I've heard that if you feel thirsty you are already dehydrated.

This seems to go against evolutionary fitness, that our bodies don't warn us when we are in danger.

Can anyone clear this up?

TIA.

I have a degree in human nutrition.
From what I was told in class, yes, when you are thirsty you are already partially dehydrated. Of course, this is the same as when you feel your tummy grumble you are already hungry. The thirsty and hungry warnings serve us just fine. Unless you are elderly or have a condition which requires close monitoring of your fluid intake, drinking when you are thirsty is not detrimental to your health.

I hope that kind of helped.

Thanks.
Now that you say it, it seems obvious to me.
I must be losing my marbles. . .