

But anyway, what about those physical traits that "distinguish us from chimpanzees" & supposedly reflect our shared heritage with pigs? There's a long list here. Many of them relate to bipedalism; to me, it's special pleading to suggest that (for example) the presence of large gluteal muscles in bipedal humans and in domestic pigs is evidence of a close evolutionary relationship (Animal Farm aside, there is a distinct lack of evidence for bipedal locomotion in suids - and strong evidence of selective breeding for large backsides in pigs destined to become bacon & pork). Hairlessness? Only in domestic pigs; anyone who's watched a huntin'&fishin' show on TV will have seen how hairy a wild boar is. Pigs & humans both have longer hind-limbs than forelimbs, & shorter digits (compared to chimps)? Well, ye-es, I guess so, but that's hardly evidence for a close relationship; one could say the same of mice... Similarly, while humans & pigs may (usually) be particular about where they defecate, well, so are other animals; rabbits, anyone? As for "snuggling [snuggling???], tears, alcoholism "being shared features in humans & pigs but not chimps... chimps & humans both have an enzyme (alcohol dehydrogenase 4) that allows us to break down ethanol, and it seems that chimps can become addicted to alcohol if given the opportunity.

Also offered in evidence are those diseases which are rare in non-human primates: "heart attack, atherosclerosis, and cancer (melanoma)". Since these are to some degree, age-related, their relative rarity (for they are not absent in apes) may be ascribed to differences in lifespan: chimps in the wild may not live long enough to develop them.

Annoyingly, I see that over at uncommondescent, this proposal has been presented as even more evidence that evolutionary biologists are Getting It Wrong!

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Perusing more of the macroevolution website, I found the suggestion that armadillos and pangolins evolved from ankylosaurs and stegosaurs (page 244 at that link). It would be interesting to hear a palaeontologist's taken on that one, but the fact that dinosaurs had diapsid skulls while mammals are synapsid doesn't help. 

 

 

 

 

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