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Research Article

A shift toward birthing relatively large infants early in human evolution

Jeremy M. DeSilva
PNAS first published January 3, 2011; https://doi.org/10.1073/pnas.1003865108
Jeremy M. DeSilva
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  1. Edited by C. Owen Lovejoy, Kent State University, Kent, OH, and approved December 2, 2010 (received for review March 23, 2010)

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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.

  • climbing
  • hominin
  • Homo
  • cooperative breeding

Footnotes

  • E-mail: jdesilva{at}bu.edu.
  • Author contributions: J.M.D. designed research, performed research, contributed new reagents/analytic tools, analyzed data, and wrote the paper.

  • The author declares no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1003865108/-/DCSupplemental.

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A shift toward birthing relatively large infants early in human evolution
Jeremy M. DeSilva
Proceedings of the National Academy of Sciences Jan 2011, 201003865; DOI: 10.1073/pnas.1003865108

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A shift toward birthing relatively large infants early in human evolution
Jeremy M. DeSilva
Proceedings of the National Academy of Sciences Jan 2011, 201003865; DOI: 10.1073/pnas.1003865108
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