"Long before it's in the papers"
June 04, 2013


Human evolution, radically reappraised

March 26, 2007
Special to World Science  
Updated March 27 

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.”

Hom­i­nid skulls. Top: Ho­mo erec­tus dat­ed to 1.75 mil­lion years ago; Mid­dle: an ear­ly "modern" Ho­mo sapi­ens dat­ed to 160,000 years ago; Bot­tom: a con­tem­po­rary hu­man. (Credits: top, Science magazine; middle, Tim White; bottom, NIH).

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 ev­o­lu­tion­ary end­point.

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 re­search­ers of ev­o­lu­tion as a pro­cess “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.”

* * *

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Human evolution has been speeding up, a new study contends—so much so, that the latest evolution ary changes largely eclipse those that accompanied modern man’s “origin.” The study, alongside a few others on which it builds, amounts to a ground-up reappraisal of traditional accounts of human evolution, which widely assumed that humans had reached a pinnacle of evolution and stopped there. The findings suggest that not only is our evolution continuing: in a sense our “origin” itself can be seen as continuing, according to a geneticist not involved in the work. Gregory Cochran of the University of Utah in Salt Lake City, Utah, a co-author of the latest 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. The traditional view of humans as a finished evolution ary product started to erode in recent years, scientists said, with a crop of studies suggesting our evolution indeed goes on. But the newest study goes further. It claims the process has actually accelerated. It also downplays the importance of a much-scrutinized era when humans considered “ana tomically modern” emerged in the fossil record, around 200,000 years ago. 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 evolution ary changes,” wrote the authors of the research, to be presented March 30 in Philadelphia at the annual meeting of the American Association of Physical Anthro pologists. The authors are anthro pologists John Hawks of the University of Wisconsin-Madison, and Gregory Cochran at the University of Utah, in Salt Lake City. 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 this study and its immediate forerunners, Lahn wrote, scientists had already noted a trend of accelerating change in the evolution ary 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.” If the study is correct, it raises new questions about how to define the “origin” of modern humans, a rather arbitrary decision anyway, Lahn remarked. The origin is “defined probably more as a matter of convenience rather than reflecting any actual watershed evolution ary 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 indi vidual 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 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 for human ancestors, Hawks and Cochran wrote in their conference presentation. “Holocene changes were similar in pattern and chronologically faster than those at the archaic-modern transition,” the time when so-called modern humans appeared, they wrote. But these changes “themselves were rapid compared to earlier hominid evolution.” Hominids are a family of primates that includes humans and their upright-walking, more ape-like, ancestors and relatives, all extinct. Hawks and Cochran also examined population growth estimates and past genetic studies to obtain an estimate the rate of production of genes that undergo positive selection—that is, genes that spread because they are beneficial. “Our estimates are consistent with genomic evidence in suggesting that the rate of generation of positively selected genes has increased as much as a hundredfold during the past 40,000 years,” they wrote. Among the most notable physical changes have been ones affecting the size of the brain case, according to Hawks and Cochran. That conclusion would fit with those of two papers published in the Sept. 9, 2005 issue of the research journal Science by Lahn and colleagues. They reported that two genes linked to brain size are rapidly evolving in humans. A “thing that should probably worry people is that brains have been getting smaller for 20,000 to 30,000 years,” said Cochran. But growth in more advanced brain areas might have compensated for this, he added, speculating 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, an eyeblink in evolutionary time. Anthropologist Jeffrey McKee of Ohio State University said the Hawks and Cochran study bears 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 environ mental niches, which would drive evolution in new directions. Lahn said he’s not convinced by the notion—also raised by Hawks and Cochran—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 evolution ary acceleration, according to anthro pologist John Kingston of Emory University in Atlanta, Ga. Evolution might speed up because we have changed our own environment, which in turn changes the evolution ary pressures. “We now control our own environment and ecology to some extent,” he said. Cochran said most likely, a powerful synergy between that and the expanded population explains the “fantastically rapid” recent evolution. Overall, the findings on accelerated evolution 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 anthro pologists of evolution as a ladder “with us as the be-all-end-all,” he said. That view went out of fashion in the 1950s but still persists “in the backs of our minds,” he added. Evidence for changing brain size in the human lineage has been widely presumed to reflect a growth in intelligence. Other changes documented in the fossil record aren’t necessarily so positive. In the June 2003 issue of the research journal Current Anthro pology, Helen Leach of the University of Otago, New Zealand wrote that skeletons from some populations in the human lineage point to 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. The new findings just emphasize a ubiquitous truth in evolution, he added: “every species is a transitional species.”