对人类来说，分娩并非一件易事，胎儿大大的脑袋很难通过产道，这对产妇和胎儿都是巨大的挑战。所幸胎儿的头骨并没有完全愈合定型，受到挤压时能够改变形状，这种情况可以持续到2周岁。一项新的研究揭示，骨缝的愈合延迟现象早在300万年前就出现在人类祖先身上了，虽然那时胎儿的头颅比现代人小得多。一个合理的解释是，直立行走改造了骨盆的结构，也使分娩变得相当困难，骨缝愈合延迟是伴随着直立行走而出现的适应现象。
猩猩和其它高等灵长类的分娩相对容易些。新生的黑猩猩脑量为155cc，只有人类的一小半，另外人类的骨盆结构也与其它灵长类不同。直立行走限制了骨盆的高度和宽度，为分娩造成了困难，这便是人类的“分娩困境(obstetric dilemma)”。头骨愈合的延迟不仅略微缓解了分娩的困难，还为人类大脑发育提供了空间：新生儿的脑量仅为400cc，成年之后竟可达1400cc。
为研究人类演化过程中，骨缝愈合现象何时出现，圣塔菲高级研究院(School for Advanced Research in Santa Fe)的人类学家D.Falk组织研究团队，考察了大量古人类化石、现代人、黑猩猩和狒狒的头骨愈合情况，并在《国家科学院院刊》(PNAS)上介绍了研究成果。其中最引人注目的材料是“汤恩幼儿(Taung child)”，这是一件4岁龄的南方古猿非洲种(Australopithecus africanus)化石，它包括了头骨和自然形成的颅内模，保存了骨缝的印痕。
研究者们使用CT扫描技术，在电脑上建立了汤恩幼儿的颅内模虚拟形象，重点考察额缝的生长情况。额缝(metopic suture)也成额中缝，将额骨分成左右两部分，自鼻根至冠状缝，幼儿两岁左右，额缝从鼻根开始愈合，像拉链一样将额骨的两部分拼接起来。研究小组发现，汤恩幼儿的额缝并没有愈合，其脑量为400cc。但它的大脑继续发育的空间并不大，因为成年南方古猿脑量也仅为460cc。
经过对比，研究者们发现，猿类如黑猩猩和狒狒胎儿，在出生后不久额缝就迅速愈合了，但无论是汤恩幼儿还是后来的人族成员，额缝都在2岁以后乳臼齿长出时才渐渐愈合。
那么，为何脑量十分有限的南方古猿和早期人类，也有骨缝推迟愈合的现象呢？Falk及其同事提出了三种假说。其一，一旦古猿开始直立行走，生育就不那么简单了，南方古猿的脑量和现代人比是很小的，但依然比猩猩多出22%。因此从南方古猿时代起，骨缝推迟愈合便是大势所趋。其二，现代人出生后大脑会加速发育，早期人族成员可能也是一样，延迟愈合为大脑留出了生长空间，虽然这种加速发育在南方古猿身上不甚明显。第三，南方古猿大脑额叶的显著加宽，虽然脑量没有明显增加，但整体形状的改变已对分娩造成困难。因此延迟愈合能缓解生产阻力。
“这些假说都很有道理，”PNAS杂志的编辑、美国肯特州立大学(Kent State University)的O.Lovejoy说。“有充足证据表明，南方古猿大脑的某些区域比今天的黑猩猩宽阔，前者经历过演化的改变。”(生物谷：Bioon.com)
doi:10.1073/pnas.1119752109
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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.
