古代人类祖先（人科动物）可能在人属进化出来之前就已经生出了较大的婴儿，并发展出了高强度、合作进行的新生儿护理方式——这是把人类和大猿区别开来的特征。Jeremy DeSilva使用来自一个全国灵长类研究中心、博物馆样本和此前研究的信息，来检验一个常见的主张：即比例来算，人类婴儿比猿的婴儿更重，同时也试图确定在人类进化过程中这种朝着更大的婴儿进化的转变发生在什么时候。DeSilva发现人类婴儿大约是母亲体重的6%，而黑猩猩的新生儿体重接近母亲体重的3%。由于更大的新生儿更难分娩而且更重而难以携带，一些科研人员已经提出，人类的抚养子女的特征——诸如父亲和家族其他成员参与新生儿护理——可能与猿人开始直立行走同时出现。然而，DeSilva提出，尽管最早的人科动物显示出了类似于今天的猿的新生儿-母亲体重比例，南方古猿属（一种如今已经灭绝的人科动物群，在大约400万年前进化出来）的雌性可能生出超过其体重5%的婴儿。DeSilva说，该研究提示，人类进化史中父母共同抚养子女的开始可能比此前科研人员认为的更早。（生物谷Bioon.com）
生物谷推荐原文出处：
PNAS doi: 10.1073/pnas.1003865108
A shift toward birthing relatively large infants early in human evolution
Jeremy M. DeSilva
Abstract
It has long been argued that modern human mothers give birth to proportionately larger babies than apes do. Data presented here from human and chimpanzee infant:mother dyads confirm this assertion: humans give birth to infants approximately 6% of their body mass, compared with approximately 3% for chimpanzees, even though the female body weights of the two species are moderately convergent. Carrying a relatively large infant both pre- and postnatally has important ramifications for birthing strategies, social systems, energetics, and locomotion. However, it is not clear when the shift to birthing large infants occurred over the course of human evolution. Here, known and often conserved relationships between adult brain mass, neonatal brain mass, and neonatal body mass in anthropoids are used to estimate birthweights of extinct hominid taxa. These estimates are resampled with direct measurements of fossil postcrania from female hominids, and also compared with estimates of female body mass to assess when human-like infant:mother mass ratios (IMMRs) evolved. The results of this study suggest that 4.4-Myr-old Ardipithecus possessed IMMRs similar to those found in African apes, indicating that a low IMMR is the primitive condition in hominids. Australopithecus females, in contrast, had significantly heavier infants compared with dimensions of the femoral head (n = 7) and ankle (n = 7) than what is found in chimpanzees, and are estimated to have birthed neonates more than 5% of their body mass. Carrying such proportionately large infants may have limited arboreality in Australopithecus females and may have selected for alloparenting behavior earlier in human evolution than previously thought.
