Human evolution, radically reappraised

http://www.world-science.net/exclusives/070326_evolution.htm

Hu­man ev­o­lu­tion has been speed­ing up tre­mend­ous­ly, a new study con­tends—so much, that the lat­est ev­o­lu­tion­ary changes seem to large­ly ec­lipse ear­l­ier ones that ac­com­pa­nied mod­ern man’s “ori­gin.”

The stu­dy, along­side oth­er recent re­search on which it builds, amounts to a sweep­ing re­ap­prais­al of tra­di­tion­al views, which tended to as­sume that hu­mans have reached an end­point of ev­o­lu­tion.

The find­ings sug­gest that not on­ly is our ev­o­lu­tion con­tin­u­ing: in a sense our very “orig­in” can be seen as on­go­ing, a ge­net­i­cist not in­volved in the study said.

Greg­o­ry Coch­ran of the Uni­ver­si­ty of Utah in Salt Lake City, Utah, a co-author of the stu­dy, said the re­search may force a rad­i­cal re­think­ing of the sto­ry of mod­ern hu­man ev­o­lu­tion. “It turns it up­side-down, pret­ty much,” he said. But skep­tics ques­tion some as­pects of the work.

The tra­di­tion­al pic­ture of hu­mans as a fi­n­ished prod­uct be­gan to erode in re­cent years, sci­en­t­ists said, with a crop of stud­ies sug­gesting our ev­o­lu­tion in­deed goes on. But the new­est in­vest­i­ga­tion goes fur­ther. It claims the pro­cess has ac­tu­al­ly ac­cel­er­at­ed.

It al­so down­plays the im­por­tance of a much-scru­ti­nized era around 200,000 years ago, when hu­mans con­sid­ered “ana­tom­i­cally mod­ern” first ap­pear in the fos­sil rec­ord. In the stu­dy, this ep­och e­merges as just part of a vast arc of ac­cel­e­rat­ing change.

“The or­i­gin of mod­ern hu­mans was a mi­nor event com­pared to more re­cent ev­o­lu­tion­ary chang­es,” wrote the au­thors of the re­search, in a pre­sent­a­tion slated for Fri­day in Phi­l­a­del­phia at the an­nu­al meet­ing of the Amer­i­can As­so­ci­a­tion of Phys­i­cal An­th­ro­po­l­o­g­ists.

The au­thors are Coch­ran and an­thro­po­l­o­gist John Hawks of the Uni­ver­si­ty of Wis­con­sin-Ma­d­i­son. The find­ings will also be sub­mitted to one or more sci­en­t­i­f­ic jour­nals, Coch­ran said.

The pro­pos­al is “truly fas­ci­nat­ing,” wrote Uni­ver­si­ty of Chica­go ge­net­i­cist Bruce Lahn in an e­mail. He was­n’t in­volved in the work, though he did con­duct ear­li­er re­search find­ing that ev­o­lu­tion may still be on­go­ing in the brain.

Even be­fore the Hawks-Cochran study and its im­me­di­ate fore­run­ners, Lahn wrote, sci­en­tists had al­ready not­ed a trend of ac­ce­le­rating change in the ev­o­lu­tion­ary line­age lead­ing to mod­ern hu­mans from ape-like an­ces­tors. But that phe­nom­e­non seemed to have oc­curred over time spans meas­ured in mil­lions of years; it was far from clear that it has con­tin­ued in the re­cent past or to­day, he added.

Hawks and Cochran, by con­t­rast, ar­gue that the trend “is vis­i­ble even in the last tens of thou­sands of years,” Lahn wrote. It “runs count­er to the feel­ing in some quar­ters that the ev­o­lu­tion of the hu­man phe­no­type [form] has slowed down or even stopped in our re­cent past.”

Defining an origin

If the study is cor­rect, it raises new ques­tions about how to de­fine the “orig­in” of mod­ern hu­mans—a rath­er ar­bi­trary de­ci­sion in any case, Lahn re­marked.

The or­i­gin is “de­fined prob­a­bly more as a mat­ter of con­ven­ience rath­er than re­flect­ing any ac­tu­al wa­ter­shed ev­o­lu­tion­ary event,” he wrote. That is, it’s “use­ful to say that any past crea­tures that are with­in cer­tain lev­els of sim­i­lar­i­ties to us to­day should be con­sid­ered as ‘the same’ as us.”

