It can’t be a coincidence that almost all of them have red blood. Why can’t it be blue? Green? Purple?
In: Biology
It *can* be blue; in fact, blue [horseshoe crab blood](https://en.wikipedia.org/wiki/Horseshoe_crab#Blood_harvesting) is immensely valuable for a number of reasons. But the difference might answer your question. Other animals with blue blood include the octopus (where it was first discovered) and a bunch of other invertebrates.
Red blood relies on [hemoglobin](https://en.wikipedia.org/wiki/Hemoglobin) to transport oxygen. It’s a chemical that can weakly bond with oxygen inside lungs, and then lose that bond inside muscles; partially because muscles have [myoglobin](https://en.wikipedia.org/wiki/Myoglobin), which will bond with oxygen more aggressively. Blue(-ish) blood relies on [hemocyanin](https://en.wikipedia.org/wiki/Hemocyanin) to do a similar thing, but mechanically differently.
It’s the haemoglobin; the way in which animal life evolved requires transportation of oxygen around the body. Blood can do that, and the substance within the red blood cells that absorbs oxygen when you breathe in is a protein called haemoglobin. It has iron in it, and oxidised iron is red.
One of the main purposes of blood is to transport oxygen across the body. This is done by having oxygen molecules stick to a substance called Haemoglobin. The Hämoglobin has a red color and make the blood red.
As many animals use Haemoglobin to transport oxygen like humans do, they all have red blood.
Because most animals have a oxygen transporting blood cell which uses the molecule haemoglobin as an oxygen transporter. Haemoglobin is famous for it’s iron (Fe) content, which imparts the reddish color, especially when oxygenated . Horseshoe crabs have copper based oxygen carry systems and their blood is blue
All life that has blood has it because it needs to transport oxygen from its environment to its tissue. To do this, it needs to bind the oxygen to something that really likes oxygen. There are only really two good candidates for this: iron and copper. When iron reacts with oxygen, it rusts and turns red. When copper reacts with oxygen, it turns turquoise. Therefore, the color of your blood is determined by which element your body uses to transport oxygen. For us, that’s iron. For other creatures like bugs and crustaceans, that’s copper.
While others have mentioned hemoglobin as the oxigen transporting molecule that gives blood its color there’s a point missing: all of these animals with “red blood” evolved from a common hemoglobin-using ancestor – hence today you see its use spread across the animal kingdom.
P.S. I’m not excluding any sort of converging evolution, where another species not having a common ancestor may have evolved to use hemoglobin as its oxigen carrying molecule.
The majority of animals on earth are insects, and they don’t have red blood, so your premise is incorrect. As other have stated, it’s red because of the haem molecule which is made of iron. But other animals have different colours as they have different molecules in their blood, and they far outnumber the red ones.
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