Giraffes, being one of the tallest mammals in the world, have a failsafe called

Question

Giraffes, being one of the tallest mammals in the world, have a failsafe called rete mirabile to prevent them from dying from excessive blood pressure while lowering their head.

Due to their long necks, a formidable amount of blood pressure is needed:

A giraffe's heart has special adaptations to enable it to pump blood up the animal's long neck to its head. A giraffe's heart has the formidable task of pumping blood at high enough pressure so that it can flow up the giraffe's neck to the brain. To accomplish this, a giraffe's heart is specially adapted. It can weigh up to 10 kg (22 lb) and generates twice the blood pressure of other large mammals. Having enough blood pressure to pump blood to the brain when the giraffe's neck is extended upward is one challenge, but when the animal lowers its head it risks injury due to excessive blood pressure. To counter this, giraffes have a pressure-regulating system known as the rete mirabile which restricts the amount of blood that rushes towards the brain when the giraffe lowers its head.

How did this evolve? I believe in evolution but I can't wrap my head around this. If an "overload" of blood would reach the giraffe's brain while it is, for instance, drinking water and either instantly killing the animal or severely wounding the animals mental and physical capabilities by rupturing the brain, how could a failsafe like this evolve?

Explanation / Answer

I think what leads to the confusion here is the assumption that the neck was evolved before the rete mirabile. Giraffes evolved from creatures with shorter necks. As evolution occurred nature was selecting for longer neck creatures (being able to eat more leaves) but at the same time creatures with longer necks had problems with blood pressure. At this point this would also not be lethal because we are talking about animals with much shorter necks, just in some way disadvantageous to the animal. So over time necks got bigger, blood pressure went up, and blood vessels developed that could better and better deal with the change of pressure. Rete mirabile also occur in other animals, so they are not a rare evolutionary phenomenon of giraffes.

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