John M. Lynch posted Entry 24 on March 24, 2004 08:32 PM.
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A paper just published in the March 25th issue of Nature (1) suggests that a mutation in a single gene (MYH16) may have been responsible for many of the cranial difference between Homo and other primates. "A gene responsible for a majority of jaw musculature was lost from human ancestors, presumably 2.4 million years ago, according to the study. Drastic reductions in these muscles may have lifted significant physical constraints on braincase volume, allowing primates with weak jaws and big brains to eventually think about their origins." (link) While support from genetics researchers seems forthcoming (link), the anthropological community seems less convinced by the results. Owen Lovejoy states "[s]uch a claim is counter to the fundamentals of evolution, [t]hese kinds of mutations probably are of little consequence." The mutation would have reduced the Darwinian fitness of those individuals," said Bernard Wood. "It only would've become fixed if it coincided with mutations that reduced tooth size, jaw size and increased brain size. What are the chances of that?" (link).

Whether or not these results pan out in the long run, they are a powerful indication of the possible effects of single mutations and should spur further comparisons of human & ape genomes.

(1) H.H. Stedman et al., “Myosin gene mutation correlates with anatomical changes in the human lineage,” Nature, 428:415-418

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Comment #82

Posted by PZ Myers on March 24, 2004 8:43 PM (e)

Errm, what? It would have only been fixed if there were other mutations that gave it a selective advantage? And that a mutation that confers a disadvantage would become fixed is “counter to the fundamentals of evolution”?

Looks like a certain anthropologist needs a refresher course in elementary population genetics.

While I’m no fan of the sweeping hypothesis in that Nature article, it certainly isn’t against evolutionary theory for drift to fix a deleterious or neutral mutation in a population.

Comment #83

Posted by RBH on March 24, 2004 8:58 PM (e)

PZ’s comments identify a common misconception of evolution as a monotonic hill-climbing process. It isn’t. And that misconception is wedded to the notion of biological evolution as search. It isn’t. It’s kind of Zen: Evolution finds ‘solutions’ without actually looking for them.


Comment #96

Posted by Reed A. Cartwright on March 25, 2004 12:16 AM (e)

*cough* pleiotropy *cough*

Comment #97

Posted by Ville on March 25, 2004 12:36 AM (e)

It’s very possible for what seems to be a mistake to pave way for further adaptation. It’s like losing rigid strength but later improving dexterity - evolution allows for some leeway - there isn’t a constant pressure which would demand immediate stop of harmful gene propagation. Too strong and straightforward adaptation to a single environment would be counterproductive for the species. Overspecialization kills. It’s good to have some slack.

Comment #112

Posted by Abiola Lapite on March 25, 2004 6:20 AM (e)

Couldn’t it be the case that the loss of function of MYH16 is simply a case of a loosening of selective restraint? More intelligent hominids were better at getting higher quality food, so they required less jaw strength to process their food. That seems more plausible to me than the idea that jaw musculature was in itself some sort of obstacle to an increase in brain size.

Comment #485

Posted by Taz on March 30, 2004 3:21 PM (e)

I think evolution cannot exist period… Ever try reading the Word of GOD??
Try it sometime

Comment #486

Posted by Andy Groves on March 30, 2004 3:40 PM (e)

Is Taz our first troll in the new-look Thumb?

Should we have him/her/it stuffed as a memento?


Comment #487

Posted by Wesley R. Elsberry on March 30, 2004 3:56 PM (e)

No, 2nd or 3rd troll, depending on sock-puppetage. But the troll density is at least much reduced from

Comment #488

Posted by Jeebus on March 30, 2004 4:00 PM (e)

Taz is a good example of a deleterious adaptation, i.e., one expressed by a deficiency in mental reasoning, which appears to be on the increase. It’s ok to have faith, Taz, but as an element to this ongoing scientific dialogue it is a non sequitur. Note that is not the same thing as saying “God is a non sequitur” (a misinterpretation I will anticipate Taz having) but that as a counter argument it is meaningless. Might as well say “Blue hyenas eat tennis balls.”

Comment #8245

Posted by Tim Duncan on October 2, 2004 4:26 PM (e)

Eleanor Rigby, picks up the rice
in the church where a wedding has been
Lives in a dream

Comment #8261

Posted by Bob Maurus on October 2, 2004 9:36 PM (e)


Only if we can get together once a year, say on Darwin’s birthday, raise a few tankards of good ale, and throw at least several rounds of darts at him.