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The giraffe neck couldn't evolve without a special circulatory system

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Claim

A giraffe's heart must be quite large (it is over 11 kg or 24 lbs) to pump blood to the giraffe's head. A series of special one-way valves in the neck regulates blood flow, and there is a special net of elastic blood vessels at the base of the brain. Without these valves and elastic blood vessels, the blood pressure in the giraffe's head would be immense when it bends over, enough to cause brain damage. All of these features -- large heart, valves in the jugular vein, and wondernet of vessels -- must be in place simultaneously or the giraffe would die. They could not have evolved gradually.

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Responses

  1. These 'special' organs are not particularly unique to giraffes:
    • All vertebrates feature a heart, and it is fairly easy to imagine one evolving in size as the demands upon it were increased in each generation (see Response 2).
    • The net of capillaries is termed the 'intracranial carotid rete mirabile', and not unique to the giraffe. All members of Artiodactyla (such as sheep, cattle, camels, pigs etc) have such a feature, and it is thought to function in cooling the blood.
    • Valves of the jugular vein are found in all mammals to prevent backflow of blood, as without them none could circulate blood to and from the head effectively. In fact, all veins, except for cranial veins, feature venous valves.
  2. Large changes that develop features in an organism, such as that of a long neck, do not happen all at once. The neck could have grown progressively longer through a series of small changes over many generations. This would, instead of making a large heart imperative immediately, gradually increase the usefulness and thus the need for a large heart, leaving a large amount of time for a larger heart and a better circulatory system to evolve along with that of a progressively longer and longer neck.
  3. "They could not have evolved gradually." - They could have evolved just as gradually as the length of the neck did, from structures that have almost none of these special properties, through structures that have less-efficient grades of these properties, to become structures that have all of these special properties.
  4. add more responses

Fallacies contained in this claim

References

  1. Darwin, Charles, 1868. Variation of Animals and Plants Under Domestication, vol. 2, chpt. 20, p. 206. London: John Murray. [3]
  2. Katsuhiro Fukuta, Hiroshi Kudo, Motoki Sasaki, Junpei Kimura, Dahlan bin Ismail, Hideki Endo (2007) "Absence of carotid rete mirabile in small tropical ruminants: implications for the evolution of the arterial system in artiodactyls" Journal of Anatomy 210 (1), 112–116.

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