Subcutaneous fat
From Wikipedia, the free encyclopedia
Subcutaneous fat is found just beneath the skin as opposed to visceral fat which is found in the peritoneal cavity. Subcutaneous fat can be measured using body fat calipers giving a rough estimate of total body adiposity. This fat aids in the process homeostasis, by forming a layer of insulation therefore stopping heat loss in the long-term.
The subcutaneous tissue is a layer of fat that lies between the dermis of the skin and underlying fascia. Subcutaneous fat insulates the body, absorbs trauma, and is a reserve energy source.[1] This tissue may be further divided into two components, the actual fatty layer, or panniculus adiposus, and a deeper vestigial layer of muscle, the panniculus carnosus.[2]
[edit] Function during human evolution
The conventional explanation of the environmental factors that drove human evolution was known as the Savannah Hypothesis and was first advanced by Raymond Dart. This postulated that the arboreal existence of the apes was replaced by a move to the savannah in order to hunt animals, even though major adaptations occurred in human ancestors long before the savannahs existed.[3] Several anthropologists, such as Bernard Wood, Kevin Hunt and Philip Tobias, have pronounced the Savannah Theory to be defunct. One of the reasons for the theory's demise is that fat has a much greater weight penalty for a running animal than the hair it supposedly replaced, and yet man carries more fat than other land mammals.
Among humans, men and women require different levels of body fat to be healthy. If a man's body fat percentage falls below about 4%, or a woman's below about 12%, then the person is likely to become seriously ill or perhaps die. However, health effects are seen well before this range. If a woman's adipose tissue constitutes less than 17% of her body weight, she ceases to menstruate.[4] The average human contains ten times as many adipocytes fat cells as would be expected in an average mammal of similar size. From this Elaine Morgan has concluded that at some point in human evolution high minimum standards for fat were established[5]. She contends that the quantity of subcutaneous fat in humans supports the other theory for the early evolution of proto-humans: the aquatic ape hypothesis (AAH), though this is still strongly disputed by many researchers. Greater insulation is needed by warm-blooded animals in water than in air.
The insulation provided by fat is strongly analogous to the sub-cutaneous fat in aquatic birds and larger aquatic mammals. This correlation was first noticed by Alister Hardy in 1930, while reading Frederic Wood Jones' Man's Place among the Mammals, which included the question of why humans, unlike all other land mammals, had fat attached to their skin. Hardy realised that this trait sounded like the blubber of marine mammals, and began to suspect that humans had ancestors more aquatic than previously imagined.
In comparison with infant apes, human infants have a remarkable level of subcutaneous fat[6]. The fat appears in the thirtieth week of pregnancy and continues increasing for the first year after birth. As well as providing insulation for a baby while its mother is in water, the additional buoyancy has been noted as another benefit of fat[5][7].
[edit] References
- ^ Marks, James G; Miller, Jeffery (2006). Lookingbill and Marks' Principles of Dermatology (4th ed.). Elsevier Inc. Page 12-13. ISBN 1-4160-3185-5.
- ^ McGrath, J.A.; Eady, R.A.; Pope, F.M. (2004). Rook's Textbook of Dermatology (Seventh Edition). Blackwell Publishing. Pages 3.1. ISBN 9780632064298.
- ^ Richard Leakey, Roger Lewin (1992). Origins Reconsidered. Little, Brown & Co. ISBN 0 349 10345 3.
- ^ Frisch, Rose E (1984). "Body Fat & Puberty". Biological Review 59: 161-188.
- ^ a b Morgan, Elaine (1997). The Aquatic Ape Hypothesis. Souvenir Press.
- ^ Schultz, AH (1969). The Life of Primates. Weidenfeld & Nicholson. ISBN 0 297 17044 9.
- ^ Scars of Evolution (On-line series of two BBC radio programmes about AAH)
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