It’s hard to make but maybe we’ll do bet­ter on an­other planet

Popular Science - - CONTENTS -

HU­MAN BA­BIES ARE BORN HALFbaked and vul­ner­a­ble, and around 303,000 women died giv­ing birth in 2015. No other pri­mate has such a bru­tal la­bor or an in­fancy so long and help­less. Some sci­en­tists blame our pre­car­i­ous re­pro­duc­tive state on 6 mil­lion years of evo­lu­tion­ary pres­sure—and the re­sult­ing biome­chan­i­cal com­pro­mises.

It all started with arm­fuls of fruit. As hu­man­ity’s African home­land be­came a mix of for­est and sa­vanna, our pre­de­ces­sors had to go vast dis­tances to gather grub. Walk­ing in long strides—without chim­p­like swag­ger or knuckle drag­ging—made it eas­ier to carry food.

Over thou­sands of gen­er­a­tions, the pelvis shifted to make our new method of lo­co­mo­tion more ef­fi­cient. The tops of the hip bones flared, and the pu­bic bone moved to­ward the bot­tom of the spine, shift­ing the shape of the birth canal. Its in­let—a baby’s first hur­dle in the ad­ven­ture of birth—widened side to side. Off­spring now needed to twist to face mom’s rear end to get their shoul­ders through.

Bipedal­ism wasn’t our only anatom­i­cal com­pro­mise. A warm­ing cli­mate made it worth­while to be long and nar­row. Lithe limbs and slim hips cre­ate a sur­face-area-to-body­mass ra­tio that makes sweat­ing more ef­fec­tive. This pres­sure to have svelte haunches af­fected the birthing out­let. In mod­ern hu­mans, the canal’s exit is wider front to back (yes, the op­po­site of its en­trance), which means ba­bies ro­tate a sec­ond time to free their shoul­ders.

In fact, keep­ing cool was prob­a­bly the big­gest driver of our baby-mak­ing woes. “Ther­moreg­u­la­tion is hard to com­pre­hend— be­cause you’re think­ing, Oh it would be hot, but we could man­age—but re­ally, that is what pushes Homo sapi­ens into truly dif­fi­cult birthing ter­ri­tory,” ex­plains May­owa Adeg­boyega, who stud­ies pelvises of pre­his­toric ho­minids at UC Davis Depart­ment of An­thro­pol­ogy.

Our ex­pand­ing brains fur­ther com­pli­cated mat­ters. Nog­gins dou­bled in size over a (rel­a­tively) speedy 2 mil­lion years, pro­vid­ing the neu­rons needed to solve com­plex prob­lems.

Bod­ies al­ready tweaked to sup­port long strides and heat ap­par­ently couldn’t han­dle the ex­o­dus of such mas­sive cra­ni­ums. That could be why new­borns have seg­mented skulls that can un­dergo an in­tense squeeze at birth and fuse into shape within the first two years of life.

Though it makes kid­dos fright­en­ingly del­i­cate, en­ter­ing the world un­der­cooked has its ad­van­tages: We’re less likely to get stuck at the ex­pense of our lives and our moth­ers’.

Newspapers in English

Newspapers from USA

© PressReader. All rights reserved.