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Experimental Manipulation of Body Size to Estimate Morphological Scaling Relationships in Drosophila

Experimental Manipulation of Body Size to Estimate Morphological Scaling Relationships in Drosophila

Published on: October 1, 2011

A shift toward birthing relatively large infants early in human evolution.

Jeremy M DeSilva¹

¹Department of Anthropology, Boston University, Boston, MA 02215, USA. jdesilva@bu.edu

Proceedings of the National Academy of Sciences of the United States of America

|January 5, 2011

Summary

Modern humans give birth to larger infants (6% body mass) than apes (3%). This study reveals Australopithecus females had large infants, suggesting a shift towards human-like birth mass ratios early in hominid evolution.

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Area of Science:

Paleoanthropology
Evolutionary Biology
Primatology

Background:

Modern humans give birth to proportionately larger infants than apes.
This difference has significant implications for human evolution, impacting birthing, social structures, and locomotion.
The timing of this evolutionary shift remains unclear.

Purpose of the Study:

To estimate birthweights of extinct hominid taxa.
To determine when human-like infant:mother mass ratios (IMMRs) evolved.
To assess the impact of infant size on early hominid behavior and evolution.

Main Methods:

Utilized conserved relationships between adult brain mass, neonatal brain mass, and neonatal body mass in anthropoids.
Estimated birthweights for extinct hominids.
Resampled estimates with fossil postcrania measurements from female hominids and compared with female body mass estimates.

Main Results:

Ardipithecus (4.4 Mya) exhibited IMMRs similar to African apes, indicating a primitive low IMMR in hominids.
Australopithecus females had significantly larger infants (estimated >5% body mass) compared to chimpanzees.
Fossil evidence from Australopithecus suggests larger infants than previously inferred.

Conclusions:

A low infant:mother mass ratio is the primitive condition for hominids.
Australopithecus females likely carried proportionately large infants, potentially limiting arboreality and promoting early alloparenting.
The shift to birthing larger infants occurred earlier in human evolution than previously assumed.