Every drop of blood in your body is doing something ancient.
It is carrying oxygen, fighting infections, clotting wounds, and patrolling for threats, a set of tasks so fundamental to animal life that nearly every creature with a backbone does some version of the same thing.
But where did blood cells come from in the first place?
How does a single fertilised egg, over hundreds of millions of years of evolution, produce something as complex and specialised as an immune system?
Researchers at Kyoto University have now published what may be the most complete answer yet to that question, and the answer is considerably more humbling than anyone expected.
