I had to look it up, but I was interested enough to do that.
Hemoglobin and Chlorophyll have similar structures. The main difference is that hemoglobin is built around iron (Fe), where as chlorophyll is built around magnesium, (Mg). The primary function for hemoglobin is to transport oxygen from the lungs to other parts of the body. Hemoglobin is composed of four elements- carbon, hydrogen, oxygen and nitrogen. All four are organized around iron. Chlorophyll is composed of the same elements, which are organized around magnesium.
Assuming animals evolved from plants or some ancestor to both, it's hard to understand the evolutionary advantage of what seems like a very simple mutation. Chlorophyll and hemoglobin being so similar, there must be a connection of some sort. But how does that mutation become an advantage?
Now here's this flower happily going about manufacturing its own food, which is far more efficient than hunting and gathering. Then one day it's DNA gets zapped by a cosmic ray, and suddenly it's making hemoglobin instead of chlorophyll. Now it can no longer make it's own food, but it has this useless new molecule, and evolution taking a long as it does, there isn't enough time for the flower to grow arms and legs and teeth and a nervous system to run those required gadgets before it dies.
So instead of a flower, it must have been a giant sequoia, with a life span of a thousand years. Now it has plenty of time to evolve into an ape. Yes, I'm beginning to understand it now.