But if the changes that ac­com­pa­nied this event are on­ly a tri­fling part of a wid­er trend, he added, it be­comes rea­son­a­ble to ask wheth­er that fur­ther de­flates the ra­tion­ale for call­ing it an or­i­gin.

“In a sense,” he wrote, one could say “the or­i­gin is still on­go­ing.”

Ev­o­lu­tion oc­curs when an in­di­vid­ual ac­quires a ben­e­fi­cial ge­net­ic mu­ta­tion, and it spreads through­out the pop­u­la­tion be­cause those with it thrive and re­pro­duce more. Cease­less repe­ti­tions of this can change spe­cies, or pro­duce new ones. As ben­e­fi­cial genes spread, harm­ful ones are weeded out; the whole pro­cess, called nat­u­ral se­lec­tion, pro­pels ev­o­lu­tion.

Hawks and Cochran an­a­lyzed mea­sure­ments of skulls from Eu­rope, Jor­dan, Nu­bia, South Af­ri­ca, and Chi­na in the past 10,000 years, a pe­ri­od known as the Hol­o­cene era. They al­so stud­ied Eu­ro­pean and West Asian skulls from the end of the Pleis­to­cene era, which lasted from two mil­lion years ago un­til the Hol­o­cene.

“A con­stel­la­tion of fea­tures” changed across the board, Hawks and Cochran wrote in their pres­en­ta­tion. “Hol­o­cene changes were si­m­i­lar in pat­tern and… faster than those at the archaic-mod­ern tran­si­tion,” the time when so-called mod­ern hu­mans ap­peared. But these changes “them­selves were rap­id com­pared to ear­li­er hom­i­nid ev­o­lu­tion.” Ho­mi­n­ids are a fam­i­ly of pri­mates that in­cludes hu­mans and their ex­tinct, more ape-like though up­right-walk­ing an­ces­tors and rel­a­tives.

Hawks and Cochran al­so ana­lyzed past ge­net­ic stud­ies to es­ti­mate the rate of prod­uction of genes that un­der­go pos­i­tive se­lec­tion—that is, genes that spread be­cause they are ben­e­fi­cial. “The rate of gene­ration of pos­i­tively se­lected genes has in­creased as much as a hun­dred­fold dur­ing the past 40,000 years,” they wrote.

There are ways to de­tect po­si­tive se­lec­tion in ge­nome data. But Mark Thom­as, a ge­net­ic an­thro­pol­o­gist at Uni­ver­si­ty Col­lege Lon­don, was skep­ti­cal that these would be enough to make Hawks’ and Cochran’s case. “The is­sue is that the most pow­er­ful meth­ods for de­tect­ing se­lec­tion are ones that lose their sen­si­tiv­i­ty go­ing more than 30,000 years back,” he said. Oth­er tech­niques can’t “dis­tin­guish be­tween se­lec­tion and pop­u­la­tion growth.”

Thom­as added that he under­stands the ske­le­tal data to show some­thing dif­ferent from what Hawks and Coch­ran say, but that he would need a ful­ler ac­count of their find­ings to make a judg­ment.

Worrisome findings?

Hawks and Coch­ran said some of the most no­ta­ble phys­i­cal changes in hu­mans have been ones af­fect­ing the size of the brain case.

A “thing that should prob­a­bly wor­ry peo­ple is that brains have been get­ting smaller for 20,000 to 30,000 years,” said Coch­ran. But brain size and in­tel­li­gence aren’t tightly linked, he added. Also, growth in more ad­vanced brain ar­eas might have made up for the shrinkage, Coch­ran said; he spec­u­lated that an al­most break­neck ev­o­lu­tion of high­er fore­heads in some peo­ples may re­flect this. A study in the Jan. 14 Brit­ish Den­tal Jour­nal found such a trend vis­i­ble in Eng­land in just the past mil­len­ni­um, he noted, a mere eye­blink in ev­o­lu­tionary time.

