Salt

You, me, your dog, and your parakeet all have salt in our blood. Under normal circumstances, its concentration is 0.9% sodium, barring electrolite imbalances and disease.

There are two probable reasons for this . One would be that this is how That Higher Being Whom We Revere designed things to be. (Nod to Heinrich Boll, who wrote a short story about a talk radio show in which the word "god" had the above substitute phrase used in its place.)

The other reason would be that 0.9% was the ocean salinity about 650 million years ago, when life began to crawl out of the seas and onto land. Lifeforms carried this salty genetic heritage with them as they populated the lands.

There need not be logical exclusion between these views. But only one of the above notions is a scientific theory.