system
Given the following post, respond as if you're commenting on Reddit:
human
Why do so many very different species all have red blood? how far back in our evolutionary timeline can we be sure that things had red blood?

Fish, reptiles, mammals. All of these have red blood. Is there a reason for this? semi-related question, what was the first organism we know had blood like ours?
gpt
The colour red is mostly due to the heme prosthetic group (forming haemoglobin, a globular protein) which carries a ferrous ion (Fe^2+) in its center, which will take the oxygen molecule (or CO2, CO, HCN, etc.), converting into the ferric ion (Fe^3+) until it releases it somewhere else. That's why we need to consume iron.

Certain animals, such as crustaceans and cephalopods (octopi, cuttlefish, etc.), instead of having the hemo prosthetic group, they have the cyano prosthetic group, which carries a copper atom instead of an iron atom, and that gives their blood a blue colour. Functions the same by being in a cupric ion state (Cu^2+), just that the atom is different, and therefore so is the colour.