Research pub­lished in the Sept. 9, 2005 is­sue of the re­search jour­nal Sci­ence by Lahn and col­leagues found that two genes linked to brain size are rap­idly evolv­ing in hu­mans.

An­thro­po­l­o­gist Jef­frey Mc­Kee of Ohio State Uni­ver­si­ty said the new find­ings of ac­ce­l­er­ated evolution bear out pre­dic­tions he made in a 2000 book The Rid­dled Chain. Based on com­put­er mod­els, he ar­gued that ev­o­lu­tion should speed up as a pop­u­la­tion grows. This is be­cause pop­u­la­tion growth cre­ates more op­por­tu­ni­ties for new mu­ta­tions; al­so, the ex­pand­ed pop­u­la­tion oc­cu­pies new en­vi­ron­men­tal niches, which would drive ev­o­lu­tion in new di­rec­tions.

Lahn said he’s not con­vinced that the ac­cel­er­at­ed phys­i­cal ev­o­lu­tion is tied to pop­u­la­tion growth. “It may be a long way be­fore” an­yone can test the truth of this, he wrote.

But oth­er fac­tors could al­so ex­plain an accele­ration, ac­cord­ing to an­thro­po­l­o­gist John Kings­ton of Em­o­ry Uni­ver­si­ty in At­lan­ta, Ga. Ev­o­lu­tion might speed up be­cause we have changed our own en­vi­ronment, which in turn changes the ev­o­lu­tion­ary pres­sures. “We now con­trol our own en­vi­ronment and ecol­o­gy to some ex­tent,” he said.

For instance, if you in­vent spears, you per­haps can af­ford to be slight­er-framed be­cause you can stand fur­ther away from wild an­i­mals, Coch­ran said. He ar­gued that a pow­er­ful syn­er­gy be­tween these sorts of changes and expand­ing pop­u­la­tion ex­plains the “fant­as­tic­ally ra­pid” re­cent evo­lu­tion.

“A very big change”

Ove­rall, the find­ings could amount to “a very big change” in tra­di­tion­al think­ing for two rea­sons, ac­cord­ing to Mc­Kee. First, he said, many re­search­ers had mis­tak­en­ly as­sumed pop­u­la­tion growth would slow down ev­o­lu­tion, be­cause new mu­ta­tions would take too long to spread through a large pop­u­la­tion.

Sec­ond, the find­ings deal a fi­nal blow to a lin­ger­ing view among an­thro­po­l­o­gists of ev­o­lu­tion as a lad­der “with us as the be-all-end-all,” he said. That idea went out of fash­ion in the 1950s but still per­sists “in the backs of our minds,” he added.

Many of the changes found in the ge­nome or fos­sil rec­ord re­flect me­tab­o­lic alt­er­a­tions to ad­just to ag­ri­cul­tur­al life, Cochran said. Oth­er changes simp­ly make us weaker.

In the June 2003 is­sue of the re­search jour­nal Cur­rent An­thro­po­l­ogy, Hel­en Leach of the Uni­ver­si­ty of Ota­go, New Zea­land wrote that skele­tons from some pop­u­la­tions in the hu­man line­age have un­der­gone a pro­gres­sive shrink­age and weak­en­ing, and re­duc­tion in tooth size, si­m­i­lar to changes seen in do­mes­ti­cat­ed an­i­mals. Hu­mans seem to have do­mes­ti­cat­ed them­selves, she ar­gued, caus­ing phys­i­cal as well as men­tal changes.

De­spite all the alte­rations, Mc­Kee said he be­lieves the no­tion of an “o­rig­in” of mod­ern hu­mans around 200,000 years ago re­mains use­ful. “It’s just a thresh­old point” at which hu­mans take on most of the phys­i­cal fea­tures we rec­og­nize, he re­marked, and as such, need­n’t be dis­carded. Coch­ran said it can still be ar­gued that the key change was lang­uage; but when this ori­gi­nated re­mains far from clear.

What­ever the imp­li­ca­tions of the recent findings, McKee added, they high­light a ubiq­ui­tous point about ev­o­lu­tion: “every spe­cies is a tran­si­tion­al spe­cies.”

Posted by Svyatoslav Igorevich on Wednesday, March 28, 2007 at 01:35 PM in
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