對人類來說,分娩并非一件易事,胎兒大大的腦袋很難通過產(chǎn)道,這對產(chǎn)婦和胎兒都是巨大的挑戰(zhàn)。所幸胎兒的頭骨并沒有完全愈合定型,,受到擠壓時能夠改變形狀,這種情況可以持續(xù)到2周歲,。一項新的研究揭示,,骨縫的愈合延遲現(xiàn)象早在300萬年前就出現(xiàn)在人類祖先身上了,雖然那時胎兒的頭顱比現(xiàn)代人小得多,。一個合理的解釋是,,直立行走改造了骨盆的結(jié)構(gòu),也使分娩變得相當困難,,骨縫愈合延遲是伴隨著直立行走而出現(xiàn)的適應(yīng)現(xiàn)象,。
猩猩和其它高等靈長類的分娩相對容易些。新生的黑猩猩腦量為155cc,,只有人類的一小半,,另外人類的骨盆結(jié)構(gòu)也與其它靈長類不同。直立行走限制了骨盆的高度和寬度,為分娩造成了困難,,這便是人類的“分娩困境(obstetric dilemma)”,。頭骨愈合的延遲不僅略微緩解了分娩的困難,還為人類大腦發(fā)育提供了空間:新生兒的腦量僅為400cc,,成年之后竟可達1400cc,。
為研究人類演化過程中,骨縫愈合現(xiàn)象何時出現(xiàn),,圣塔菲高級研究院(School for Advanced Research in Santa Fe)的人類學(xué)家D.Falk組織研究團隊,,考察了大量古人類化石、現(xiàn)代人,、黑猩猩和狒狒的頭骨愈合情況,,并在《國家科學(xué)院院刊》(PNAS)上介紹了研究成果。其中最引人注目的材料是“湯恩幼兒(Taung child)”,,這是一件4歲齡的南方古猿非洲種(Australopithecus africanus)化石,,它包括了頭骨和自然形成的顱內(nèi)模,保存了骨縫的印痕,。
研究者們使用CT掃描技術(shù),,在電腦上建立了湯恩幼兒的顱內(nèi)模虛擬形象,重點考察額縫的生長情況,。額縫(metopic suture)也成額中縫,將額骨分成左右兩部分,,自鼻根至冠狀縫,,幼兒兩歲左右,額縫從鼻根開始愈合,,像拉鏈一樣將額骨的兩部分拼接起來,。研究小組發(fā)現(xiàn),湯恩幼兒的額縫并沒有愈合,,其腦量為400cc,。但它的大腦繼續(xù)發(fā)育的空間并不大,因為成年南方古猿腦量也僅為460cc,。
經(jīng)過對比,,研究者們發(fā)現(xiàn),猿類如黑猩猩和狒狒胎兒,,在出生后不久額縫就迅速愈合了,,但無論是湯恩幼兒還是后來的人族成員,額縫都在2歲以后乳臼齒長出時才漸漸愈合,。
那么,,為何腦量十分有限的南方古猿和早期人類,也有骨縫推遲愈合的現(xiàn)象呢,?Falk及其同事提出了三種假說,。其一,,一旦古猿開始直立行走,生育就不那么簡單了,,南方古猿的腦量和現(xiàn)代人比是很小的,,但依然比猩猩多出22%。因此從南方古猿時代起,,骨縫推遲愈合便是大勢所趨,。其二,現(xiàn)代人出生后大腦會加速發(fā)育,,早期人族成員可能也是一樣,,延遲愈合為大腦留出了生長空間,雖然這種加速發(fā)育在南方古猿身上不甚明顯,。第三,,南方古猿大腦額葉的顯著加寬,雖然腦量沒有明顯增加,,但整體形狀的改變已對分娩造成困難,。因此延遲愈合能緩解生產(chǎn)阻力。
“這些假說都很有道理,,”PNAS雜志的編輯,、美國肯特州立大學(xué)(Kent State University)的O.Lovejoy說。“有充足證據(jù)表明,,南方古猿大腦的某些區(qū)域比今天的黑猩猩寬闊,,前者經(jīng)歷過演化的改變。”(生物谷:Bioon.com)
doi:10.1073/pnas.1119752109
PMC:
PMID:
Metopic suture of Taung (Australopithecus africanus) and its implications for hominin brain evolution
Dean Falka,b,1, Christoph P. E. Zollikoferc, Naoki Morimotoc, and Marcia S. Ponce de Leónc,1
The type specimen for Australopithecus africanus (Taung) includes a natural endocast that reproduces most of the external morphology of the right cerebral hemisphere and a fragment of fossilized face that articulates with the endocast. Despite the fact that Taung died between 3 and 4 y of age, the endocast reproduces a small triangular-shaped remnant of the anterior fontanelle, from which a clear metopic suture (MS) courses rostrally along the midline [Hrdlička A (1925) Am J Phys Anthropol 8:379–392]. Here we describe and interpret this feature of Taung in light of comparative fossil and actualistic data on the timing of MS closure. In great apes, the MS normally fuses shortly after birth, such that unfused MS similar to Taung’s are rare. In humans, however, MS fuses well after birth, and partially or unfused MS are frequent. In gracile fossil adult hominins that lived between ∼3.0 and 1.5 million y ago, MS are also relatively frequent, indicating that the modern human-like pattern of late MS fusion may have become adaptive during early hominin evolution. Selective pressures favoring delayed fusion might have resulted from three aspects of perinatal ontogeny: (i) the difficulty of giving birth to large-headed neonates through birth canals that were reconfigured for bipedalism (the “obstetric dilemma”), (ii) high early postnatal brain growth rates, and (iii) reorganization and expansion of the frontal neocortex. Overall, our data indicate that hominin brain evolution occurred within a complex network of fetopelvic constraints, which required modification of frontal neurocranial ossification patterns.