Fatness at Birth

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Human Uniqueness Compared to "Great Apes": 
Relative Difference
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Humans are among the fattest mammals at birth  reflecting rapid fat deposition during the third trimester of pregnancy. For most mammals, roughly 2-3% of birth weight is fat. Chimpanzee newborns average 3% of birth weight as body fat.  The human level of newborn fattness exceeds that of all documented mammals other than a single seal species, which is born with a comparable level. Most human baby fat is subcutaneous white adipose tissue, rather than heat-producing brown fat. The evolutionary origins of the distinctive human pattern of fat deposition is not known, but several explanations have been suggested. Subcutaneous white fat is an insulator in some mammals; it was traditionally assumed that humans evolved extra baby fat to compensate for the loss of fur. The evolutionary importance of fat as a source of insulation in humans is debated, and there is minimal evidence that human baby fat evolved to serve this role. More recently it has been proposed that human baby fat evolved to protect brain energetics during starvation or nutritionally-stressful childhood infections. In humans, the unusually large brain consumes the majority of the body's energy at birth, and is rapidly damaged by a shortfall of energy or oxgen. Human metabolism shows evidence for an evolved capacity to feed the brain on body fat during starvation, as evidenced for instance by the human brain's unusual ability to utilize fat-derived ketone bodies in lieu of glucose. Testing whether human baby fat evolved as an energetic buffer for the brain awaits data on brain size and body composition from a wider array of mammal and especially primate species.

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Timing

Timing of appearance of the difference in the Hominin Lineage as a defined date or a lineage separation event. The point in time associated with lineage separation events may change in the future as the scientific community agrees upon better time estimates. Lineage separation events are currently defined as:

  • the Last Common Ancestor (LCA) of humans and old world monkeys was 25000 thousand (25 million) years ago
  • the Last Common Ancestor (LCA) of humans and chimpanzees was 6000 thousand (6 million) years ago
  • the emergence of Homo ergaster was 2000 thousand (2 million) years ago
  • the Last Common Ancestor (LCA) of humans and neanderthals was 400 thousand years ago
  • the common ancestor of modern humans was 100 thousand years ago

Possible Appearance: 
6,000 Thousand Years
Probable Appearance: 
3,000 Thousand Years
Definite Appearance: 
100 Thousand Years
The Human Difference: 

Humans are characterized by having an unusually high percentage of subcutaneous fat as compared to visceral fat. This is especially apparent in newborns, roughly 15% fat at birth, compared to around 3% in chimpanzee newborns. Few data have been published on newborn body composition in other species of non-human primates, but there is circumstantial and some quantitative evidence that primates are typically mammalian in this respect. The human level of newborn fattness exceeds that of all documented mammals other than a single seal species, which is born with a comparable level. Most human baby fat is subcutaneous white adipose tissue, rather than heat-producing brown fat. The evolutionary origins of the distinctive human pattern of fat deposition is not known, but several explanations have been suggested. Subcutaneous white fat is an insulator in some mammals, particularly aquatic species, so it was traditionally assumed that humans evolved extra baby fat to compensate for the loss of fur. The evolutionary importance of fat as a source of insulation in humans is debated, however, and there is minimal evidence that human baby fat evolved to serve this role.

 

Mechanisms Responsible for the Difference: 

Late in the final trimester, human infants accumulate up to 500 grams of additional fat, accounting for around 90% of total weight gain in the final weeks of gestation. A greater proportion of body fat in human newborns seems to reflect the lengthening of the gestational period in humans as compared to other apes, around 237 days in chimpanzees versus roughly 266 days in humans. This additional fat provides insulation for temperature regulation, as well as offering protection from nutritional stress. Infants born prematurely have substantially less body fat, and may be more vulnerable to the effects of early nutritional stress due to insufficient maternal milk production, particularly prevalent in nulliparous (first-time) mothers.

Possible Selection Processes Responsible for the Difference: 

Infants born prematurely have substantially less body fat, and may be more vulnerable to the effects of early nutritional stress due to insufficient maternal milk production, particularly prevalent in nulliparous (first-time) mothers. Infant subcutaneous fat stores may be important for maintaining the growth and development of the human brain, providing ketones, an optimal energy source for use by neural tissues in the absence of glucose. Early in development, ketones may actually be of principle importance as a fuel source.

References

  1. Metabolic correlates of hominid brain evolution., Leonard, William R., Robertson Marcia L., J Snodgrass Josh, and Kuzawa Christopher W. , Comp Biochem Physiol A Mol Integr Physiol, 2003 Sep, Volume 136, Issue 1, p.5-15, (2003)
  2. Survival of the fattest: fat babies were the key to evolution of the large human brain., Cunnane, Stephen C., and Crawford Michael A. , Comp Biochem Physiol A Mol Integr Physiol, 2003 Sep, Volume 136, Issue 1, p.17-26, (2003)
  3. Fast and reproducible method for the direct quantitation of adipose tissue in newborn infants., Harrington, T A. M., Thomas E L., Modi N, Frost G, Coutts G A., and Bell J D. , Lipids, 2002 Jan, Volume 37, Issue 1, p.95-100, (2002)
  4. Adipose tissue in human infancy and childhood: an evolutionary perspective., Kuzawa, C W. , Am J Phys Anthropol, 1998, Volume Suppl 27, p.177-209, (1998)
  5. Body Composition and Energy Needs during Growth, Holliday, MA , Human Growth: A Comprehensive Treatise, New York, p.101-117, (1986)